NCD095458527_20000328_FCX Inc. (Statesville)_FRBCERCLA RD_Final Remedial Design Report OU-3-OCR

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FINAL REMEDIAL DESIGN REPORT FOR
OPERABLE UNIT THREE (OU3)
FCX-STATESVILLE SUPERFUND SITE
STATESVILLE, NORTH CAROLINA
prepared for
El Paso Energy Corporation
1001 Louisiana Street
Houston, TX 77002
March 2000
27-60313.011
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Environmental Engineering & Consul(ing
227 French Londing Drive
Nashville. Tennessee 37228• 1605
Tel: [phone]fl
Fax: [phone]
March 28, 2000
Mr. McKenzie Mallary
North Site Management Branch
EPARegion4
Atlanta Federal Center
61 Forsyth Street
Atlanta, GA 30303
RE: Final Remedial Design Report for Operable Unit Three (OU3)
FCX-Statesville Superfund Site, Statesville, North Carolina
Dear Ken:
27-60313.011
Enclosed are three copies of the "Final Remedial Design Report for Operable Unit
Three (OU3), FCX-Statesville Superfund Site, Statesville, North Carolina". This report
has two attachments: the Technical Specifications and the Design Plans. The response
to comments on the Pre-Final Remedial Design (RD) Report has been added to this
report. For your convenience, one of the reports is provided in a three-ring binder.
If you have any questions regarding this document, please call me at [phone] or
Mr. Roger Towe of El Paso Energy Corporation at [phone].
Sincerely,
Brown and Caldwell
Kenton H. Oma, P.E.
Assistant Technical Director
Design and Solid Waste
cc: N. Testerman, NCDENR
R. McKeen, Weston
R. Towe, El Paso
H. Mitchell, Jr., Beaunit
J. Wright, Burlington
N. Prince, ESC
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TABLE OF CONTENTS
Letter of Transmittal
Table of Contents
List of Tables
List of Figures
Engineering Certification Pages
Response to Comments on the Pre-Final Remedial Design Report
1.0 INTRODUCTION
I. I Site Background
1.1.1 Site Description
1.1.2 Site History
1.1.3 Site Conditions
1.2 Remedial Design Objectives
1.2.1 Soil Design Objectives
1.2.2 Groundwater Design Objectives
1.3 Description of OU3 Remediation Technologies
1.3.1 Soil Vapor Extraction
1.3 .2 Air Sparging with Soil Vapor Extraction
1.3.3 Monitored Natural Attenuation
2.0 PRE-DESIGN INVESTIGATION
2.1 Installation of Groundwater Monitoring Wells
2.2 Results of Groundwater Sampling and Analyses
2.3 Evaluation ofNatural Attenuation
2.4 Pilot Test Results
3.0 DESIGN ANALYSIS
3. I Design Approach
3.2 Design for Monitored Natural Attenuation
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TABLE OF CONTENTS (Continued)
3.3 Phase I Design
3.3.1 Wells and Monitoring Probes
3.3.2 Soil Vapor Extraction System
3.3.3 Air Sparging System
3.3.4 Data Acquisition and Process Control
3.3.5 Facility Modifications and Equipment Locations
3.4 Phase I Design Documents
3.4.1 Technical Specifications and Design Plans
3.4.2 Preliminary Construction Cost Estimate
3.5 Phase I Performance Testing
3.5.1 Baseline Sampling and SVE Radius of Influence
Measurements
3.5.2 Source Area Monitoring
3.5.3 Soil Vapor Extraction
3.5.4 Soil Vapor Extraction with Air Sparging
3.5.5 Performance Evaluation Report
3.6 Phase II Design (If Required)
4.0 REMEDIAL ACTION
4.1 Project Delivery Strategy
4.2 Remedial Action Work Plan
4.3 Remedial Action
4.4 Permit Requirements
4.4.1 Air Emissions from Soil Vapor Extraction
4.4.2 Soil Erosion and Sediment Control
4.4.3 Well Construction Permits
4.4.4 Handling Potentially Contaminated Soil
4.5 Preliminary Schedule
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TABLE OF CONTENTS (Continued)
APPENDICES
Appendix A -
Appendix B -
Appendix C -
Technical Protocol for Evaluating Natural Attenuation of
Chlorinated Solvents in Ground Water
Design Calculations
Preliminary Construction Cost Estimate
ATTACHMENTS
Attachment I -Technical Specifications
Attachment 2 -Design Plans
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LIST OF TABLES
Follows
Table No. Title Page No.
3-1 Monitoring Wells Selected for Semi-Annual Groundwater Sampling,
Operable Unit Three (003), FCX-Statesville Superfund Site 3-2
3-2 Summary of Chemical Analyses and Analytical Method References
for Semi-Annual Groundwater Sampling, Operable Unit Three
(003), FCX-Statesville Superfund Site 3-2
LIST OF FIGURES
]ligure No. Title
1-1 Site Location Map
1-2 Site Layout
2-1 Monitoring Well Location Map
2-2 Isoconcentration Map for PCE in Shallow Groundwater
2-3 Isoconcentration Map for PCE in Intermediate Groundwater
2-4 Isoconcentration Map for TCE in Shallow Groundwater
2-5 lsoconcentration Map for TCE in Intermediate Groundwater
2-6 Locations of Pilot Test Wells and Monitoring Probes
3-1 Design Layout of Phase I SVE Wells, Air Sparging Wells, and
Monitoring Probes
3-2 Conceptual Layout of Phase II SVE Wells in Relation to Phase I
SVE Wells
4-1 Preliminary Schedule for Remedial Action for Operable Unit Three
(003), FCX-Statesville Superfund Site
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PROFESSIONAL ENGINEER'S CERTIFICATION
This FCX Operable Unit Three (OU3) Remedial Design has· been prepared under the
direction and supervision of a qualified, State of North Carolina licensed, Professional
Engineer. Mr. Robert E. Ash, IV, P.E., of Brown and Caldwell was responsible for the
overall preparation of the Design. Portions of the Remedial Design were prepared by
subcontractors. Mr. Mathew B. Dozier, P.E. and Mr. M. Hall Oakley, P.E. of Smith
Seckman Reid were responsible for the electrical, and the structural portions of the design,
respectively. Professional Engineer's certifications are included in this section.
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PROFESSIONAL ENGINEER'S CERTIFICATION
This is to certify that the FCX Operable Unit Three (OU3) Remedial Design for the FCX
Superfund Site in Statesville, North Carolina was prepared under my direction and
superv1s10n.
R.obert·E. Ash, IV, P.E.
North Carnlina Registration No. 23295
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PROFESSIONAL ENGINEER'S CERTIFICATION
This is to certify that the electrical design of the FCX OU3 Remedial Design for the FCX
Superfund Site in Statesville, North Carolina was prepared under my direction and
supervision.
Matt Dozie P.E. Date
North Carolina Registration No. 18634
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PROFESSIONAL ENGINEER'S CERTIFICATION
This is to certify that the Structural Design of the FCX OU3 Remedial Design for the
FCX Superfund Site in Statesville, North Carolina was prepared under my direction and
supefVls1on.
all Oakley, P.E.
North Carolina Registration
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RESPONSE TO COMMENTS ON THE
PRE-FINAL REMEDIAL DESIGN REPORT
FOR OPERABLE UNIT THREE (OU3)
FCX-STATESVILLE SUPERFUND SITE
STATESVILLE, NORTH CAROLINA
Dated September 1999
Following are comments received from the USEP A. Each comment is followed by a response.
SPECIFIC COMMENTS -PRE-DESIGN INVESTIGATION REPORT
1.
2.
Page E-7 (ES-7), First Bullet: The statement is made that it is difficult to predict where
sparged air and entrained VOCs may migrate and that therefore, critical placement of SVE
well should be considered. Please provide in the design report a discussion of how
"leakage" or unintended migration of sparged air and entrained VOCs during particularly
Phase 1 (prior to any operating experience or opportunity for modification) will be detected,
measured, and controlled.
Response:
Detection, measurement, and control of unintended migration will be addressed in a Phase 1
Performance Testing Plan. The elements of this plan will be discussed in the Remedial
Action Work Plan (RAWP). We anticipate that the Phase 1 Performance Testing Plan will
be issued during the Phase 1 construction. In general, unintended migration will be
addressed by the following: installation of 10 soil vapor extraction (SVE) wells and only two
air sparging wells; monitoring the facility with portable instruments; and measuring vacuum
influence at the monitoring probes and under the facility slab.
Page 3-8, Paragraph 1: Please discuss the line of evidence for concluding that mercury is
an anomaly in well W-Ss rather than being site relates (relatnd).
Response:
Mercury was not detected in the groundwater samples from well W-Ss during the remedial
investigation (RI) and is not a known site contaminant based on historic activities at the
textile facility.
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3. Page 4-5, Paragraph 2: Provide some evidence to support the observation that daughter
products are found.
Response:
Groundwater samples contain PCE daughter products, including TCE, cis-1,2-DCE, and
some vinyl chloride. In particular, the cis-1,2-DCE is not known to have an industrial use,
so its presence within OU3 is attributed to the anaerobic degradation of PCE. This is
further discussed in the USEPA Protocol provided as Appendix A of the Pre-Final Remedial
Design Report.
SPECIFIC COMMENTS -PRE-FINAL DESIGN REPORT
4. Page 1-5, Paragraph 2: The statement is made that institutional controls will be addressed in
explanation of significant difference currently under the consideration by the USEP A.
Review of this document does not include review of that explanation of significant
difference.
Response:
The explanation of significant difference (ESD) has not been finalized. It was not intended
for the RD to include the ESD. The RD addresses the remediation of the OU3 source area
by AS/SVE and the plume by monitored natural attenuation. The ESD is intended to
address institutional controls.
5. Page 1-6, Paragraph 2: The statement is made that the ability of SVE to remove semi-
volatile compounds and non-volatile compounds is limited to the extent of (that) these
compounds are biodegradable. It should be noted that certain semi-volatile compounds are
sufficiently volatile to be ~emoved partially by SVE systems. Please indicate whether any
such compounds are present.
Response:
In general, semi-volatile organic compounds (SVOCs) can be removed by SVE systems,
although the removal is inefficient. As summarized in Table 34 of the Remedial
Investigation (RI) Report, there have been very few detected SVOCs in groundwater
(1,2,4-trichlorobenzene was reported in wells W-16s and W-17s). Tables 17 and 18
document SVOCs in the site soil. Most detections of SVOCs were P AHs and most of these
were "J" qualified. These results are summarized in Table 19 of the RI Report.
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6.
7.
8.
Page 1-9, Paragraph 3: Line 3 references "the protocol", however, the complete reference
for this protocol is not provided until later in the same paragraph. Please provide the
complete reference the first time it is cited.
Response:
The complete reference, "Technical Protocol for Evaluating Natural Attenuation of
Chlorinated Solvents in Ground Water", is provided in Appendix A of the Pre-Final RD
Report. This editorial comment will not be addressed in the RD report because it was
determined in discussions with the USEP A that the Pre-Final RD did not need to be issued
agam.
Page 2-2, Second Bullet: Please describe how the depth of the media of lower
permeability was detected or measured and whether there is any concern that the lower
permeability unit was penetrated by the well drilling activity.
Response:
The estimated depth of the lower permeability unit was determined during the installation of
deep monitoring well W-20d. This was accomplished by advancing the well boring into the
bedrock using a 6-inch air-hammer and interval packer testing on ten-foot intervals as the
boring was advanced. Interval packer tests were conducted from a depth of 122 feet to
202 feet. Based on the interval packer tests, fracture permeability was detected to a depth of
162 feet. Between the depths of 162 feet and 202 feet, no fracture permeability was
detected. The interval packer testing demonstrated that the base of the upper fractured zone
terminated at a depth of approximately 162 feet below the ground surface at W-20d. Below
a depth of 162 feet, no permeability was observed, showing that at least 40 feet of potential
aquitard separates the upper fractured unit from the deeper groundwater units.
Page 2-3, Third Bullet: Please describe how iron and manganese would result from natural
attenuation mechanisms.
Response:
Oxidized forms of iron and manganese serve as electron acceptors during biodegradation as
discussed in the USEPA Protocol (Appendix A of the Pre-Final RD). The process converts
the insoluble higher oxidized states of iron and manganese to the more soluble reduced
forms. As a result, the dissolved phase concentrations of these two metals increase in many
biologically active aquifers.
9. Page 2-4, Second Bullet: Please discuss whether the reported reductive dechlorination
products could in fact be original contaminants at the site.
Response:
The degradation sequence is PCE to TCE to cis-1,2-DCE to vinyl chloride. The
cis-1,2-DCE is not known to have an industrial use so its presence within OU3 is attributed
to the anaerobic degradation of PCE, and not a site contaminant. As well as being a PCE
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10.
11.
12.
daughter product, TCE does occur as an impurity in PCE and its presence at OU3 could be
attributed, in part, to this. Since cis-1,2-DCE is present, TCE must also be present as a
daughter product of PCE.
Page 2-7, Fourth Bullet: Please discuss what data from the pilot test demonstrated that air
sparging may inhibit natural attenuation.
Response:
It is known that oxygen inhibits reductive dechlorination (see USEPA Protocol, Appendix A
of Pre-Final RD). If dissolved oxygen in groundwater moves away from the zone of active
air sparging, then anaerobic biodegradation can become inhibited. A helium tracer test
during the pilot test indicated some lateral movement of the sparged air. The lateral
movement was also indicated by vacuum/pressure measurements at the monitoring probes.
As part of the Phase I performance testing, any lateral movement will be minimized by the
AS/SVE operations based on data collected during the testing. ·
Page 2-7, Third Bullet and Page 3-1, Second Paragraph: The report indicates that a
phase approach will be used to design and implement the treatment system and that careful
placement of SVE wells using a phase approach should be considered to maximize a capture
of injected air. Please describe how the Phase 1 system will be implemented to minimize the
likelihood of escaping air from the SVE system.
Response:
In general, unintended migration will be addressed during Phase 1 by the following:
installation of 10 soil vapor extraction (SVE) wells and only two air sparging wells;
monitoring the facility with portable instruments; and measuring vacuum influence at the
monitoring probes and under the facility slab. Based upon the radius of influence
anticipated for the SVE and air sparging wells as determined during the AS/SVE pilot test,
the area of influence for the SVE wells should extend beyond the area of influence of the air
sparging wells. A Phase 1 Performance Testing Plan will be issued during the Phase 1
construction and will address the monitoring and measurements that will be performed.
Page 3-1, Paragraph 3: The text indicates that the criteria for evaluating the performance
of Phase I AS/SVE are listed in the RD. Please indicate clearly where these can be found.
Please note the general criteria provided on page 3-2 including extracted soil/ gas
concentrations, mass removal rates, and total contaminant mass removal will not necessarily
meet the objective of establishing the completeness of capture of sparged air established in
the previous paragraph.
Response:
The general criteria are provided in the same paragraph on page 3-2. As will be discussed in
the Phase 1 Performance Testing Plan, performance evaluation will be based on vacuum
influence, mass removal, residual VOC concentrations at individual monitoring locations,
impact on groundwater quality, etc. Completeness of capture will be assessed based on
..
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13.
14.
15.
16.
vacuum distribution and helium tracer testing at SVE wells, monitoring wells, and at the
surface. Various surface locations will be monitored for VOCs.
Page 3-2, Third Bullet: The text references a Quality Assurance Project Plan. Please
indicate whether this is an existing plan or is yet to be prepared.
Response:
The Quality Assurance Project Plan was prepared by Aquaterra, Inc. for the RI/FS. This
plan was incorporated by reference for the pre-design investigation work and will be
incorporated by reference for work related to the RD/RA.
Table 3-1: Please clarify the selection of monitor wells to be used for natural attenuation
background wells. Several of the wells such as W-12s and W-12i appear to be within the
contaminated zone. In addition, please provide groundwater flow direction information to
support the use of these as presumed background wells.
Response:
A groundwater divide exists at the site with groundwater flowing to both the north and
south. A groundwater divide is located approximately at the railroad tracks. There is no up-
gradient portion of the plume (refer to Figures 2-3, 2-4, 2-5, and 2-6 in the Pre-Final RD
Report). Therefore the selected background wells are located to the east and west of the
plume. Samples from these wells have shown trace concentrations of V OCs but are
minimally impacted and thus are satisfactory to serve as background wells for evaluating
geochemical/biopararneter conditions.
Page 3-3, Second Paragraph: The text indicates that the SVE system and combined SVE
and air sparging systems will be operated long enough to permit data collection and
evaluation of the Phase I installation. Please provide an initial estimate of how long that
operating period will be.
Response:
The Phase I performance testing will include an estimated six months of SVE without air
sparging followed by one month of combined AS/SVE. The extended SVE operations
period is planned to allow the SVE system to approach a more steady-state voe removal
condition so that when the combined AS/SVE testing is conducted, changes in VOC
concentrations from the extracted vapors can be more directly attributed to the air sparging.
Actual testing periods may vary based on performance data.
Page 3-4, Paragraph 2: Please provide the rationale for selection of 10,000 parts per billion
isoconcentration line as the basis for the Phase I design.
Response:
The objective of the AS/SVE is to remove voe mass from the source area. The PeE data
in the shallow groundwater indicate that the PeE concentrations decrease rapidly away from
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17.
18.
19.
20.
the 10,000-ppb isoconcentration. This provides evidence that the PCE source is within the
10,000-ppb isoconcentration, and as such, the Phase I design is targeted within this area
beneath the textile plant.
Page 3-8, Section 3.4.2: Please briefly state which factors determine the range of capital on
O&M costs (i.e., which design and or operating variables would result in higher or low(er}
end range costs.
Response:
This information is provided in Appendix C of the RD report. The cost table in
Appendix C gives a low and high estimate for the quantity of each capital cost element and a
low and high estimate for the unit cost of each capital cost element. The cost table presents
an estimated low and high annual cost for each O&M element.
Page 3-11, Paragraph 1: The text references helium tracer tests and groundwater
measurements to be taken during the operation phase. Please indicate where the detailed
test procedures and plans for these tests will be provided.
Response:
The detailed test procedures will be located in the Phase I Performance Testing Plan that will
be prepared.
Page 3-12, Paragraph 2: Based upon the description in this section, it appears that a
Phase II system is intended to fill gaps in the Phase I system, rather than to expand the zone
of influence beyond the Phase I system. Please clarify that this is the case and provide the
basis for limiting the treatment zone to 10,000-ppb isoconcentration line rather than
expanding the system beyond that area.
Response:
The Phase II design, if required, is intended to fill in the gaps in the AS/SVE system. The
extent of a Phase II system will be based upon the Phase I testing data that will be obtained
including the groundwater quality results of source-area sampling. The basis for limiting the
AS/SVE treatment zone to the 10,000-ppb isoconcentration line is presented in the
response to question 16.
Page 4-1, Paragraph 3: For the remedial action work plan, please indicate where within
this structure the quality assurance project plan for analytical data will be contained.
Response:
The Quality Assurance Project Plan was prepared by Aquaterra, Inc. for the RI/FS. This
plan was incorporated by reference for the pre-design investigation work and will be
incorporated by reference for work related to the RA.
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21. Page 4-4, Section 4.4 Permit Requirements: Please indicate whether during operations if
other waste streams will be generated and whether permits for their management are
required. Likely waste streams may include liquid condensate from the vapor separator on
the SVE system. In general, this would not likely require a permit if the material were stored
on site for short periods before off-site disposal. Please clarify.
Response:
Liquid condensate is not expected since the AS/SVE system is mostly indoors; however, the
design incorporates provisions for collecting condensate should it be present. The
condensate will be placed in containers and appropriately disposed of off-site in the same
manner as the liquid IDW that is generated during the groundwater sampling events. A
permit is not anticipated to be required for this activity. Spent granular activated carbon
(GAC) will be removed from the site and disposed of or regenerated as appropriate.
ATTACHMENT 2 -DRAWINGS
22. DWG 60313 C2 Construction Plan. Since the AS/SVE systems will be under the
structure, how has the design accounted for any high permeability zones around buried lines
or the subsurface fill under the slab? If the structure is slab on grade (see sheet M2) there
will presumably be a high permeability (e.g., crushed stone subgrade?) zone under the slab.
Will this affect air flow, contribute to short-circuiting or otherwise affect air capture? Should
a sub-slab ventilation system be used to insure sparged air does not escape through this
zone?
23.
Response:
The high permeability zone under the building slab is anticipated to cause the SVE system to
perform as if no slab was present at all. Potential short-circuiting will be investigated during
the Phase I testing by taking vacuum measurements at the well and probe locations and by
performing a helium tracer test. Since the design calls for 10 SVE wells, it is not anticipated
that a sub-slab ventilation system will be needed.
DWG 60313 Pl Process Flow Diagram. The following items related to the PFD should
be clarified in the Design Report (not necessarily on the PFD).
1. Please provide a discussion in the Design report of the method and strategy for
balancing and control of the various SVE and AS wells.
Response:
The AS and SVE wells will be balanced by first determining achievable flow rates
and VOC concentrations at the SVE wells. The SVE wells will be adjusted in an
effort to maximize the VOC mass removal rate while maintaining a vacuum at the
well and probe locations. Details will be provided in the Phase I Performance
Testing Plan.
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2.
3.
Please discuss how liquid from the moisture separator will be managed.
Response:
Liquid condensate is not expected since the AS/SVE system is mostly indoors;
however, the design incorporates provisions for collecting condensate should it be
present. The condensate will be placed in containers and appropriately disposed of
off-site in the same manner as the liquid IDW that is generated during the
groundwater sampling events. A permit is not anticipated to be required for this
act1v1ty.
Please indicate whether GAC canisters are disposable or will be regenerated or
refilled on site. (If the latter, should the PFD show this component) Please discuss
if the.GAC will be operated in convention{aO lead lag mode (which appears to be the
case based on sheet Ml), and if so, should the PFD include this component.
Response:
The granular activated carbon (GAC) vessels are not disposable and will remain on
site. Spent GAC will be removed from the vessels and disposed of or regenerated
off site as appropriate. The GAC vessels will be operated in series Qead-lag mode) as
indicated on this drawing.
24. DWG 60313 P2. It is not the intent of this review to address specific equipment component
or methods selected by the Design Engineer. It is assumed the control philosophy relative
to process controls will be discussed in the O&M plan. Therefore, there are not comments
· on the detail of this drawing.
Response:
No response required.
25. DWG 60313 Ml, Details. This shows what appears to be a new compressor. The Design
Report indicates plant air will be used. Please clarify.
Response:
A new compressor that is independent of the plant air and plant electrical supply will
provide air to the air sparging wells.
26. DWG 60313 M2, Details. According to this drawing, the SVE well (002/M2) also contains
the sparge component -it appears to have the same below ground construction as the
AS/SVE well. Is this intended or should the SVE well be shown without the AS
component? Also please note that some detail from the AS/SVE well is not shown on the
SVE well (nominal borehole diameter, SVE extraction line size).
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27.
Response:
The below ground construction is intended to be the same for the air sparging wells, SVE
wells, and monitoring probes. The nominal borehole diameter applies to all three well types
and the SVE extraction line size applies to the AS/SVE wells and the SVE wells.
DWG 60313, Sheet M2 Details shows the SVE line exiting the well through the manhole
and well cap while Sheet M3 shows the line exiting laterally from the manhole through a
trench. Is this intended or does the configuration vary among the wells? Is the latter
configuration specific to EW9 and EWlO due to above ground obstructions while Sheet M2
configuration applies to all others?
Response:
The SVE wells EW-9 and EW-10 require a different configuration at the surface than do the
other wells. Well EW-9 is located in a main hallway and well EW-10 is located outside the
building in a driveway. Both these areas must be kept clear of obstructions and as such,
these wells require different configurations at the surface.
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1.0 INTRODUCTION
This Pre-Final Remedial Design (RD) Report provides a design and a preliminary
schedule for performing the Remedial Action (RA) for Operable Unit Three (OU3) of the
FCX-Statesville Superfund Site (the Site) in Statesville, North Carolina. The RD work is
being performed in accordance with the "Remedial Design Work Plan for Operable Unit
Three, FCX-Statesville Superfund Site, Statesville, North Carolina," dated July 1998 by
ECKENFELDER INC. (now Brown and Caldwell). The OU3 RD addresses the
remediation of the soils and groundwater associated with the property currently owned by
Burlington Industries, Inc. (Burlington). Operable Units One (OU!) and Two (OU2)
address soil and groundwater contamination associated with the FCX property, which is
located to the south of the Burlington property. The OU3 RD for the primary remedy is
being conducted by El Paso Natural Gas d/b/a El Paso Energy Corporation (El Paso).
Section I. I of this introduction provides background infonnation and a brief overview of
the Site conditions; Section 1.2 presents the RD objectives; and Section 1.3 provides a
description of the remedial technologies included in the selected remedy for OU3.
Included in Section 2.0 of this report is a summary description of the pre-design
investigation (PD!), which was performed in support of the RD ("Pre-Design
Investigation Report for Operable Unit Three, FCX-Statesville Superfund Site,
Statesville, North Carolina" dated March 1999 by Brown and Caldwell). The data from
the PD! are summarized in the PD! report and are not reproduced in this RD Report.
Section 3.0 presents the design analysis (also considered to be the design criteria) for
OU3. Section 4.0 discusses the RA, including the project delivery strategy, potential
permit requirements, and the preliminary schedule.
1.1 SITE BACKGROUND
The background information includes a Site description, a Site history, and a brief
overview of the Site conditions.
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1.1.1 Site Description
The OU3 Site is located in Iredell County approximately 1.5 miles west of downtown
Statesville, North Carolina (see Figure 1-1). The Site consists of the soil, groundwater,
sediment, and surface water contamination emanating from the textile plant property
currently owned by Burlington. The property is approximately 15 acres in size. Two
large buildings consisting of a warehouse (approximately 60,000 square feet in size) and
the textile plant building (approximately 275,000 square feet in size) are present on the
Burlington property (see Figure 1-2).
Land immediately surrounding the Site is predominantly industrial with a variety of other
uses ranging from commercial to residential with associated school and church facilities.
Farther from the Site, rural land in the Statesville area is used for timber farming, grain
crops farming, and dairy farming.
The Site lies within the geologic belt known as the Blue Ridge-Inner Piedmont Belt,
which is situated in the Inner Piedmont Physiographic Province in western-central North
Carolina. This province is characterized as gently rolling slopes. The Blue Ridge-Inner
Piedmont Belt consists of metamorphic rocks including gneisses and schists. These rocks
have weathered to form a relatively thin overburden of saprolite, which is observed
throughout the Site.
Groundwater at the Site is observed within the saprolite and underlying bedrock.
Saprolite forms the uppermost hydrogeologic unit. Groundwater occurs within the pore
spaces of the saprolite under water table conditions. Groundwater within the fractured
bedrock unit occurs under unconfined or semi-confined conditions. Site information
indicates that the two units are in hydraulic communication. Groundwater gradients
observed on-Site indicate that groundwater in the saprolite and bedrock appears to be
flowing both to the north and to the south from the textile plant.
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I ..., ...,
0 "' ci 2
'-' 2 ~ 0
2000 0 2000 4000 ·------' . SCALE FEET
SOURCE: U.S.G.S. TOPOGRAPHIC MAP, STATESVIU.E WEST QUADRANGLE, NC
FIGURE 1-1
SITE LOCATION MAP
FCX-STATESVILLE SUPERFUND SITE
STATESVILLE, NORTH CAROLINA
60313.011 9/99
BROWN AND
CALDWELL Nashville, Tenneaaee ·
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LEGEND
---PROPERTY LINE
200 0
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FEET
SITE LAYOUT
TE OU3 RFUND SI , X-STATESVILLE SN1;fR\H CAROLINA 9/99 FC STATESVILLE,
60313.011
ROWN AND
~ALDWELL Nashville, T1mnessee
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1.1.2 Site History
The textile plant was constructed at the OU3 Site in 1927. From 1955 to 1977, the textile
plant was operated by Beaunit Mills, later known as Beaunit Corporation (Beaunit). In
1967, Beaunit became a subsidiary of El Paso. In April 1977, Beaunit sold substantially
all of its assets, including the plant, to Beaunit II, Inc. As a part of that transaction,
Beaunit changed its name to BEM Holding Corporation (BEM), and Beaunit II, Inc.
changed its name to the Beaunit Corporation. In July 1978, the textile plant was sold by
the Beaunit Corporation (formerly Beaunit II, Inc.) to Beaunit Fabrics Corporation
(Beaunit Fabrics). In 1981, Burlington purchased certain assets, including the textile
plant, from Beaunit Fabrics. Burlington presently owns the textile plant; the plant was
closed in I 999, and Burlington is currently offering the plant for sale.
In June 1993, the United States Environmental Protection Agency (USEPA) Region 4
signed an Administrative Order on Consent for OU3 with Burlington, as well as the
former property owner, El Paso; hence the OU3 Site Group (Group) consists of El Paso
and Burlington. The USEPA Region IV issued the Final Record of Decision (ROD) for
OU3 in September I 996. The Consent Decree (CD) for OU3 was lodged on
December 18, 1997, and became final on April I, 1998.
1.1.3 Site Conditions
Several media and constituents of interest are associated with OU3. The pnmary
constituents of interest present within OU3 include perchloroethylene (PCE), also called
tetrachloroethene, and other chlorinated hydrocarbons. The groundwater contains
primarily PCE and other volatile organic compounds (VOCs). On-Site soil contains
primarily VOCs and to a lesser extent, inorganics and polynuclear aromatic hydrocarbons
(PAHs). Surface water and sediment associated with an intermittent stream originating
from the seep to the north of the Burlington textile plant also contains some inorganic
constituents, polychlorinated biphenyls (PCBs), and VOCs. However, it was determined
that the constituent concentrations posed no risk and no remediation is required.
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1.2 REMEDIAL DESIGN OBJECTIVES
The overall objective of the RD is to develop a design for the selected remedy as defined
by the ROD, consistent with the requirements of the CD and the Statement of Work
(SOW). The remedy to be designed, as defined in the SOW, includes treatment of VOC-
containing soil in the vadose zone with soil vapor extraction (SVE), and treatment of the
VOC-containing groundwater zone with air sparging. Monitored natural attenuation will
also assist in treatment of the VOCs at the Site. The PD! was performed and included
installation of additional monitoring wells, groundwater sampling, an evaluation of
natural attenuation, and a pilot-test of SVE and air sparging. The PD! has provided
additional data needed for the Pre-Final RD development. The additional data that was
obtained provides information critical to preparing the design of the selected remedial
components.
The design objectives for soils and groundwater in OU3 are provided below.
1.2.1 Soil Design Objectives
Elevated levels of several constituents, primarily VOCs, are present in the soil of OU3.
No cleanup levels have been established for on-Site impacted soil; however, the objective
of the soil RD is to minimize the potential for vapor transport and infiltration of VOCs
from the soil into the groundwater using SVE technology.
1.2.2 Groundwater Design Objectives
Groundwater containing VOCs has been identified in the shallow saprolite and
intermediate bedrock aquifers. Air sparging was selected in the ROD to treat
groundwater constituents of concern by removing VOC mass and controlling migration
in order to meet Federal Maximum Contaminant Levels (MCLs) or the North Carolina
Groundwater Standards, whichever are more protective. These will be referred to as the
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11
ROD MeLs in this report. The objective of the RD for groundwater is to design an air
sparging remediation for impacted groundwater based upon the results of the air sparging
and SVE (AS/SVE) pilot test that was conducted as part of the PD!.
Sampling and analysis of the groundwater will be performed to monitor the OU3 RA
performance as well as to monitor the extent and effectiveness of natural attenuation.
Institutional controls will be addressed in an Explanation of Significant Difference (ESD)
currently under consideration by the US EPA. The purpose of the institutional control is
to prohibit the consumption of impacted groundwater (associated with OU3) from
drinking water wells.
1.3 DESCRIPTION OF OU3 REMEDIATION TECHNOLOGIES
Various technologies were reviewed in the ROD for remediation of OU3 including air
sparging, SVE, and groundwater extraction and treatment. The remediation technologies
for OU3 selected by the ROD include SVE, air sparging with SVE, and monitored natural
attenuation.
1.3.1 Soil Vapor Extraction
Soil vapor extraction uses the induced movement of air through the vadose zone to
volatilize and remove voes. In the most commonly practiced method of application, a
blower (e.g., a vacuum source) is attached to an SVE well which is screened across the
impacted interval of the vadose zone. The blower creates a partial vacuum (reduced
pressure) within the well and induces air flow from the surrounding soils towards the
SVE well. As the air moves through the impacted soils, the portion of the voes that is
present in the vapor phase flows towards the SVE well and is removed through the well
along with the extracted air. The voes associated with the soils and present as free
phase liquids ( either between the soil particles or present as a layer on top of the
groundwater) will gradually partition (volatilize) into the surrounding soil gas and will be
extracted with the recovered air. When appropriate, based on regulations and voe
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concentrations in the extracted air, an off-gas treatment system is incorporated as part of
the SVE process.
The SVE technology has been used widely at sites regulated by the Comprehensive
Environmental Response Compensation and Liability Act (CERCLA), Resource
Conservation Recovery Act (RCRA), Department of Defense (DOD), Department of
Energy (DOE), and state mandates. The technology has been applied to the remediation
of chlorinated solvents, non-chlorinated solvents, and lighter petroleum hydrocarbon
blends. The SVE technology was developed principally to remove VOCs. Its
applicability to semi-volatile organic compounds (SVOCs) and non-volatile compounds
is limited to the extent that these compounds are biodegradable by aerobic
. . m1croorgamsms.
The SVE technology is attractive because it is applied in situ, requires minimal disruption
to normal site activities, and can be implemented beneath buildings, roadways, parking
lots, and other man-made structures. Furthermore, the process removes contaminant
mass with minimal potential to spread the contamination. In some cases SVE can also
serve to control vapor migration. Many VOCs are relatively easily vaporized from the
absorbed and/or free phase. The SVE process contributes to the long-term improvement
of groundwater quality by removing VOC mass from the vadose zone.
As a result of having been used at a large number of sites under a wide variety of
conditions, design protocols for SVE are available and there are numerous equipment
vendors who can provide components or pre-assembled packaged systems. Installation
and operation of SVE systems are relatively straightforward making the technology cost-
effective for many site conditions especially for very large soil treatment areas.
The SVE technology has been implemented as part of multicomponent remedial systems
in conjunction with air sparging and monitored natural attenuation as well as other
technologies. When used as an integral part of air sparging, SVE can remove existing
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mass from the vadose zone as well as capture voes stripped from the saturated zone as a
result of air sparging.
1.3.2 Air Sparging with Soil Vapor Extraction
Air sparging introduces air into groundwater to remove voes. This is accomplished
through injection of air under pressure through small-diameter wells that are screened
within or below the contaminated interval and/or near the base of the aquifer. The
injected air moves radially outward and upwards from the well screen towards the
groundwater surface in discrete channels. The voes are stripped from the aqueous
phase into the gas phase. Once the voes reach the vadose zone, they are typically
removed by SVE.
The introduction of air necessarily introduces oxygen into groundwater. This increases
the oxidation/reduction potential (Eh) of the groundwater and promotes aerobic
degradation of aerobically degradable compounds such as benzene and toluene. Added
oxygen can also promote cometabolism of some chlorinated solvents provided one of
several compounds (e.g., toluene, methane, etc.) is also present. At the same time, the
addition of oxygen can interfere with the reductive dechlorination (natural attenuation) of
chlorinated solvents including PeE. Sparging can be performed using nitrogen rather
than air if anaerobic conditions need to be maintained.
Air sparging is most effective when operated in a pulse mode (air injection in individual
wells operated using an off/on cycle). When air flow is initiated, air moves through the
soils opening discrete channels. When the air flow is interrupted, the air channels fill
with water. This movement of water in and out of channels, as well as the mounding that
occurs when air flow is initiated, serves to mix the groundwater and enhance the removal
ofVOes.
The effectiveness of air sparging is dependent upon many factors including the Henry's
law constant of the specific constituents as well as both the initial concentration of
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constituents and their respective cleanup levels. Of particular importance are the details
of the site hydrogeology. Small differences in soil permeability can significantly affect
the flow paths of air within the saturated zone. As a result, individual wells may
remediate relatively small or large areas based on differences in permeability that are not
necessarily evident from well installation logs. Pilot testing, as was performed during the
PD!, is typically required to determine the area of influence of individual wells. There
can be large variability across even a small site, which will not necessarily be identified
by the pilot testing.
1.3.3 Monitored Natural Attenuation
Natural processes that reduce the mass and concentrations of the VOCs present in OU3
have been observed in Site groundwater samples. For this reason, monitored natural
attenuation is being evaluated to determine the extent to which it may complement the
active remediation technologies of SVE and air sparging being implemented at the Site.
Monitored natural attenuation, also referred to as intrinsic remediation, relies on the
natural restorative capacity of aquifers to control migration and reduce the masses of
constituents of concern. The mechanisms that contribute to natural attenuation include
adsorption, diffusion, dispersion, volatilization, and degradation.
The hydraulic conductivity, gradient, and porosity of the aquifer determine the rate of
groundwater flow. As the groundwater moves through the aquifer, mixing of affected
groundwater with clean groundwater occurs as a result of dispersion and, to a lesser
extent, diffusion. These processes result in somewhat lower constituent concentrations
and marginally broader plumes. As the constituents move through the aquifer they
adsorb to the aquifer materials, especially when appreciable organic content is present,
and subsequently desorb (dissolve). The adsorption/desorption process retards the rate at
which constituents move through the aquifer. As a result, constituent migration is slower
than otherwise would occur as a consequence of groundwater flow through the aquifer.
The processes of diffusion, dispersion, and retardation moderate constituent
concentrations but do not cause a reduction in constituent mass. Chemical and biological
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degradation reactions reduce both mass and concentrations of the degradable organic
constituents. For chlorinated aliphatic hydrocarbons [ e.g., PCE and trichloroethene
(TCE)], the process of degradation in groundwater occurs largely through anaerobic (in
the absence of oxygen) biodegradation. The specific process is referred to as reductive
dechlorination. In this process, the chlorine atoms on chlorinated ethenes are sequentially
replaced with hydrogen atoms. This anaerobic biodegradation process is as follows:
PCE ➔ TCE ➔ DCE ➔ Vinyl Chloride ➔ Ethene
In addition to the anaerobic process, dichloroethene (DCE), vinyl chloride, and ethene
can also be biodegraded aerobically, ultimately yielding chloride ions, carbon dioxide,
and water.
The reductive dechlorination process requires the presence of other degradable organic
compounds and species referred to as electron acceptors, and appropriate geochemical
conditions. According to the protocol, these parameters, as well as the presence and
distribution of the chlorinated solvents and degradation products, should be measured
from appropriate monitoring wells and interpreted as part of the natural attenuation
evaluation. The USEP A has issued appropriate guidance in the document entitled
"Technical Protocol for Natural Attenuation of Chlorinated Solvents in Ground Water"
which was developed in conjunction with the U.S. Air Force Center for Environmental
Excellence (AFCEE), hereafter called the AFCEE document. USEP A Region 4 has
incorporated that document into the recently issued "Draft Region 4 Approach to Natural
Attenuation of Chlorinated Solvents." Both of the documents provide useful guidance to
evaluate specific sites for the potential for monitored natural attenuation to be
incorporated into the Site remedy. Another USEPA document, OSWER Directive
9200.4-17 entitled "Use of Monitored Natural Attenuation at Superfund, RCRA
Corrective Action, and Underground Storage Tank Sites" clarifies USEPA's policy
regarding the use of "monitored natural attenuation" for the remediation of contaminated
soil and groundwater. The OSWER directive was appended to the PDI report (Brown
and Caldwell 1999) and the AFCEE document is reproduced in Appendix A.
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It is necessary to evaluate the extent to which the combined effects of the several natural
attenuation processes are able to limit constituent migration and reduce constituent
masses. This is accomplished through the use of fate and transport models such as
BIOSCREEN.
Long term monitoring of Site contaminants and natural attenuation parameters are
required for monitored natural attenuation. The primary objective of long term
monitoring is to observe whether the natural attenuation processes along with any active
remediation are serving to reduce or limit expansion of the plume.
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2.0 PRE-DESIGN INVESTIGATION
A Pre-Design Investigation (PD!) Report was prepared and describes investigation work
that was performed in support of preparation of the OU3 RD. This section provides a
brief summary of the PD!. For more detailed information, refer to the PD! report "Pre-
Design Investigation Report for Operable Unit Three (OU3), FCX-Statesville Superfund
Site, Statesville, North Carolina" dated March 1999 by Brown and Caldwell. The PD!
work included installation of additional groundwater monitoring wells, sampling and
analysis of groundwater from selected monitoring wells, an evaluation of natural
attenuation at the Site, and a pilot test of air sparging and SVE.
2.1 INSTALLATION OF GROUNDWATER MONITORING WELLS
Groundwater within the saprolite and intermediate bedrock aquifers associated with the
Site generally flows both to the north and to the south creating two potential transport
mechanisms from the Site. Additional shallow and intermediate (saprolite and bedrock)
groundwater monitoring wells were required to define the horizontal and vertical extent
of the constituents of concern in the OU3 groundwater.
Two new wells, W-3\s and W-3\i, were installed as a couplet to further delineate the
downgradient extent of the groundwater plume to the north (see Figure 2-1 ). The well
couplet consists of a shallow monitoring well screened within the saprolite and an
intermediate monitoring well screened within the underlying bedrock unit. A third new
well, W-32i, screened within the upper bedrock (intermediate zone), was installed to
further delineate the downgradient extent of the groundwater plume to the south. To
further evaluate the vertical extent of the groundwater plume to the north, a monitoring
well, W-20d, was installed in the plume to the north adjacent to the existing monitoring
well couplet W-20s and W-20i.
P.\PRO.l'.60313.0! lldn02,doc 2-1
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-$-Shallow Monitoring Well Location
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♦
Intermediate Monitoring Well Location
Qeep Monitoring Well Location
FIGURE 2-1
MONITORING WELL
LOCATION MAP
FCX-STATESVILLE SUPERFUND SITE, OU3
STATESVILLE, NORTH CAROLINA
60313.011 9/99
BROWN AND
CALDWELL Nashville, Tennessee
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2.2 RESULTS OF GROUNDWATER SAMPLING AND ANALYSES
Groundwater samples were collected from on-Site and off-Site wells. The sample
analyses provided data to further delineate the horizontal and vertical extent of
constituents of concern, to evaluate metals concentrations, and to measure_ biodegradation
parameters within the groundwater plume. As part of this sampling, potable water from
two residential groundwater drinking wells located downgradient of the Site was sampled
in order to establish a broader database ofgroundwater quality.
The groundwater samples were analyzed for the designated parameters, which sometimes
differed between wells and sampling events depending on the purpose of the sample.
The chemical tests and analytical parameters (including the basis for selecting those
parameters) are presented in the POI report and include VOCs, metals, pesticides, and a
suite of natural attenuation parameters.
In general, the VOC results from this POI were consistent with results from the Remedial
Investigation (RI) in 1994 through 1996, with some exceptions. The RI results are
presented in the report entitled "Final Remedial Investigation Report, FCX-Statesville
Superfund Site, Operable Unit 3, Statesville, North Carolina," dated July 1996 by
Aquaterra, Inc. A summary of the groundwater results and observations is as follows.
• No VOCs or pesticides were reported that exceeded the ROD MCLs for OU3 in
the three new downgradient monitoring wells (W-3ls and W-3li to the north
and W-32i to the south). This indicates that the horizontal extent of the plume
has been defined in the downgradient directions to the north and south of OU3.
• At monitoring well W-20d, which was installed to assess the vertical extent of
the groundwater plume to the north of the Site, the concentrations of PCE and
1,2-dichloropropane were elevated. Interval packer testing during installation of
W-20d indicated that there are at least 40 feet of media with significantly lower
permeability that separate the upper fractured unit from the deeper groundwater
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unit (the well was screened above the media with lower permeability). As a
result, the groundwater quality data from W-20d are considered representative
of the vertical extent of the groundwater plume at this location.
No VOCs were detected in the two residential water supply wells that were
sampled downgradient of the Site.
• The PD! sample results showed PCE concentrations at downgradient monitoring
wells W-20s and W-29i to be higher than the RI sample results. The higher
results in these wells may represent changes in groundwater elevations,
sampling techniques, laboratory procedures, or may represent processes
occurnng within the aquifer. Similar variations in sampling results were
observed for W-30i, which was used as a control well. Alternatively, the PDI
sample results potentially may indicate a slight expansion of the groundwater
plume over the period of time between the RI sampling and the PD! sampling
events. Typically however, eight sampling events are required to identify
statistically significant trends in groundwater sampling data. Only one to four
sampling events have been conducted at the Site. Therefore it is premature to
conclude, before any more sampling events have been performed, that these
variations have any significance.
• Concentrations of aluminum, iron, and manganese in the slow purge and
unfiltered samples were elevated. Even though slow purge sampling was used,
fine mica flakes from the saprolite formation were observed in the samples.
The aluminum concentrations observed in the samples are considered to reflect
these fine suspended particles, rather than indicating impact to groundwater
from OU3. The iron and manganese concentrations may reflect natural
background conditions and/or suspended sediments. In some areas, iron and
manganese appear to result from natural attenuation mechanisms.
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• Mercury was observed in excess of the ROD MCL in one monitoring well,
W-Ss, and is considered an anomaly unrelated to the manufacturing process at
the facility and is not considered to be a Site-wide issue.
Data from the PDI was used in conjunction with the RI data to revise the PCE and TCE
isoconcentration maps. Each isoconcentration line was revised using the maximum
concentration at a given sampling well that was available from the data. Figures 2-2 and
2-3 show the revised isoconcentrations for PCE in the shallow and intermediate zones,
respectively; Figures 2-4 and 2-5 show the revised isoconcentrations for TCE in the
shallow and intermediate zones, respectively.
2.3 EVALUATION OF NATURAL ATTENUATION
Monitored natural attenuation of the constituents of concern has become widely accepted
as a remedy or as a component of a remedy in conjunction with some form of source
control. The evaluation of natural attenuation performed during the PD! provided a
qualitative understanding of the biodegradation and physical processes, as well as an
attempt to quantify the contributions from the biodegradation and physical processes.
The evaluation process was applied to what might be considered four plume areas. These
consist of the shallow saprolite saturated interval to the north and to the south of the
groundwater divide, and the intermediate bedrock saturated interval to the north and to
the south of the groundwater divide. Evidence that natural attenuation is occurring at the
Site is as follows.
• The groundwater quality data from the RI and the PDI were evaluated to
identify the presence and relative concentrations of constituents of concern
(especially PCE) and reductive dechlorination products. Reductive
dechlorination products are present across the plume. In some areas the ratio of
the reductive dechlorination products relative to the parent compound, PCE, is
fairly high, suggesting extensive reductive dechlorination. Trends in
concentrations over time and along the groundwater flow path provide a
P:\PROJ\603 !J.01 !\drs02.doc 2-4
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SCALE FEET
Leaend
z-_.-+-
Shallow Monitoring Well Location
Intermediate Monitoring Well Location
Deep Monitoring Well Location
/
--10-Groundwater lsoconcentration Contour
[ x J
( X )
PCE Results in [ppb) from Pre-Design
Investigation (PDI)
PCE Results in (ppb) from Remedial
Investigation {RI)
1. Maximum observed PCE concentrations ore used
on isoconcentrotion mop.
FIGURE 2-2
ISOCONCENTRATION MAP FOR
PCE IN SHALLOW GROUNDWATER
FCX-STATESVILLE SUPERFUND SITE, OU3
STATESVILLE, NORTH CAROLINA
60313.011 9/99
BROWN AND
CALDWELL Nashville, Tennessee
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F~ET
Legend
z----+-
Shallow Monitoring Well Location
Intermediate Monitoring Well Location
Deep Monitoring Well Location
--10-Groundwater lsoconcentrotion Contour
[ X l
( X )
1'I_Qk;_
PCE Results in [ppb] from Pre-Design
Investigation (PDI)
PCE Results in {ppb) from Remedial
Investigation (RI)
1. Maximum observed PCE concentrations ore used
on isoconcentration mop.
FIGURE 2-3
ISOCONCENTRATION MAP FOR
PCE IN INTERMEDIATE GROUNDWATER
FCX-STATESVILLE SUPERFUNO SITE, OU3
STATESVILLE, NORTH CAROLINA
60313.011 9/99
BROWN AND
CALDWELL Nashville, Tennessee
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0
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<:) z
j: < "' • 0
300 0 300 600
SCALE FEET
/iQk
1. Maximum observed TCE concentrations ore used
on isoconcentrotion mop.
FIGURE 2-4
ISOCONCENTRATION MAP FOR
TCE IN SHALLOW GROUNDWATER
FCX-STATESVILLE SUPERFUNO SITE, OU3
STATESVILLE, NORTH CAROLINA
60313.011 9/99
BROWN AND
CALDWELL Nashville, Tennessee
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z-.--.-
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1. Maximum observed TCE concentrations ore used
on isoconcentration map.
FIGURE 2-5
ISOCONCENTRATION MAP FOR
TCE IN INTERMEDIATE GROUNDWATER
FCX-STATESVILLE SUPERFUND SITE, OU3
STATESVILLE, NORTH CAROLINA
60313.011 9/99
BROWN AND
CALDWELL Nashville, Tennessee
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semi-quantitative understanding of the extent to which reductive dechlorination
is limiting the migration of groundwater constituents in the downgradient
direction.
• Another indication of natural attenuation is whether the plume has reached a
dynamic equilibrium or steady state condition, i.e., are the mechanisms that
retard migration and destroy constituent mass in an approximate equilibrium
with the mechanisms of dissolution and advection that result in migration? The
site-wide water quality data (with a few exceptions that require additional
sampling to determine if variations have any significance) suggest a fairly
constant plume. This is based on a comparison of PCE concentrations reported
during the RI sampling events (1994 through I 996) to those reported during the
PDI sampling events (1998 and 1999). Some variation was observed and is
anticipated due to normal variability associated with groundwater
characterization. The relative stability of the plume is not surprising since
chlorinated solvent plumes where biodegradation is occurring typically reach
equilibrium over time.
• A USEPA protocol was used to rank the Site for natural attenuation of
chlorinated solvents. The ranking of those wells located midway in the plume
provides sufficient evidence of reductive dechlorination. The evaluation of
natural attenuation suggests that the natural processes will continue to limit the
migration ofVOCs.
Active remediation of the source area may alter the Site geochemistry. As a result,
natural attenuation mechanisms, especially biodegradation, may be impacted. For
example, air sparging introduces oxygen to the groundwater. To the extent oxygen is
dissolved, reductive dechlorination would be inhibited. This impact might be limited to
the source area at least over the near future if air sparging were implemented. Air
sparging has the potential to slightly impact natural attenuation in directions away from
the source area.
P:\PRO.l'.6031J.0l l\drs02.doc 2-5
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2.4 PILOT TEST RES UL TS
A pilot test was conducted in the apparent source area at OU] to evaluate air sparging
and SVE. Air sparging was performed within the saprolite at two depths, 50 feet and
66 feet. Five monitoring probe clusters were installed around the air sparging wells and
the SVE well. The pilot test objectives were to investigate and measure the physical
characteristics of the soil and aquifer in the vadose and saturated zones, respectively, in
relationship to the operation of SVE only and air sparging with SVE. As part of the pilot
test, helium tracer testing was performed. In addition, a pneumatic permeability test of
the vadose zone beneath the textile plant was performed using a second SVE well located
inside the building. The locations of the two air sparging wells, the two SVE wells, and
the five monitoring probe clusters are shown in Figure 2-6.
In general, the pilot test identified significant heterogeneity in both the saturated zone and
the vadose zone with respect to air sparging and SVE. The observed heterogeneity
indicated that an observational ( or phased) approach to the design and implementation of
air sparging and SVE at the Site would be required. A summary of the pilot test results
and observations is as follows:
• The vadose zone soil is highly heterogeneous, as shown by the wide range of
vacuum readings in the monitoring probes that were 20 feet or less from the
SVE well. Thus, SVE performance in the vicinity of any well (whether outside
of the buil~ing or inside the building) is expected to be asymmetrical, e.g., air
flow and the lateral distance of influence will not be the same in all directions
and will not be predictable.
• The pneumatic permeability range of the vadose zone soil in the SVE well is
almost identical to that of the well located inside the building. Therefore,
performance of SVE with wells underneath the building can be expected to
behave similarly with regard to achievable flow rates and wellhead vacuum for
P:IPROJ\60313.0! 1\drs02.doe 2-6
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AS-1 G GAS-2
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MP-2 A
LEGEND:
G Air Sparging Well
0 SVE Well
A Monitoring Probe Cluster
MP-1
0 Groundwater Monitoring Well
0 10 20 30 --
0 W-9s
0 W-9i
NOTE:
W-16i 0 0 W-16s
~
SVE-2
(location inside building)
Burlington
Textile Plant
Pilot test well and monitoring probe locations are approximate
(not surveyed).
FIGURE 2-6
LOCATIONS OF
40 -----
PILOT TEST WELLS AND
MONITORING PROBES
STATESVILLE. NORTH CAROLINA
FCX-STATESVILLE SUPERFUND SITE scale feet 60313.011 . 9/99
BROWN AND
CALDWELL Nashville, Tennessee,
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similarly designed wells. Subsurface infrastructure at the Site is anticipated to
have at least some influence on the performance of an SVE system at the Site.
• Air injection was possible at relatively low flow rates at depths of 50 feet and
66 feet in the saprolite. However, SVE was not effective in completely
capturing the sparged air using a single SVE well in the study area. This is
evidenced by the pressurization of some of the vadose zone monitoring probes,
the limited helium capture by the extraction well, and the presence of helium in
some of the vadose zone monitoring probes. Air sparging reduced the radius of
influence of a single SVE well from the range of 22 to 59 feet to the range of 12
to 54 feet. This may be less important where an array of SVE wells is installed.
• The saturated zone is highly heterogeneous with regard to air flow patterns from
the injected air from both the shallow depth and deep depth air sparging wells.
There appear to be horizontal confining layers within the saturated zone which
inhibit injected air movement to the vadose zone, especially at the deeper
sparging depth of 66 feet. Further evidence of the heterogeneity of the saturated
zone is supplied by the variability of the VOC results from the pre-and post-test·
groundwater sampling and by the variability of response of measured
groundwater upwelling at the monitoring probes during air sparging.
• Based on the heterogeneity of the saturated zone, it is difficult to predict where
sparged air and thus entrained VOCs may move, especially at the deeper sparge
depth of 66 feet. Consequently, careful placement of SVE wells using a phased
approach should be considered to maximize the capture of injected air.
• Air sparging has the potential to inhibit natural attenuation if the injected air
traverses long distances in the saturated zone or if dissolved oxygen traverses
downgradient.
P:\rRQJ\60313,01 l\drsO:Z.drn:: 2-7
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• The voe data indicate that voes were being removed during SVE without air
sparging and during SVE with air sparging. The variability of the data and the
types of data collected do not allow a quantitative calculation of the mass of
voes removed from the vadose zone or the groundwater.
P:\PRQJ\60J 13.01 l\dr102.doc 2-8
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3.0 DESIGN ANALYSIS
This section of the Pre-Final RD Report presents an analysis of the remedial design. The
design analysis consists of the design approach, the design for monitored natural
attenuation, the Phase I design, the Phase I performance testing, and the Phase II design.
3.1 DESIGN APPROACH
The remedial design consists of two parts: monitored natural attenuation and AS/SVE.
Monitored natural attenuation is ongoing as part of the remedy for groundwater outside
the source area. The monitored natural attenuation design is described in Section 3 .2.
The installation and operation of an AS/SVE system is to address the source area. Based
on the heterogeneity of the Site observed during the POI, an observational (or phased)
approach to design and construction of an AS/SVE system for the source area will be .
utilized. This type of phased approach consists of the installation of an initial system
using well separations that are based upon anticipated Site conditions. The Phase I
installation would then be operated and monitored to evaluate system performance prior
to the Phase II installation, if required. The operation information from each phase of
installation will be considered and incorporated into subsequent phases as appropriate.
The RD is based on using this observational (phased) approach assuming that there will
be one or two phases of installation ( e.g., Phase I and Phase II). The process equipment
(i.e. compressor, blowers, air emissions control system, etc.) have been sized to
accommodate contingent air sparging and/or SVE well installations which may be
necessary if Phase II is required. Piping manifolds have also been designed with spare
ports and additional flow capacity to accommodate contingent wells. The air sparging
wells and SVE wells associated with the RD will be located in the PCE source area which
is underneath the existing textile plant building. The design documents have been
prepared for Phase I of the RD. The criteria for evaluating the performance of the Phase I
AS/SVE are listed in the RD. The criteria for determining the end point (i.e., when to
permanently shut the system down) for the AS/SVE will be addressed in the RA
\\BCNSH03\PROJECTS\PRO1\60313.0 l l\drsOJ.doc 3-l
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Operation and Maintenance (O&M) Plan. The criteria for evaluating the performance of
Phase I and for determining the shut down point for AS/SVE will include evaluation of
extracted soil gas concentrations, mass removal rates, and total contaminant mass
removal.
In addition to monitored natural attenuation away from the source area, the natural
attenuation parameters in the source area will be monitored to determine if the
implementation of air sparging alters or acts to hinder the natural attenuation observed to
be occurring at the Site.
3.2 DESIGN FOR MONITORED NATURAL ATTENUATION
The design for monitored natural attenuation will be implemented by performing semi-
annual sampling of selected monitoring wells for natural attenuation parameters. The
selected monitoring wells, sample frequency, and analysis will be reevaluated and
adjusted as appropriate after the first two years of monitoring and again after five years of
monitoring. As data are evaluated, the list of wells may change to accommodate
fluctuations in groundwater levels and conditions during testing. In addition,
modifications to the sampling plan are anticipated in order to monitor the effectiveness of
the source control measure once the remedy is in place. The tasks associated with
monitored natural attenuation at the Site include the following:
• sampling of the monitoring wells listed in Table 3-1,
• field measurements and laboratory analysis of the samples using the analytical
method references shown in Table 3-2,
Quality Assurance/Quality Control (QA/QC) sampling and analysis in
accordance with a Quality Assurance Project Plan,
• validation of the analytical reports for metals and VOCs, and
\\BCNSH0J\PROJECTS\pRQJ\6031 J.0 l l\dn03 .doc 3-2
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Groundwater Zone
Shallow:
Intermediate:
Deep:
TABLE 3-1
MONITORING WELLS SELECTED FOR
SEMI-ANNUAL GROUNDWATER SAMPLING,
OPERABLE UNIT THREE (OU3),
FCX-ST A TESVILLE SUPERFUND SITE
Monitoring Well'
W-Ss
W-12sh
W-17s
W-18sh
W-19s
W-20s
W-22s
W-24s
W-3ls
W-Si
W-1 0ib
W-12ib
W-20i
W-22i
W-28i
W-29i
W-30i
W-3 li
W-32i
W-20d
'The analytical methods for the analyses of groundwater samples are given in Table 3-2.
"Natural attenuation background well.
P:\PROJ\60313.01 l\drt0J0l.doc Page 1 of l
-· -
Sam pie Evaluation
Field Measurements:
- -
TABLE 3-2
SUMMARY OF CHEMICAL ANALYSES AND ANALYTICAL METHOD REFERENCES FOR
SEMI-ANNUAL GROUNDWATER SAMPLING,
OPERABLE UNIT THREE (OU3),
FCX-STATES_VILLE SUPERFUND SITE
Chemical Test/Analyte Parameter
Carbon dioxide
Iron (II)
Manganese (II)
Sulfide
Conductivity
Oxidation-reduction potential (ORP)
pH
Dissolved oxygen (DO)
Temperature
Analytical Reference Methoda
Hach KitC
Hach KitC
Hach KitC
Hach KitC
ASTM Method D-1125-82
ASTM Method D-1498-76
ASTM Method D-1293-84
CHEMETRICS Kitc
NAd
Laboratory Analyses: Chloride USEPA Method 325.2
Aquaterra QAPP Table 3
Aquaterra QAPP Table 3
Aquaterra QAPP Table 3
USEPA Method 353.2
Iron (total)
Manganese (total)
Aluminum (total)
Nitrate/nitrite
Sulfate
Ethane, ethene, and methanee
TCL voes
Alkalinity ( carbonate/bicarbonate )f
Dissolved total organic carbon (TOC)
Volatile fatty acids
USEPA Method 375.4/9038
USEPA Method 8015-Modified
Aquaterra QAPP Table 2
Standard Methods 2320B
USEPA Method 415.1
Standard Methods 5560C
DQO Leveib
II
II
II
I
II
II
II
II
II
III
IV
IV
IV
III
III
Ill
IV
III
Ill
Ill
•Sample preservatives, when required by the method, will be added to sample containers at the analytical laboratory prior to sampling. Contract Required
Detection Limits (CRDLs) will be according to the contract laboratory procedure (CLP) methods referenced in the Aquaterra QAPP Tables 2 and 3.
bDQOs (Data Quality Objectives) and QA/QC frequencies per "Environmental Investigations Standard Operating Procedures and Quality Assurance Manual",
May 1996, USEPA Region 4. Level I~ Field Screening; Level II~ Field Analyses; Level III~ Screening Data with Definitive Confirmation; Level IV~
Definitive Data.
CMethod will be per manufacture's procedures.
dNot Applicable.
eAnalysis will be subcontracted to Microseeps Incorporated, Pittsburgh, Pennsylvania.
fsamples to be collected in zero headspace containers to prevent exchange of carbon dioxide between the samples and the atmosphere.
P:IJ>ROJ\60J IJ.0J l\dn0302 doc Page I of 1
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• interpretation and reporting of the analytical results to the USEP A.
3.3 PHASE I DESIGN
The first phase will include installation of an SVE system, air sparging system, and a
network of monitoring probes for checking the impact of the selected remedy on the
source area. Figure 3-1 shows the proposed locations of the SVE wells, air sparging
wells, and monitoring probes. Design calculations for the Phase I design are presented in
Appendix B. The calculations include sizing and performance calculations for the piping,
SVE system, air sparging system, and granular activated carbon (GAC) system. The
details of the Phase I design are provided in the technical specifications and the design
plans, which are in Attachments I and 2, respectively.
As part of Phase I, the SVE system and combined SVE and air sparging systems will be
operated long enough to permit data collection and evaluation of the Phase I installation.
The decision as to the need for and/or extent of a Phase II installation would be made
once the data are evaluated and interpreted from Phase I.
3.3.1 Wells and Monitoring Probes
The SVE wells, air sparging wells, and monitoring probes have been designed using a
uniform design with like materials, screen depth, construction, etc. The depth and screen
length design for the SVE and air sparging wells is based on data from the POI and on the
anticipated heterogeneity of the soil. Each well and probe contains two concentric wells:
an outer well screened in the vadose zone and an inner well screened in the groundwater.
This concentric design will allow for collection of both groundwater samples and vadose
zone soil gas samples from one well installation. In this way the construction is
simplified and the number of available monitoring points is maximized. The locations of
the wells and probes are based on the known source area. The detailed designs for the
wells and probes are in the RD technical specifications and design plans (see
I\BCNSHOJ\PROJECTS\PROJ\603 13.011\drsOJ.doc 3-3
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0 0
II
Textile Plant
Warehouse
/ \/
...
/'
I
____c-$:q,L__c....__...,. \
\
Plant
/ '----
/
...
0 0
[7
PIEDMONT STREET
r-~~ j I I N I _j ' c II { I I , I I I ---...._f I ----
·------
LEGEND
ed E
f Influence
,!_; .
I "J'-.._ Q !
jf!,-
1 I ~
L-
100 - -. ----SCALE
Proposed SVE Well Location
(Illustrated with assumed 50' SVE Radius of Influence)
Proposed Air Sparging and SVE Well Location
(Illustrated with assumed 50' SVE Radius of Influence)
Proposed Monitoring Probe Location
Tetrachloroethene (PCE) Shallow
Groundwater lsoconcentration (ppb) Contours
(Dashed where Inferred)
lsoconcentrotion contour information token
from Figure 2-2.
2. Assumed 50' radius of influence for homogeneous
conditions. Actual zone of influence will be dependent on
heterogeneity of vadose zone.
200
FEET
FIGURE 3-1
DESIGN LAYOUT OF PHASE I
SVE WELLS, AIR SPARGING WELLS
AND MONITORING PROBES
FCX-STATES~LLE SUPERFUND SITE, OU3
STATES~LLE, NORTH CAROLINA
60313.011 9/99
BROWN AND
CALDWELL Nashville, Tennessee
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Attachments 1 and 2, respectively). Each of the wells or probes is discussed further in
subsequent paragraphs.
The Phase I installation of SVE will cover that area bounded by the I 0,000 ~Lg/L PCE
isoconcentration line depicted in Figure 3-1, plus a buffer area to accommodate recovery
of the injected air. It is proposed that ten SVE wells be installed in a triangular grid with
an assumed radius of influence for SVE of 50 feet (i.e. 100-foot separation). The radius
of influence was selected based on the calculated radius of influence data from the PD!
report (see Table 5-9, Section 5.5.9, POI Report). The outer SVE wells will be placed
approximately 100 feet beyond the air sparging wells. The exact locations of these wells
will be dependent upon the obstructions and operations inside the plant. Therefore, the
locations (and possibly the number of wells) may change from what is depicted in
Figure 3-1.
Two air sparging wells will be installed during Phase I at the time of the SVE well
installation. The air sparging wells will be installed as concentric wells with SVE wells
in the central portion of the plume 10,000 µg/L PCE isoconcentration line (see
Figure 3-1 ). The air sparging wells will be screened in the shallow aquifer
(approximately 50 feet below ground surface based on the POI pilot test results). As
stated previously, the air sparging and SVE wells will be installed in a triangular grid
pattern to the extent practicable.
Monitoring probes will be installed within the anticipated area of influence as part of the
Phase I installation. The proposed locations of the monitoring probes are shown in
Figure 3-1. The monitoring probes will be used to gather data to be used in evaluating
the system performance and the need for additional wells in the Phase II installation, if
determined necessary.
I\BCNSH03\PROJECTS\PROJ\603 IJ.Ol l \drsOJ.doc 3-4
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3.3.2 Soil Vapor Extraction System
The SVE system will consist of instrumentation at the SVE wells, associated piping and
supports to transport extracted gas, various control instrumentation, a packaged SVE
vacuum blower system, a packaged GAC system for off-gas treatment, and a stack for
treated off-gas discharge. A packaged SVE vacuum blower system was chosen since
there are several vendors who can easily supply the packaged system according to the
design specifications. The packaged SVE system consists of a liquid separator,
particulate filter, air bleed-in valve, and two blowers. The two blowers included in the
design have been chosen with both the Phase I design requirements and the potential
requirements that a Phase II design would have. During the Phase I performance testing,
one blower will be used with the second available as a backup to be used as needed.
Each blower has been designed to deliver an extraction flow rate of 250 standard cubic
feet per minute (scfm) with an anticipated average flow of 25 scfm at 75 inches water
column (in.W.C.) vacuum at each of the ten Phase I SVE wells.
The blowers will provide vacuum to the SVE wells through a piping manifold (or header)
system. The piping system has been designed with a common header that includes
connections to the proposed ten Phase I SVE wells. The piping common header also
includes ten additional ports for future installation of up to. ten Phase II SVE wells. The
pipe size has been designed to accommodate the potential Phase II design flow rate of
500 scfm, if full implementation of Phase II is necessary. The extracted vapors will be
piped to an air emissions control device. Even though the extracted vapors are not
expected to exceed any VOC discharge standards, the RD includes an air emissions
control device. Control technologies that have been evaluated include carbon adsorption,
thermal oxidation, and catalytic oxidation. A packaged GAC system has been chosen as
the control technology based on the application and cost effectiveness.
\\BCNSH0J\pROJECTSIPROJ\60313.01 l\drs0J.doe 3-5
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3.3.3 Air Sparging System
A packaged air sparging system has been specified which consists of an air compressor; a
filter to supply clean, oil-free air; and a regulator. Piping, valves, instrumentation, and
other ancillary equipment have also been designed to support the air sparging portion of
the remedy. The air sparging system has been designed based on data obtained during
the PD!. The system has been designed to supply a maximum flow of 10 scfm and a
maximum pressure of 50 pounds per square inch gage (psig) to each air sparging well.
The actual air sparge flow rate to each well is anticipated to be less than or equal to
5 scfm. The piping to deliver the air for sparging consists of a common header that has
ports for the two Phase I air sparging wells and that has two additional ports for the
Phase II air sparging wells if necessary. It is anticipated that air sparging will be
performed intermittently using pulsed flow. The design will allow for independent
control of each air sparging well with either pulsed or continuous flow. The process
control system will be programmed to provide automatic control of the air pulsing
sequence to each air sparging well.
3.3.4 Data Acquisition and Process Control
A control system is included for operation of the AS/SVE system. The control system
will have automatic data acquisition and process control capabilities using a
programmable logic controller (PLC). Remote access to authorized users will be
available through the PLC. The PLC has been designed to receive and archive process
data, and produce summary data reports. The PLC will be set up to indicate alarm status
and make automatic adjustments or shut downs as required for the alarm condition. An
example of a situation that would require automatic shut down is if there were a
malfunction or failure of the SVE system, an automatic shut down of the air sparging
system would be necessary since air sparging will not be performed without SVE. An
example of the type of data calculation that the PLC will perform is to calculate process
flow rates in scfm by correcting for process temperature, process pressure, and
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barometric pressure. The PLC has been designed to support the Phase I design and have
excess capacity to support the Phase II design, if it is necessary.
3.3.5 Facility Modifications and Equipment Locations
Several modifications to the facility are included in the Phase I design. Figure 3-1 shows
the proposed locations of the wells and probes. The wells and probes will be located
inside the building except for one SVE well that will be outside, adjacent to the west wall
of the building. The equipment area, shown on Figure 3-1, will contain the packaged
SVE system, packaged air sparging system, packaged GAC system, and the data
acquisition and process control system. An overhead door will be installed in the exterior
west wall of the plant building to allow access to the equipment area, and fences and
gates have been designed for securing the equipment area. The SVE and air sparging
well heads have been located near walls or columns where practicable. The piping and
instrumentation wiring for the system will be overhead inside the plant. The equipment
area will have its own dedicated power supply and lighting so that the AS/SVE system
can be operated independently from the Burlington plant operations.
3.4 PHASE I DESIGN DOCUMENTS
The Phase I design documents are described in this section and include technical
specifications, design plans, and a preliminary construction cost estimate.
3.4.1 Technical Specifications and Design Plans
The technical specifications and design plans are included as Attachments I and 2,
respectively. The technical specifications and design plans have been prepared m a
typical, standardized Construction Specifications Institute (CS!) Master Format. The
design plans consist of a cover sheet and drawings. The technical specifications and
design plans provide detail sufficient for bidding and actual construction by a qualified
contractor.
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3.4.2 Preliminary Construction Cost Estimate
A preliminary construction cost estimate for the Phase I design has been prepared based
on the technical specifications. Appendix C provides a summary of the preliminary
construction cost estimate. This estimate assumes the capital and O&M costs for the
Phase I design. The total capital costs are estimated to range from $510,000 to
$1,000,000. Of this, direct capital costs are estimated to be $350,000 to $690,000.
Annual O&M costs ranged from $190,000 to $320,000. The natural attenuation
monitoring annual costs ranged from $60,000 to $100,000 based on the ongoing semi-
annual groundwater monitoring program.
3.5 PHASE I PERFORMANCE TESTING
The Phase I performance testing will begin during the construction as a part of
operational testing for Phase I. The performance testing will include: baseline sampling
of soil, groundwater, and soil vapor; source area monitoring; SVE testing; and testing of
air sparging with SVE. Once the performance testing is completed, a performance
evaluation report will be prepared which will present and discuss the results of the
testing. The report will include an evaluation of the necessity of modifications to the
operating parameters for the Phase I design, the necessity of a Phase II design, and/or the
necessity for an additional contingency design. The RA work plan will include a Field
Sampling Plan that will provide the details for the sampling and analysis that will be
conducted as part of the Phase I performance testing. The components of the Phase I
performance testing are described in this section.
3.5.1 Baseline Sampling and SVE Radius of Influence Measurements
During the construction of Phase I, baseline sampling of soil, groundwater, and soil
vapors will be performed. This sampling will be coordinated with the construction.
During installation of the SVE wells, air sparging wells, and monitoring probes, soil
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samples will be collected and will be analyzed for voes. Once the wells and probes are
installed and developed, baseline groundwater samples will be collected from each
location. In addition, groundwater samples will be collected from the pilot test air sparge
wells, AS-I and AS-2, to evaluate rebound in those wells since the performance of the
pilot test in August 1998 (see Figure 2-6 for the locations of wells AS-I and AS-2). The
baseline and rebound samples will be analyzed for the parameters listed in Table 3-2
which include voes and natural attenuation parameters.
During initial operation testing and system check-out, the extracted vapors from each
SVE well will be monitored for voe concentrations to provide baseline prior to start-up
of the SVE system. As each SVE well is operated independently, vacuum data will also
be collected from adjacent SVE wells and monitoring probes to calculate the radius-of-
influence at each SVE well location. The voe concentrations and the radius of
influence calculations for each SVE well will provide a basis for evaluating the SVE
performance testing results.
3.5.2 Source Area Monitoring
Source area monitoring will consist of monitoring groundwater in the source area by
periodic sampling during the performance testing activities. Groundwater samples will
be collected from the SVE wells (i.e., water samples from the inner concentric portion of
the well screened within the groundwater zone, not the outer SVE concentric portion
screened in the vadose zone), air sparging wells, and from selected monitoring probes.
One round of groundwater samples will be collected immediately before air sparging
begins. A second round of sampling has been planned for immediately after the air
sparging test (see Section 3.5.4) to measure immediate impacts to the groundwater by air
sparging. A third round of sampling has been planned for approximately two to three
months after the initial air sparging test to measure the rebound effect and to assist with
the evaluation of potential long-term effects on natural attenuation. The groundwater will
be tested for voes and natural attenuation parameters. The baseline data from the start-
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up of the system will be used to determine if additional adjustments to the parameter list
are necessary.
3.5.3 Soil Vapor Extraction
The SVE system will be operated without air sparging for a trial period. The trial period
is anticipated to be between three and six months using the observational approach. The
actual duration of the SVE performance testing will depend on when the voe
concentrations in the extracted soil gas have decreased and stabilized sufficiently to allow
for measurement of a significant change due to air sparging. It is important to have the
voe concentrations stabilized so that when the air sparging performance testing is
conducted, it is possible to determine if a rise in voe concentration is due to the air
sparging. The SVE system will be operated using the eight SVE wells and the two
SVE/AS wells for a total often wells. The testing will include monitoring the following:
• operational parameters including flow rates, vacuum, and temperature at each
SVE well, at the packaged SVE system, and at the packaged GAe system;
• vacuum and voe concentrations at the monitoring probes;
• combined concentration of voes extracted from the ten SVE wells; and
• concentration of voes between the two GAe vessels of the off-gas treatment
system to check for breakthrough.
The operational protocol for SVE will be included in an O&M plan as part of the RA
work plan. After start-up and reaching steady state, the SVE system will be evaluated to
determine the rate and effectiveness of voe removal from the vadose zone. Provisions
will be made to adjust the operational protocol during this trial period if necessary.
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3.5.4 Soil Vapor Extraction with Air Sparging
After the initial trial period of the SVE system, air sparging will be operated with SVE
for another trial period. The anticipated trial period for AS/SVE is two to four weeks.
The actual duration of the AS/SVE performance testing will depend on the results as the
test is being conducted. The radius of influence for each air sparging well will be
measured and the combined radius of influence for both wells will be measured. Helium
tracer tests and groundwater upwelling measurements will be used to estimate the air
sparging radius of influence. The voes removed using air sparging with SVE will be
measured and the performance of the remedy will be evaluated to determine the system
effectiveness. The operation of the system will include evaluation of the data and
operational adjustments to improve the system performance. Provisions will be made to
adjust the operational protocol if necessary.
3.5.S Performance Evaluation Report
The performance evaluation report will be prepared after the performance testing has
been performed. The report will include the following:
• description of the testing;
• results of the source area monitoring of groundwater for voes and natural
attenuation parameters;
• estimated radius of influence for the individual SVE wells;
• results of the helium tracer test during air sparging;
• estimated radius of influence for the air sparging wells;
• evaluation of the effectiveness of the Phase I system;
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• recommendations for changes to the operation of the Phase I system; and
• recommended Phase II design approach and/or contingency design approach (if
necessary).
The evaluation of the overall performance of the system will consider the performance of
the installed equipment, the effectiveness of AS/SVE in reducing VOC concentrations in
the source area, and the effect of SVE with air sparging on the natural attenuation
processes within OU3. The data will also be evaluated to plan for monitoring the
rebound of various parameters within the groundwater as the long-term effects of SVE
with air sparging are evaluated. The data generated as a result of the Phase I performance
testing will assist with developing an approach and criteria for judging when the system
can be permanently shut down.
3.6 PHASE II DESIGN (IF REQUIRED)
If implementation of Phase II is determined to be necessary, then additional SVE. wells,
air sparging wells, and/or monitoring probes would be installed. The Phase II conceptual
design includes installation of SVE wells at the midpoints between the locations of some
or all of the Phase I SVE well locations. The selected locations would be adjusted as
needed based upon the Phase I trial results and the physical constraints within the textile
plant. Figure 3°2 illustrates how the Phase JI wells might be located in relationship to the
Phase I grid layout. The anticipated radius-of-influence of the Phase JI SVE wells would
be approximately 29 feet (as compared to 50 feet for the Phase I SVE wells). If this
Phase II grid layout were to be completely implemented, then ten additional SVE wells
would be installed for a total of 20 SVE wells (from both Phase I and Phase II). The
exact locations of these wells will be dependent upon obstructions and operations inside
the plant. Therefore, the locations (and possibly the number of wells) may change. The
need for and location of additional air sparging wells and monitoring probes would be
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1-----100' ----
LEGEND
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Contingent Phase II SVE
Wells to be located
approximately at the
center of Phase t welts
(if required)
1,-Phase I SVE Well Location
0 Phase II SVE Well Location
r SVE Radius of Influence
I i I I
FlGURE 3-2
CONCEPTUAL LAYOUT OF
PHASE II SVE WELLS. IN RELATION
TO PHASE I SVE WELLS
FCX-STATESVILLE SUPERFUND SITE, OU3
STATESVILLE, NORTH CAROLINA
60313.011 9/99
BROWN AND
CALDWELL Nashville, Tennessee
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determined based on the Phase I data and only those Phase II wells which were needed
would be installed.
If the evaluation of the performance testing of the SVE and air sparging determines that a
Phase II design will not adequately address site remediation objectives, then a
contingency design will be incorporated. Since the Phase I evaluation will be based on
data collected during the performance testing, the need for and nature of a contingency
design will not be evaluated until after the performance testing.
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4.0 REMEDIAL ACTION
The Remedial Action (RA) includes a project delivery strategy, evaluation of permitting
requirements, and a preliminary schedule for implementing the selected remedy.
Implementation of the RA will be coordinated with the facility owner. The installations
of wells, piping, and other devices may require field adjustment, depending upon the
operations within the textile plant.
4.1 PROJECT DELIVERY STRATEGY
The project delivery strategy begins with approval of the RD by the USEPA and the
North Carolina Department of Environment and Natural Resources (NCDENR). Once
the RD is approved, work will begin to implement the RA for the source area and for the
outer plume area. The first step of the RA is preparation of the RA Work Plan which is
described in Section 4.2. An RA contractor will be selected to perform the construction
of the Phase I design as defined by the technical specifications and design plans. Upon
completion of the Phase I construction, a certification report will be prepared and
submitted to the USEPA and the NCDENR. The Phase I performance testing will begin
during the Phase I construction and will continue after the construction is complete. A
performance evaluation test report will be prepared and submitted to the USEP A and the
NCDENR with recommendations for the necessity of modifications to the Phase I design,
the necessity of proceeding with the Phase II design, and the necessity of considering a
contingency design. Monitored natural attenuation will be ongoing during the RA Work
Plan preparation and the implementation of the RA.
4.2 REMEDIAL ACTION WORK PLAN
The RA Work Plan will be submitted to the USEPA and the NCDENR in accordance
with the revised RA schedule. The RA Work Plan will include the following elements:
• RA Schedule
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• Permitting Plan
• Construction Management Plan (CMP)
• Construction Quality Assurance Plan (CQAP)
• Field Sampling Plan
• Contingency Plan
• Project Delivery Strategy
• Groundwater Monitoring Plan
• O&M Plan
4.3 REMEDIAL ACTION
The remedial action will include the following elements:
• Procurement of the RA contractor to implement the RA will begin during the
review and approval of the Final RD.
• Acquisition of the approvals outlined later in this section of the RD Report will
be pursued concurrent with the RA contractor procurement. Work products
developed to meet substantive requirements will be submitted to the appropriate
regulatory agencies. The USEPA and NCDENR will be copied on these
submittals as required.
• A contract will be awarded to the RA contractor within approximately 120 days I of initiating contractor procurement. It is anticipated that the RA contractor will
employ several subcontractors to complete portions of the RA as necessary.
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The task of construction observation and construction quality assurance (CQA)
will be performed by a contractor. During the RA activities the CQA contractor
will serve as the Site Representative of the Project Coordinator.
• Once the contract has been awarded and the notice to proceed has been issued, a
pre-construction meeting will be held. The purpose of the meeting will be to
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detail the project requirements including submittal requirements, project
schedule, quality assurance/quality control, and project close-out. It is
anticipated that this meeting will occur at the Site.
• It is expected that the RA contractor will prepare a major portion of the project
submittals for the materials to be installed as part of the RA prior to
mobilization at the Site. Project submittals will be performed in accordance
with the Contract Documents.
• It is expected that RA activities will be continuous (for Phase I) from the initial
mobilization by the RA contractor through completion and demobilization from
the Site.
• The Phase I contract may include unit costs for anticipated Phase II work
elements. Alternatively, a separate contract may be let if Phase II work is
necessary.
• It is anticipated that the RA contractor will be responsible for procurement of all
major equipment. Prior to procurement, equipment and materials to be
used/installed as part of the RA will be submitted to the Project Coordinator for
review and approval as specified in the Contract Documents.
• The RA contractor will be responsible for the health and safety of all on Site
personnel. Air emission and spill control requirements will be addressed in the
health and safety plan developed by the RA contractor in accordance with the
specifications. The RA contractor will be responsible for developing and
submitting a health and safety plan to the Project Coordinator prior to
commencement of the work.
• The CQAP will be included as part of the Final RA submittal.
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• Weekly construction meetings and a monthly progress meeting will be held at
the Site. The RA contractor is required to develop a· detailed construction
schedule that includes project meetings and major milestones as described in the
Contract Documents.
• Within ninety (90) days after the RA has been completed, a pre-certification
inspection with representatives from USEPA and NCDENR is anticipated to
occur. If no outstanding work items are identified, the pre-certification
inspection will serve as the final certification inspection. If outstanding issues
are identified, the project will proceed and a final certification inspection will be
scheduled. Within thirty (30) days after the final certification inspection, a
written report certifying that the RA has been completed in accordance with the
requirements of the Consent Decree will be submitted to the USEPA. The
report will include record drawings depicting installations of the work and will
document major deviations from the approved RD. The RA Certification
Report will be certified by a Professional Engineer licensed in the State of North
Carolina as well as the Project Coordinator or his delegate.
• It is anticipated that the O&M Plan developed as part of the RA will be revised
once the RA has been completed and the RA Certification Report has been
submitted. The O&M Plan will be revised into a manual and include more
detailed procedures to be implemented during the O&M period. It is anticipated
that the post-RA O&M Manual will also include warranty and reference
information for materials installed during construction.
4.4 PERMIT REQUIREMENTS
Construction and operation of the AS/SVE system requires, or potentially reqmres,
permits or agreements from various agencies or parties. The following potential permit
requirements or approvals were identified and evaluated:
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• Operations
Air emissions from the treatment system
• Construction
Soil erosion and sediment control
Well Construction Permits
Handling potentially contaminated soil
Local Building Permits
Agencies and parties contacted to establish jurisdictions and permit requirements were:
• NCDENR
• Iredell County, North Carolina
A discussion of each issue follows, including whether or not a permit or agreement is
required and which agency or party requires the permit or agreement. The RA contractor
will be responsible for local building permits as part of construction.
4.4.1 Air Emissions from Soil Vapor Extraction
The NCDENR regulates em1ss10ns to the atmosphere in accordance with regulations
established pursuant to the Clean Air Act. Our review of the relevant regulations and
discussions with NCDENR personnel revealed that as long as the VOC emissions are less
than 5 tons per year, there are no requirements applicable to the discharge of off gas
vapors containing VOCs· at the Site. However, NCDENR procedure requires that a letter
containing emissions calculations be submitted for its review and determination of the
need, or lack thereof, to permit the emissions. Even though the extracted vapors are not
expected to exceed any VOC discharge standards, the RA will include an air emissions
control device.
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4.4.2 Soil Erosion and Sediment Control
The NCDENR regulates soil eros10n and sediment release related to construction
activities. A review of the relevant regulations with NCDENR personnel revealed that
there are no permits required for operations that result in disturbance of less than one acre
of total land area. Activities at the Site are anticipated to disturb less than one acre.
Iredell County confirmed that it has no ordinances that regulate soil erosion or sediment
release. Nevertheless, these activities will be performed in accordance with appropriate
storm water management practices.
4.4.3 Well Construction Permits
Based on conversations with NCDENR prior to the POI activities, well construction
permits are not required due to the fact that this Site is regulated under CERCLA.
However, the substantive requirements for well construction will be met.
4.4.4 Handling Potentially Contaminated Soil
Potentially contaminated soil will likely be encountered during well construction. In
conversations with NCDENR, we have established that no permits or petitions are
required for this activity. The impacted excavated soil will be placed into drums or roll-
off containers to be characterized and disposed of appropriately. Based on North
Carolina Regulation 15A NCAC 2H .0217(a)(9), disposal of "drilling muds, cuttings and
well water from the development of wells" are deemed permitted in accordance with NC
General Statute 143 215.l(d), and thus no individual Division permit need be issued.
However, if the drill cuttings/mud or well purge water have been contaminated by
hazardous waste constituents, the Division of Waste Management, Hazardous Waste
Section should be contacted to determine the regulatory status of the contaminated
material.
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4.5 PRELIMINARY SCHEDULE
The Preliminary RA Schedule is included as Figure 4-1. An actual construction schedule
will be required to be prepared by the RA contractor and actual schedule durations may
vary. Variations will be dependent on the number and complexity of phases required and
on the owner's operations within the facility. The schedule has been developed assuming
six months of SVE testing and four weeks of air sparging for the Phase I performance
testing. The dates included on the schedule are dependent upon regulatory review and
approval consistent with those on the schedule.
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FIGURE 4-1 PRELIMINARY SCHEDULE FOR REMEDIAL ACTION FOR OPERABLE UNIT THREE (OU3), FCX-STATESVILLE SUPERFUND SITE
Task Name Start End !999 2000
Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May USEPA & NCDENR Annroval offinal RD Reoort Dec/06/99 Dec/06/99 lo
7 Remedial Action Work Plan Dec/09/99 Mar/16/00 ,,'/// "/ /,·///_,/ .,, . .,.,/.,.,/ Prepare RA Work Plan Dec/09/99 Feb/11/00 'r'.
Submit RA Work Plan to USEPA & NCDENR Feb/11/00 Feb/11/00 c;s = USEPA & NCDENR Review of RA Work Plan Feb/11/00 Mar/16/00 . USEPA & NCDENR Annroval of RA Work Plan Mar/16/00 Mar/16/00 g Remedial Action Contractor Procurement Mar/16/00 Aug/14/00 ,v . .,,-,-. //>'// • , . /,•///. '/ ./,-/. r RA Contractor Proclll"ement Mar/16/00 Ju]/24/00
~ Submit Notice of Award to RA Contractor Jul/24/00 Jul/24/00 Negotiate Contract with RA Contractor Jul/24/00 Aug/14/00 ~ i!llll, Phase l Remedial Construction Aug/14/00 Jan/30/01 ,.,.,,,,, //_✓ /// .,,,,_,,., ,· ,,.,.,,. . ,, .,.,,.,,.,,. •//j / Notice to Proceed Aug/14/00 Aug/15/00 ~ Contractor Mobilization Aug/15/00 Sen/05/00 --~ Phase I Remedial Construction Seo/05/00 Jan/09/01 Substantial Comoletion Jan/09/01 Jan/09/01 l Site Walk Through with USEPA and NCDENR Jan/09/01 Jan/09/01 J Perform Punch List Items Jan/09/01 Jan/3010 I ~--Phase I Remedial Construction Project Closc'Out Jan/30/01 Jan/30/01 c; Preoare Phase I Certification Report Jan/30/01 Mar/14/01 i, Submit Phase I Certification Rot. to USEPA and NCDENR Mar/14/01 Mar/14/01 ~ Phase I Performance Testing Dec/22/00 Jan/02/02
/ ,'/,' ' , , "/. // './//,'. Source Area Monitoring Dec/22/00 Jan/09101 lm ~ Phase I Soil Vaoor Extraction Jan/09/01 Jul/17/01 Source Area Monitoring Jul/] 7 /01 Jul/24/01 Phase I Air Sparging/Soil Vapor Extraction Jul/24/01 Aug/21/01 Source Area Monitoring Aug/21/01 AuonS/01 Phase l Performance Testine Reoort Aug/28/01 Jan/02/02 Monthlv Progress R=orts Aug/10/99 Jan/02/02
P:/PROJ/60313.0I l/PDRSCHED.TLP
Note: 1be schedule is dependent on the actual duration ofUSi::PA and NCDENR reviews and actual approval dates.
2001 2002
Jun Jul Aug Sep Oct Nov Dec Jan
'///'/ ."// ·"// '//////, , '//// ✓//··///, .-///,'/ "/// "//
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APPENDIX A
TECHNICAL PROTOCOL FOR
EVALUATING NATURAL ATTENUATION OF
CHLORINATED SOLVENTS IN GROUND WATER
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APPENDIX A
Technical Protocol for Evaluating Natural Attenuation
of Chlorinated Solvents in Groundwater (EP A/600/R-98/128)
P:\PROJ\60507\APPENDIX.doc
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&EPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPN600/R-98/128
September 1998
Technical Protocol for
Evaluating Natural
Attenuation of Chlorinated
Solvents in Ground Water
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TECHNICAL PROTOCOL FOR EVALUATING NATURAL
ATTENUATION OF CHLORINATED SOLVENTS IN
GROUND WATER
by
Todd H. Wiedemeier
Parsons Engineering Science, Inc.
Pasadena, California
Matthew A. Swanson, David E. Moutoux, and E. Kinzie Gordon
Parsons Engineering Science, Inc.
Denver, Colorado
John T. Wilson, Barbara H. Wilson, and Donald H. Kampbell
United States Environmental Protection Agency
National Risk Management Research Laboratory
Subsurface Protection and Remediation Division
Ada, Oklahoma
Patrick E. Haas, Ross N. Miller and Jerry E. Hansen
Air Force Center for Environmental Excellence
Technology Transfer Division
Brooks Air Force Base, Texas
Francis H. Chapelle
United States Geological Survey
Columbia, South Carolina
!AG #RW57936164
Project Officer
John T. Wilson
National Risk Management Research Laboratory
Subsurface Protection and.Remediation Division
Ada, Oklahoma
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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NOTICE
The information in this document was developed through a collaboration between the U.S.
EPA (Subsurface Protection and Remediation Division, National Risk Management Research
Laboratory, Robert S. Kerr Environmental Research Center, Ada, Oklahoma [SPRD]) and the U.S.
Air Force (U.S. Air Force Center for Environmental Excellence, Brooks Air Force Base, Texas
[AFCEE]). EPA staff were primarily responsible for development of the conceptual framework for
the approach presented in this document; staff of the U.S. Air Force and their contractors also
provided substantive input. The U.S. Air Force was primarily responsible for field testing the
approach presented in this document. Through a contract with Parsons Engineering Science, Inc.,
the U.S. Air Force applied the approach at chlorinated solvent plumes at a number of U.S. Air
Force Bases. EPA staff conducted field sampling and analysis with support from ManTech
Environmental Research Services Corp., the in-house analytical support contractor for SPRD.
All data generated by EPA staff or by ManTech Environmental Research Services Corp. were
collected following procedures described in the field sampling Quality Assurance Plan for an in-
house research project on natural attenuation, and the analytical Quality Assurance Plan for ManTech
Environmental Research Services Corp.
This protocol has undergone extensive external and internal peer and administrative review by
the U.S. EPA and the U.S. Air Force. This EPA Report provides technical recommendations, not
policy guidance. It is not issued as an EPA Directive, and the recommendations of this EPA Report
are not binding on enforcement actions carried out by the U.S. EPA or by the individual States of
the United States of America. Neither the United States Government (U.S. EPA or U.S. Air Force),
Parsons Engineering Science, Inc., or any of the authors or reviewers accept any liability or
responsibility resulting from the use of this document. Implementation of the recommendations of
the document, and the interpretation of the results provided through that implementation, are the
sole responsibility of the user.
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
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FOREWORD
The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's
land, air, and water resources. Under a mandate of national environmental laws, the Agency strives
to formulate and implement actions leading to a compatible balance between human activities and
the ability of natural systems to support and nurture life. To meet these mandates, EPA's research
program is providing data and technical support for solving environmental problems today and
building a science knowledge base necessary to manage our ecological resources wisely, understand
how pollutants affect our health, and prevent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for investigation
of technological and management approaches for reducing risks from threats to human health and
the environment. The focus of the Laboratory's research program is on methods for the prevention
and control of pollution to air, land, water, and subsurface resources; protection of water quality in
public water systems; remediation of contaminated sites and ground water; and prevention and
control of indoor air pollution. The goal of this research effort is to catalyze development and
implementation of innovative, cost-effective environmental technologies; develop scientific and
engineering information needed by EPA to support regulatory and policy decisions; and provide
technical support and information transfer to ensure effective implementation of environmental
regulations and strategies.
The site characterization processes applied in the past are frequently inadequate to allow an
objective and robust evaluation of natural attenuation. Before natural attenuation can be used in the
remedy for contamination of ground water by chlorinated solvents, additional information is required
on the three-dimensional flow field of contaminated ground water in the aquifer, and on the physical,
chemical and biological processes that attenuate concentrations of the contaminants of concern.
This document identifies parameters that are useful in the evaluation of natural attenuation of
chlorinated solvents, and provides recommendations to analyze and interpret the data collected
from the site characterization process. It will also allow ground-water remediation managers to
incorporate natural attenuation into an integrated approach to remediation that includes an active
remedy, as appropriate, as well as natural attenuation.
Clinton W. Hall, Director
Subsurface Protection and Remediation Division
National Risk Management Research Laboratory
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TABLE OF CONTENTS
Notice ........................................................................................................................................... ii
Foreword ..................................................................................................................................... iii
Acknowledgments ..................................................................................................................... viii
List of Acronyms and Abbreviations .. : ....................................................................................... ix
Definitions .................................................................................................................................. xii
SECTION I INTRODUCTION .................................................................................................. I
I. I APPROPRIATE APPLICATION ON NATURAL ATTENUATION ........................ 2
1.2 ADVANTAGES AND DISADVANTAGES .............................................................. 4
1.3 LINES OF EVIDENCE .............................................................................................. 6
1.4 SITE CHARACTERIZATION ................................................................................... 7
1.5 MONITORING .......................................................................................................... 9
SECTION 2 PROTOCOL FOR EVALUATING NATURAL ATTENUATION ...................... 11
2.1 REVIEW AVAILABLE SITE DATA AND DEVELOP PRELIMINARY
CONCEPTUAL MODEL ........................................................................................ 13
2.2 INITIAL SITE SCREENING ................................................................................... 15
2.2.1 Overview of Chlorinated Aliphatic Hydrocarbon Biodegradation ................... 15
2.2.1. l Mechanisms of Chlorinated Aliphatic Hydrocarbon Biodegradation ..... 23
2.2.1.1.1 Electron Acceptor Reactions (Reductive Deha!ogenation) ............... 23
2.2. 1.1.2 Electron Donor Reactions ................................................................. 25
2.2.1.1.3 Cometabolism ................................................................................... 25
2.2.1.2 Behavior of Chlorinated Solvent Plumes ................................................ 26
2.2. 1.2.1 Type I Behavior ................................................................................ 26
2.2.1.2.2 Type 2 Behavior ................................................................................ 26
2.2.1.2.3 Type 3 Behavior ................................................................................ 26
2.2.1.2.4 Mixed Behavior ................................................................................ 27
2.2.2 Bioattenuation Screening Process ..................................................................... 27
2.3 COLLECT ADDITIONAL SITE CHARACTERIZATION DATA IN
SUPPORT OF NATURAL ATTENUATION AS REQUIRED ............................... 34
2.3.1 Characterization of Soils and Aquifer Matrix Materials .................................. 37
2.3.2 Ground-water Characterization ........................................................................ 38
2.3.2.1 Volatile and Semivolatile Organic Compounds ..................................... 38
2.3.2.2 Dissolved Oxygen .................................................................................. 38
2.3.2.3 Nitrate .................................. : .................................................................. 39
2.3.2.4 Iron (II) ................................................................................................... 39
2.3.2.5 Sulfate .................................................................................................... 39
2.3.2.6 Methane ................................... : .............................................................. 39
2.3.2.7 Alkalinity ................. : .............................................................................. 39
2.3.2.8 Oxidation-Reduction Potential ............................................................... 40
2.3.2.9 Dissolved Hydrogen ............................................................................... 40
2.3.2.10 pH, Temperature, and Conductivity ....................................................... 41
2.3.2.11 Chloride .................................................................................................. 42
2.3.3 Aquifer Parameter Estimation .......................................................................... 42
2.3.3.1 Hydraulic Conductivity .......................................................................... 42
2.3.3.1.1 Pumping Tests in Wells ..................................................................... 43
2.3.3.1.2 Slug Tests in Wells ............................................................................ 43
2.3.3.1.3 Downhole Flowmeter ........................................................................ 43
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2.3.3.2 Hydraulic Gradient .................................................................................. 44
2.3.3.3 Processes Causing an Apparent Reduction in
Total Contaminant Mass ........................................................................ 44
2.3.4 Optional Confirmation of Biological Activity .................................................. 45
2.4 REFINE CONCEPTUAL MODEL, COMPLETE PRE-MODELING
CALCULATIONS, AND DOCUMENT INDICATORS OF NATURAL
ATTENUATION ...................................................................................................... 45
2.4.1 Conceptual Model Refinement ......................................................................... 46
2.4.1.1 Geologic Logs .......................................................................................... 46
2.4.1.2 Cone Penetrometer Logs .......................................................................... 46
2.4.1.3 Hydrogeologic Sections ........................................................................... 46
2.4.1.4 Potentiometric Surface or Water Table Map(s) ....................................... 47
2.4. 1.5 Contaminant and Daughter Product Contour Maps ................................ 47
2.4.1.6 Electron Acceptor, Metabolic By-product, and
Alkalinity Contour Maps ........................................................................ 47
2.4.2 Pre-Modeling Calculations ............................................................................... 48
2.4.2.1 Analysis of Contaminant, Daughter Product, Electron Acceptor,
Metabolic By-product, and Total Alkalinity Data .................................. 48
2.4.2.2 Sorption and Retardation Calculations .................................................... 49
2.4.2.3 NAPL/Water Partitioning Calculations ................................................... 49
2.4.2.4 Ground-water Flow Velocity Calculations .............................................. 49
2.4.2.5 Biodegradation Rate-Constant Calculations ............................................ 49
2.5 SIMULATE NATURAL ATTENUATION USING SOLUTE FATE AND
TRANSPORT MODELS ......................................................................................... 49
2.6 CONDUCT A RECEPTOR EXPOSURE PATHWAYS ANALYSIS ...................... 50
2.7 EVALUATE SUPPLEMENTAL SOURCE REMOVAL OPTIONS ....................... 50
2.8 PREPARE LONG-TERM MONITORING PLAN .................................................. 50
2.9 PRESENT FINDINGS ............................................................................................. 52
SECTION 3 REFERENCES ............................................... : ................................................... 53
APPENDIX A ........................................................................................................................ Al-I
APPENDIXB ........................................................................................................................ Bl-I
APPENDIX C ........................................................................................................................ Cl-I
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No.
2.1
2.2
2.3
2.4
2.5
2.6
2.7
FIGURES
Title Page
Natural attenuation of chlorinated solvents flow chart .................................................. 12
Reductive dehalogenation of chlorinated ethenes .......................................................... 24
Initial screening process flow chart ................................................................................ 28
General areas for collection of screening data ............................................................... 31
A cross section through a hypothetical release .............................................................. 36
A stacked plan representation of the plumes that may develop from the
hypothetical release ........................................................................................................ 36
Hypothetical long-term monitoring strategy .................................................................. 51
TABLES
No. Title Page
1. Contaminants with Federal Regulatory Standards ........................................................ xiv
2. 1 Soil, Soil Gas, and Ground-water Analytical Protocol .................................................. 16
2.2 Objectives for Sensitivity and Precision to
Implement the Natural Attenuation Protocol ................................................................. 21
2.3 Analytical Parameters and Weighting for Preliminary Screening for
Anaerobic Biodegradation Processes ............................................................................. 29
2.4 Interpretation of Points Awarded During Screening Step 1 ........................................... 32
2.5 Range of Hydrogen Concentrations for a Given Terminal
Electron-Accepting Process ........................................................................................... 41
Vil
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ACKNOWLEDGMENTS
The authors would like to thank Dr. Robert Hinchee, Doug Downey, and Dr. Guy Sewell for their
contributions and their extensive and helpful reviews of this manuscript. Thanks also to Leigh
Alvarado Benson, R. Todd Herrington, Robert Nagel, Cindy Merrill, Peter Guest, Mark Vesseley,
John Hicks, and Saskia Hoffer for their contributions to this project.
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BRA
BRAC
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CAP
CERCLA I cfm
CFR
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CSM
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DERP
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DQO
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HSSM I HSWA
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LIST OF ACRONYMS AND ABBREVIATIONS
American Association of Railroads
Air Force Base
Air Force Center for Environmental Excellence
American Society for Testing and Materials
below ground surface
baseline risk assessment
Base Realignment and Closure
benzene, toluene, ethylbenzene, xylenes
corrective action plan
Comprehensive Environmental Response, Compensation and Liability
Act
cubic feet per minute
Code of Federal Regulations
chemical of potential concern
cone penetrometer testing
conceptual site model
dilution/attenuation factor
Defense Environmental Restoration Program
Dense Nonaqueous Phase Liquid
dissolved oxygen
Department of Defense
data quality objective
engineering evaluation/cost analysis
feasibility study
gallons per day
standard (Gibbs) free energy
high-density polyethylene
Hydrocarbon Spill Screening Model
Hazardous and Solid Waste Amendments of 1984
inside-diameter
investigation derived waste
Installation Restoration Program
liter
lower explosive limit
light nonaqueous-phase liquid
leaking underground fuel tank
management action plan
maximum corltaminant level
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I MDL method detection limit
µg microgram
µg/kg microgram per kilogram
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µg/L microgram per liter
mg milligram
mg/kg milligrams per kilogram
mg/L milligrams per liter
I mglm' milligrams per cubic meter
mmHg millimeters of mercury
MOC method of characteristics
I MOGAS motor gasoline
NAPL nonaqueous-phase liquid
I NCP National Contingency Plan
NFRAP no further response action plan
NOAA National Oceanographic and Atmospheric Administration
NOEL no-observed-effect level I NPL National Priorities List
OD outside-diameter
I ORP oxidation-reduction potential
OSHA Occupational Safety and Health Administration
OSWER Office of Solid Waste and Emergency Response
I PAH polycyclic aromatic hydrocarbon
PEL permissible exposure limit
I POA point-of-action
POC point-of-compliance
POL petroleum, oil, and lubricant
ppmv parts per million per volume
I psi pounds per square inch
PVC polyvinyl chloride
I QA quality assurance
QC quality control
I RAP remedial action plan
RBCA risk-based corrective action
RBSL risk-based screening level
redox reduction/oxidation
I RFI RCRA facility investigation
RI remedial investigation
RME reasonable maximum exposure
I RPM remedial project manager
SAP sampling and analysis plan
I SARA Supcrfund Amendments and Reauthorization Act
scfm standard cubic feet per minute
SPCC spill prevention, control, and countermeasures
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I SSL soil screening level
SSTL site-specific target level
SVE soil vapor extraction
I SVOC semi volatile organic compound
TC toxicity characteristic
TCLP toxicity-characteristic leaching procedure
I TI technical impracticability
TMB trimethylbenzene
TOC total organic carbon
I TPH total petroleum hydrocarbons
TRPH total recoverable petroleum hydrocarbons
TVH total volatile hydrocarbons
I TVPH .total volatile petroleum hydrocarbons
TWA time-weighted-average
I UCL upper confidence limit
us United States
USGS US Geological Survey
UST underground storage tank
I voes volatile organic compounds
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DEFINITIONS
Aerobe: bacteria that use oxygen as an electron acceptor.
Anabolism: The process whereby energy is used to build organic compounds such as enzymes and
nucleic acids that are necessary for life functions. In essence, energy is derived from catabolism,
stored in high-energy intermediate compounds such as adenosine triphosphate (ATP), guanosine
triphosphate (OTP) and acetyl-coenzyme A, and used in anabolic reactions that allow a cell to
grow.
Anaerobe: Organisms that do not require oxygen to live.
Area of Attainment: The area over which cleanup levels will be achieved in the ground water. It
encompasses the area outside the boundary of any waste remaining in place and up to the boundary
of the contaminant plume. Usually, the boundary of the waste is defined by the source control
remedy. Note: this area is independent of property boundaries or potential receptors -it is the
plume area which the ground water must be returned to beneficial use during the implementation of
a remedy.
Anthropogenic: Man-made.
Autotrophs: Microorganisms that synthesize organic materials from carbon dioxide.
Catabolism: The process whereby energy is extracted from organic compounds by breaking them down
into their component parts.
Coefficient of Variation: Sample standard deviation divided by the mean.
Cofactor: A small molecule required for the function of an enzyme.
Cometabolism: The process in which a compound is fortuitously degraded by an enzyme or cofactor
produced during microbial metabolism of another compound.
Daughter Product: A compound that results directly from the biodegradation of another. For example
cis-1,2-dichloroethenc (cis-1,2-DCE)is commonly a daughter product of trichloroethene (TCE).
Dehydrohalogenation: Elimination of a hydrogen ion and a halide ion resulting in the formation of an
alkene.
Diffusion: The process whereby molecules move from a region of higher concentration to a region of
lower concentration as a result of Brownian motion.
Dihaloelimination: Reductive elimination of two halide substituents resulting in formation of an alkene.
Dispersivity: A property that quantifies mechanical dispersion in a medium.
Effective Porosity: The percentage of void volume that contributes to percolation; roughly equivalent to
the specific yield.
Electron Acceptor: A compound capable of accepting electrons during oxidation-reduction reactions.
Microorganisms obtain energy by transferring electrons from electron donors such as organic
compounds (or sometimes reduced inorganic compounds such a"s sulfide) to an electron acceptor.
Electron acceptors arc compounds that are relatively oxidized and include oxygen, nitrate,
iron (III), manganese (IV), sulfate, carbon dioxide, or in some cases the chlorinated aliphatic
hydrocarbons such as perchloroethene (PCE), TCE, DCE, and vinyl chloride.
Electron Donor: A compound capable of supplying (giving up) electrons during oxidation-reduction
reactions. Microorganisms obtain energy by transferring electrons from electron donors such as
organic compounds (or sometimes reduced inorganic compounds such as sulfide) to an electron
acceptor. Electron donors arc compounds that are relatively reduced and include fuel
hydrocarbons and native organic carbon.
Electrophile: A reactive species that accepts an electron pair.
Elimination: Reaction where two groups such as chlorine and hydrogen are lost from adjacent carbon
atoms and a double bond is formed in their place.
Epoxidation: A reaction wherein an oxygen molecule is inserted in a carbon-carbon double bond and an
epoxide is formed.
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Facultative Anaerobes: microorganisms that use (and prefer) oxygen when it is available, but can also use
alternate electron acceptors such as nitrate under anaerobic conditions when necessary.
Fermentation: Microbial metabolism in which a particular compound is used both as an electron donor
and an electron acceptor resulting in the production of oxidized and reduced daughter products.
Heterotroph: Organism that uses organic carbon as an external energy source and as a carbon source.
Hydraulic Conductivity: The relative ability of a unit cube of soil, sediment, or rock to transmit water.
Hydraulic Head: The height above a datum plane of the surface of a column of water. In the
groundwater environment, it is composed dominantly of elevation head and pressure head.
Hydraulic Gradient: The maximum change in head per unit distance.
Hydrogenolysis: A reductive reaction in which a carbon-halogen bond is broken, and hydrogen replaces
the halogen substituent.
Hydroxylation: Addition of a hydroxyl group to a chlorinated aliphatic hydrocarbon.
Lithotroph: Organism that uses inorganic carbon such as carbon dioxide or bicarbonate as a carbon
source and an external source of energy.
Mechanical Dispersion: A physical process of mixing along a flow path in an aquifer resulting from
differences in path length and flow velocity. This is in contrast to mixing due to diffusion.
Metabolic Byproduct: A product of the reaction be.tween an electron donor and an electron acceptor.
Metabolic byproducts include volatile fatty acids, daughter products of chlorinated aliphatic
hydrocarbons, methane, and chloride.
Monooxygenase: A microbial enzyme that catalyzes reactions in which one atom of the oxygen molecule
is incorporated into a product and the other atom appears in water.
Nucleophile: A chemical reagent that reacts by forming covalent bonds with electronegative atoms and
compounds.
Obligate Aerobe: Microorganisms that can use only oxygen as an electron acceptor. Thus, the presence
of molecular oxygen is a requirement for these microbes.
Obligate Anaerobes: Microorganisms that grow only in the absence of oxygen; the presence of molecular
oxygen either inhibits growth or kills the organism. For example, methanogens are very sensitive
to oxygen and can live only under strictly anaerobic conditions. Sulfate reducers, on the other
hand, can tolerate exposure to oxygen, but cannot grow in its presence (Chapelle, 1993).
Pe,formance Evaluation Well: A ground-water monitoring well placed to monitor the effectiveness of
the chosen remedial action.
Porosity: The ratio of void volume to total volume of a rock or scdimenl.
Respiration: The process of coupling oxidation of organic compounds with the reduction of inorganic
compounds, such as oxygen, nitrate, iron (III), manganese (IV), and sulfate.
Solvolysis: A reaction in which the solvent serves as the nucleophile.
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Table i: Contaminants with Federal Regulatory Standards Considered in this Document
Abbreviation Chemical Abstracts Service CAS Other Names Molecular
(CAS) Name Number Formula
PCE tetrachloroethene 127-18-4 oerchloroethvlene; tetrachloroethvlene C,CI,
TCE trichloroethene 79-01-6 trichloroethylene C,HCI,
1,1-DCE I, 1-dichloroethene 75-35-4 1,1-dichloroethylene; vinylidine chloride C2H2CI,
trans-1,2-DCE (E)-1,2-dichloroethene 156-60-5 trans-l ,2-dichloroethene;trans-1,2-dichloroethylene C,H,CI,
cis-1,2-DCE 156-59-2 cis-1,2-dichloroethene; cis-1,2-dichloroethvlene C,H,CI,
vc chloroethene 75-01-4 vinyl chloride; chloroethylene C,H,CI
1,1, 1-TCA 1, I, I -trichloroethane 71-55-6 C,H,CI,
1,1,2-TCA I, 1,2-trichloroethane 79-00-5 C,H,CI,
1,1-DCA I, 1-dichloroethane 75-34-3 C2H,CI,
1,2-DCA 1,2-dichloroethane 107-06-02 C,H,C)z
CA chloroethane 75-00-3 C2H5CI
CF trichloromethane 67-66-3 chloroform CHCl3
CT tetrachloromethane 56-23-5 carbon tetrachloride CCI.
Methylene Chloride dichloromethane 75-09-2 methylene dichloride CH,CI,
CB chlorobenzene 108-90-7 C.H,CI
1,2-DCB 1,2-dichlorobenzene 95-50-1 o-dichlorobenzene C.H,Cl,
1,3-DCB 1,3-dichlorobenzene 541-73-1 m-dichlorobenzene C.H,CI,
1,4-DCB 1,4-dichlorobenzene 106-46-7 p-dichlorobenzene C.H.Cl2
1,2,3-TCB 1,2,3-trichlorobenzene 87-61-6 C.H,CI,
1,2,4-TCB 1,2,4-trichlorobenzene 120-82-1 C.l-13Cl3
1,3,5-TCB 1,3,5-trichlorobenzene 108-70-3 C.H3Cl3
1,2,3,5-TECB 1,2,3,5-tetrachlorobenzene 634-90-2 1,2,3,5-TCB C.H,CI,
1,2,4,5-TECB 1,2,4,5-tetrachlorobenzene 95-94-3 C.H,CI,
HCB hexachlorobenzene 118-74-1 c.c1.
EDB 1,2-dibromoethane 106-93-4 ethylene dibromide; dibromoethane C2H.Br2
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SECTION 1
INTRODUCTION
Natural attenuation processes (biodegradation, dispersion, sorption, volatilization) affect the
fate and transport of chlorinated solvents in all hydrologic systems. When these processes are
shown to be capable of attaining site-specific remediation objectives in a time period that is reasonable
compared to other alternatives, they may be selected alone or in combination with other more
active remedies as the preferred remedial alternative. Monitored Natural Attenuation (MNA) is a
term that refers specifically to the use of natural attenuation processes as part of overall site
remediation. The United States Environmental Protection Agency (U.S. EPA) defines monitored
natural attenuation as (OSWER Directive 9200.4-17, 1997):
The term "monitored natural attenuation," as used in this Directive, refers
to the reliance on natural attenuation processes (within the context of a carefully
controlled and monitored clean-up approach) to achieve site-specific remedial
objectives within a time frame that is reasonable compared to other methods. The
"natural attenuation processes" that are at work in such a remediation approach
include a variety of physical, chemical, or biological processes that, under favorable
conditions, act without human intervention to reduce the mass, toxicity, mobility,
volume, or concentration of contaminants in soil and ground water. These in-situ
processes include, biodegradation, dispersion, dilution, sorption, volatilization, and
chemical or biological stabilization, transformation, or destruction of contaminants.
Monitored natural attenuation is appropriate as a remedial approach only
when it can be demonstrated capable of achieving a site's remedial objectives within
a time frame that is reasonable compared to that offered by other methods and
where it meets the applicable remedy selection program for a particular OSWER.
program. EPA, therefore, expects that monitored natural attenution typically will
be used in conjunction with active remediation measures (e.g., source control), or
as a follow-up to active remediation measures that have already been implemented.
The intent of this document is to present a technical protocol for data collection and analysis
to evaluate monitored natural attenuation through biological processes for remediating ground
water contaminated with mixtures of fuels and chlorinated aliphatic hydrocarbons. This document
focuses on technical issues and is not intended to address policy considerations or specific regulatory
or statutory requirements. In addition, this document does not provide comprehensive guidance on
overall site characterization or long-term monitoring of MNA remedies. Users of this protocol
should realize that different Federal and State remedial programs may have somewhat different
remedial objectives. For example, the CERCLA and RCRA Corrective Action programs generally
require that remedial actions: I) prevent exposure to contaminated ground water, above acceptable
risk levels; 2) minimize further migration of the plume; 3) minimize further migration of
contaminants from source materials; and 4) restore the plume to cleanup levels appropriate for
current or future beneficial uses, to the extent practicable. Achieving such objectives could often
require that MNA be used in conjunction with other "active" remedial methods. For other cleanup
programs, remedial objectives may be focused on preventing exposures above acceptable levels.
Therefore, it is imperative that users of this document be aware of and understand the Federal and
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State statutory and regulatory requirements, as well as policy considerations that apply to a specific
site for which this protocol will be used to evaluate MNA as a remedial option. As a general
practice (i.e., not just pertaining to this protocol), individuals responsible for evaluating remedial
alternatives should interact with the overseeing regulatory agency to identify likely characterization
and cleanup objectives for a particular site prior to investing significant resources. The policy
framework within which MNA should be considered for Federal cleanup programs is described in
the November 1997 EPA Directive titled, "Use of Monitored Natural Attenuation at Superfund,
RCRA Corrective Action and Underground Storage Tank Sites" (Directive No. 9200.4-17).
This protocol is designed to evaluate the fate in ground water of chlorinated aliphatic
hydrocarbons and/or fuel hydrocarbons. Because documentation of natural attenuation requires
detailed site characterization, the data collected under this protocol can be used to compare the
relative effectiveness of other remedial options and natural attenuation. This protocol should be
used to evaluate whether MNA by itself or in conjunction with other remedial technologies is
sufficient to achieve site:specific remedial objectives. In evaluating the appropriateness of MN A,
the user of this protocol should consider both existing exposure pathways, as well as exposure
pathways arising from potential future uses of the ground water.
This protocol is aimed at improving the characterization process for sites at which a remedy
involving monitored natural attenuation is being considered. It contains methods and recommended
strategies for completing the remedial investigation process. Emphasis is placed on developing a
more complete understanding of the site through the conceptual site model process, early pathways
analysis, and evaluation of remedial processes to include MNA. Understanding the contaminant.
flow field in the subsurface is essential for a technically justified evaluation of an MNA remedial
option; therefore, use of this protocol is not appropriate for evaluating MNA at sites where the
contaminant flow field cannot be determined with an acceptable degree of certainty (e.g., complex
fractured bedrock, karst aquifers).
In practice, natural attenuation also is referred to by several other names, such as intrinsic
remediation, intrinsic bioremediation, natural restoration, or passive bioremediation. The goal of
any site characterization effort is to understand the fate and transport of the contaminants of concern
over time in order to assess any current or potential threat to human health or the environment.
Natural attenuation processes, such as biodegradation, can often be dominant factors in the fate and
transport of contaminants. Thus, consideration and quantification of natural attenuation is essential
to a more thorough understanding of contaminant fate and transport.
1.1 APPROPRIATE APPLICATION ON NATURAL ATTENUATION
The intended audience for this document includes Project Managers and their contractors,
scientists, consultants, regulatory personnel, and others charged with remediating ground water
contaminated with chiorinated. aliphatic hydrocarbons or mixtures of fuel hydrocarbons and
chlorinated aliphatic hydrocarbons. This protocol is intended to be used within the established
regulatory framework appropriate for selection of a remedy at a particular hazardous waste site
(e.g., the nine-criteria analysis used to evaluate remedial alternatives in the CERCLA remedy
selection process). It is not the intent of this document to replace existing U.S. EPA or state-
specific guidance on conducting remedial investigations.
The EPA does not consider monitored natural attenuation to be a default or presumptive
remedy at any contaminated site (OSWER Directive 9200.4-17, 1997), as its applicability is highly
variable from site to site. In order for MNA to be selected as a remedy, site-specific determinations
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will always have to be made to ensure that natural attenuation is sufficiently protective of human
health and the environment.
Natural attenuation in ground-water systems results from the integration of several subsurface
attenuation mechanisms that are classified as either destructive or nondestructive. Biodegradation
is the most important destructive attenuation mechanism, although abiotic destruction of some
compounds does occur. Nondestructive attenuation mechanisms include sorption, dispersion, dilution
from recharge, and volatilization. The natural attenuation of fuel hydrocarbons is described in the
Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural
Attenuation of Fuel Contamination Dissolved in Groundwater, published by the Air Force Center
for Environmental Excellence (AFCEE) (Wiedemeier et al., 1995d). This document differs from
the technical protocol for intrinsic remediation of fuel hydrocarbons because it focuses on the
individual processes of chlorinated aliphatic hydrocarbon biodegradation which are fundamentally
different from the processes involved in the biodegradation of fuel hydrocarbons.
For example, biodegradation of fuel hydrocarbons, especially benzene, toluene, ethyl benzene,
and xylenes (BTEX), is mainly limited by electron acceptor availability, and generally will proceed
until all of the contaminants biochemically accessible to the microbes are destroyed. In the experience
of the authors, there appears to be an adequate supply of electron acceptors in most, if not all,
hydrogeologic environments. On the other hand, the more highly chlorinated solvents such as
perchloroethene (PCE) and trichloroethene (TCE) typically are biodegraded under natural conditions
via reductive dechlorination, a process that requires both electron acceptors (the chlorinated aliphatic
hydrocarbons) and an adequate supply of electron donors. Electron donors include fuel hydrocarbons
or other types of anthropogenic carbon (e.g., landfill leachate) or natural organic carbon. If the
subsurface environment is depleted of electron donors before the chlorinated aliphatic hydrocarbons
are removed, biological reductive dechlorination will cease, and natural attenuation may no longer
be protective of human health and the environment. This is the most significant difference between
the processes of fuel hydrocarbon and chlorinated aliphatic hydrocarbon biodegradation.
For this reason, it is more difficult to predict the long-term behavior of chlorinated aliphatic
hydrocarbon plumes than fuel hydrocarbon plumes. Thus, it is important to have a good
understanding of the important natural attenuation mechanisms. Data collection should include all
pertinent parameters to evaluate the efficacy of natural attenuation. In addition to having a better
understanding of the processes of advection, dispersion, dilution from recharge, and sorption, it is
necessary to better quantify biodegradation. This requires an understanding of the interactions
between chlorinated aliphatic hydrocarbons, anthropogenic or natural carbon, and inorganic electron
acceptors at the site. Detailed site characterization is required to adequately document and understand
these processes. The long-term monitoring strategy should consider the possibility that the behavior
of a plume may change over time and monitor for the continued availability of a carbon source to
support reductive dechlorination.
An understanding of the attenuation mechanisms is also important to characterizing exposure
pathways. After ground water plumes come to steady state, sorption can no longer be an important
attenuation mechanism. The most important mechanisms will be biotransformation, discharge
through advective flow, and volatilization. As an example, Martin and Imbrigiotta ( 1994) calibrated
a detailed transport and fate model to a release of pure TCE at Picatinny Arsenal, in New Jersey.
The plume was at steady state or declining. Ten years after surface spills ceased, leaching of
contaminants from subsurface DNAPLs and desorption from fine-grained layers were the only
processes identified that continued to contribute TCE to ground water. Desorption ofTCE occurred
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at a rate of 15 to 85 mg/second. Anaerobic biotransformation consumed TCE at a rate of up to 30
mg/second, advective flow and discharge of TCE to surface water accounted for up to 2 mg/
second, and volatilization of TCE accounted for 0.1 mg/second. In this case, recharge of
uncontaminated water drove the plume below the water table, which minimized the opportunity for
volatization to the unsaturated zone. As a result, discharge to surface water was the only important
exposure pathway. Volatilization will be more important at sites that do not have significant
recharge to the water table aquifer, or that have NAPLs at the water table that contain chlorinated
organic compounds.
Chlorinated solvents are released into the subsurface as either aqueous-phase or nonaqueous
phase liquids. Typical solvent releases include nonaqueous phase relatively pure solvents that are·
more dense than water and aqueous rinseates. Additionally, a release may occur as a mixture of
fuel hydrocarbons or sludges and chlorinated aliphatic hydrocarbons which, depending on the
relative proportion of each compound group, may be more or less dense than water. If the NAPL
is more dense than water, the material is referred to as a "dense nonaqueous-phase liquid," or
DNAPL. If the NAPL is less dense than water the material is referred to as a "light nonaqueous-
phase liquid," or LNAPL. Contaminant sources generally consist of chlorinated solvents present
as mobile NAPL (NAPL occurring at sufficiently high saturations to drain under the influence of
gravity into a well) and residual NAPL (NAPL occurring at immobile, residual saturations that are
unable to drain into a well by gravity). In general, the greatest mass of contaminant is associated
with these NAPL source areas, not with the aqueous phase.
When released at the surface, NAPLs move downward under the force of gravity and tend to
follow preferential pathways such as along the surface of sloping fine-grained layers or through
fractures in soil or rock. Large NAPL releases can extend laterally much farther from the release
point than would otherwise be expected, and large DNAPL releases can sink to greater depths than
expected by following preferential flow paths. Thus, the relative volume of the release and potential
migration pathways should be considered when developing the conceptual model for the distribution
of NAPL in the subsurface.
As water moves through NAPL areas (recharge in the vadose zone or ground water flow in an
aquifer), the more soluble constituents partition into the water to generate a plume of dissolved
contamination and the more volatile contaminants partition to the vapor phase. After surface
releases have stopped, NAPLs remaining in the subsurface tend to "weather" over time as volatile
and soluble components are depleted from NAPL surfaces. Even considering this "weathering"
effect, subsurface NAPLS continue to be a source of contaminants to ground water for a very long
time. For this reason, identification and delineation of subsurface zones containing residual or
free-phase NAPL is an important aspect of the site conceptual model to be developed for evaluating
MNA or other remediation methods.
Removal, treatment or containment of NAPLs may be necessary for MNA to be a viable
remedial option or to decrease the time needed for natural processes to attain site-specific remediation
objectives. In cases where removal of mobile NAPL is feasible, it is desirable to remove this
source material and decrease the time required to reach cleanup objectives. Where removal or
treatment ofNAPL is not practical, source containment may be practicable and necessary for MNA
to be a viable remedial option.
1.2 ADVANTAGES AND DISADVANTAGES
In comparison to engineered remediation technologies, remedies relying on monitored natural
attenuation have the following advantages and disadvantages, as identified in OSWER Directive
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9200.4-17, dated November 1997. (Note that this an iterim, not a final, Directive which was released
by EPA for use. Readers are cautioned to consult the final version of this Directive when it becomes
available.)
The advantages of monitored natural attenuation (MNA) remedies are:
• As with any in situ process, generation of lesser volume of remediation wastes reduced potential
for cross-media transfer of contaminants commonly associated with ex situ treatment, and
reduced risk of human exposure to contaminated media;
• Less intrusion as few surface structures are required;
• Potential for application to all or part of a given site, depending on site conditions and cleanup
objectives;
• Use in conjunction with, or as a follow-up to, other (active) remedial measures; and
• Lower overall remediation costs than those associated with active remediation.
The potential disadvantages of monitored natural attenuation (MNA) include:
• Longer time frames may be required to achieve remediation objectives, compared to active
remediation;
• Site characterization may be more complex and costly;
• Toxicity of transformation products may exceed that of the parent compound;
• Long-term monitoring will generally be necessary;
• Institutional controls may be necessary to ensure long-term protectiveness;
Potential exists for continued contamination migration, and/or cross-media transfer of
contaminants;
• Hydrologic and geochemical conditions amenable to natural attenuation are likely to change
over time and could result in renewed mobility of previously stabilized contaminants, adversely
impacting remedial effectiveness; and
• More extensive education and outreach efforts may be required in order to gain public
· acceptance of monitored natural attenuation.
At some sites the same geochemical conditions and processes that lead to biodegradation of
chlorinated solvents and petroleum hydrocarbons can chemically transform naturally occmTing
manganese, arsenic and other metals in the aquifer matrix, producing forms of these metals that
are more mobile and/or more toxic than the original materials. A comprehensive assessment of
risk at a hazardous waste site should include sampling and analysis for these metals.
This document describes ( l) those processes that bring about natural attenuation, (2) the site
characterization activities that may be performed to conduct a full-scale evaluation of natural
attenuation, (3) mathematical modeling of natural attenuation using analytical or numerical solute
fate and transport models, and (4) the post-modeling activities that should be completed to ensure
successful evaluation and verification of remediation by natural attenuation. The objective is to
quantify and provide defensible data to evaluate natural attenuation at sites where naturally occurring
subsurface attenuation processes are capable of reducing dissolved chlorinated aliphatic hydrocarbon
and/or fuel hydrocarbon concentrations to acceptable levels. A comment made by a member of the
regulatory community summarizes what is required to successfully implement natural attenuation:
A regulator looks for the data necessary to determine that a proposed
treatment technology, if properly installed and operated, will reduce the contaminant
concentrations in the soil and water to legally mandated limits. In this sense, the
use of biological treatment systems calls for the same level of investigation,
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demonstration of effectiveness, and monitoring as any conventional [ remediation/
system (National Research Council, 1993).
When the rate of natural attenuation of site contaminants is sufficient to attain site-specific
remediation objectives in a time period that is reasonable compared to other alternatives, MNA
may be an appropriate remedy for the site. This document presents a technical course of action that
allows converging lines of evidence to be used to scientifically document the occurrence of natural
attenuation and quantify the rate at which it is occurring. Such a "weight-of-evidence" approach
will greatly increase the likelihood of successfully implementing natural attenuation at sites where
natural processes are restoring the environmental quality of ground water.
1.3 LINES OF EVIDENCE
The OSWER Directive 9200.4-17 ( 1997) identifies three lines of evidence that can be used to
estimate natural attenuation of chlorinated aliphatic hydrocarbons, including:
( 1) Historical ground water and/or soil chemistry data that demonstrate a clear and meaningful
trend of decreasing contaminant mass and/or concentration over time at appropriate
monitoring or sampling points. (In the case of a ground water plume, decreasing
concentrations should not be solely the result of plume migration. In the case of inorganic
contaminants, the primary attenuating mechanism should also be understood.)
(2) Hydro geologic and geochemical data that can be used to demonstrate indirectly the type(.,")
of natural attenuation processes active at the site, and the rate at which such processes
will reduce contaminant concentrations to required levels. For example, characterization
data may be used to quantify the rates of contaminant sorption, dilution, or volatilization,
or to demonstrate and quantify the rates of biological degradation processes occurring at
the site.
( 3) Data from field or microcosm studies ( conducted in or with actual contaminated site
media) which directly demonstrate the occurrence of a particular natural attenuation
process at the site and its ability to degrade the contaminants of concern (typically used to
demonstrate biological degradation processes only).
The OSWER Directive provides the following guidance on interpreting the lines of evidence:
Unless EPA or the implementing state agency determines that historical
data (Number I above) are of sufficient quality and duration to support a decision
to use monitored natural attenuation, EPA expects that data characterizing the
nature and rates of natural attenuation processes at the site (Number 2 above)
should be provided. Where the latter are also inadequate or inconclusive, data
from microcosm studies (Number 3 above) may also be necessary. In general,
more supporting information may be required to demonstrate the efficacy of
monitored natural attenuation at those sites with contaminants which do not readily
degrade through biological processes ( e.g., most non-petroleum compounds,
inorganics), at sites with contaminants that transform into more toxic and/or mobile
forms than the parent contaminant, or at sites where monitoring has been performed
for a relatively short period of time. The amount and type of information needed for
such a demonstration will depend upon a number of site-specific factors, such as the
size and nature of the contamination problem, the proximity of receptors and the
potential risk to those receptors, and other physical characteristics of the
environmental setting ( e.g., hydrogeology, ground cover, or climatic conditions).
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The first line of evidence does not prove that contaminants are being destroyed. Reduction in
contaminant concentration could be the result of advection, dispersion, dilution from recharge,
sorption, and volatilization (i.e., the majority of apparent contaminant loss could be due to dilution).
However, this line of evidence is critical for determining if any exposure pathways exist for current
or potential future receptors.
In order to evaluate remediation by natural attenuation at most sites, the investigator will
have to determine whether contaminant mass is being destroyed. This is done using either, or
both, of the second or third lines of evidence. The second line of evidence relies on chemical and
physical data to show that contaminant mass is being destroyed, not just being diluted or sorbed to
the aquifer matrix. For many contaminants, biodegradation is the most important process, but for
certain contaminants nonbiological reactions are also important. The second line of evidence is
divided into two components:
• Using chemical analytical data in mass balance calculations to show that decreases in
contaminant and electron acceptor/donor concentrations can be directly correlated to
increases in metabolic end products/daughter compounds. This evidence can be used to
show that electron acceptor/donor concentrations in ground water are sufficient to facilitate
degradation of dissolved contaminants. Solute fate and transport models can be used to
aid mass balance calculations and to collate and present information on degradation.
• Using measured concentrations of contaminants and/or biologically recalcitrant tracers in
conjunction with aquifer hydrogeologic parameters such as seepage velocity and dilution
to show that a reduction in contaminant mass is occurring at the site and to calculate
biodegradation rate constants.
The biodegradation rate constants are used in conjunction with the other fate and transport
parameters to predict contaminant concentrations and to assess risk at downgradient performance
evaluation wells and within the area of the dissolved plume.
Microcosm studies may be necessary to physically demonstrate that natural attenuation is
occurring. Microcosm studies can also be used to show that indigenous biota are capable of degrading
site contaminants at a particular rate. Microcosm studies for the purpose of developing rate
constants should only be undertaken when they are the only means available to obtain biodegradation
rate estimates. There are two important categories of sites where it is difficult or impossible to
extract rate constants from concentrations of contaminants in monitoring wells in the field. In
some sites, important segments of the flow path to receptors are not acce·ssible to monitoring because
of landscape features (such as lakes or rivers) or property boundaries that preclude access to a site
for monitoring. In other sites that are influenced by tides, or the stage of major rivers, or ground
water extraction wells, the ground water plume trajectory changes so rapidly that it must be described
in a statistical manner. A "snapshot" round of sampling cannot be used to infer the plume velocity
in calculations of the rate of attenuation.
1.4 SITE CHARACTERIZATION
The OSWER Directive 9200.4-17 (1997) describes EPA requirements for adequate site
characterization.
Decisions to employ monitored natural attenuation as a remedy or remedy
component should be thoroughly and adequately supported with site-specific
characterization data and analysis. In general, the level of site characterization
necessary to support a comprehensive evaluation of natural attenuation is more
detailed than that needed to support active remediation. Site characterizations for
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natural attenuation generally warrant a quantitative understanding of source mass;
ground water flow; contaminant phase distribution and partitioning between soil,
ground water, and soil gas; rates of biological and non-biological transformation;
and an understanding of how all of these factors are likely to vary with time. This
information is generally necessary since contaminant behavior is governed by
dynamic processes which must be well understood before natural attenuation can
be appropriately applied at a site. Demonstrating the efficacy of this remediation
approach likely will require analytical or numerical simulation of complex
attenuation processes. Such analyses, which are critical to demonstrate natural
attenuation's ability to meet remedial action objectives, generally require a detailed
conceptual site model as a foundation.
A conceptual site model is a three-dimensional representation that conveys
' what is known or suspected about contamination sources, release mechanisms, and
the transport and fate of those contaminants. The conceptual model provides the
basis for assessing potential remedial technologies at the site. "Conceptual site
model" is not synonymous with "computer model;" however, a computer model
may be helpful for understanding and visualizing current site conditions or for
predictive simulations of potential future conditions. Computer models, which
simulate site processes mathematically, should in turn be based upon sound
conceptual site models to provide meaningful information. Computer models typically
require a lot of data, and the quality of the output from computer models is directly
related to the quality of the input data. Because of the complexity of natural systems,
models necessarily rely on simplifying assumptions that may or may not accurately
represent the dynamics of the natural system.
Site characterization should include collecting data to define (in three spatial
dimensions over time) the nature and distribution of contamination sources as well
as the extent of the ground water plume and its potential impacts on receptors.
However, where monitored natural attenuation will be considered as a remedial
approach, certain aspects of site characterization may require more detail or
additional elements. For example, to assess the contributions of sorption, dilution,
and dispersion to natural attenuation of contaminated ground water, a very detailed
understanding of aquifer hydraulics, recharge and discharge areas and volumes,
and chemical properties is required. Where biodegradation will be assessed,
characterization also should include evaluation of the nutrients and electron donors
and acceptors present in the ground water, the concentrations of co-metabolites
and metabolic by-products, and perhaps specific analyses to identify the microbial
populations present. The findings of these, and any other analyses pertinent to
characterizing natural attenuation processes, should be incorporated into the
conceptual model of contaminant fate and transport developed for the site.
Development of an adequate database during the iterative site characterization process is an
important step in the documentation of natural attenuation. Site characterization should provide
data on the location, nature, phase distribution, and extent of contaminant sources. Site
characterization also should provide information on the location, extent, and concentrations of
dissolved contamination; ground water geochemical data; geologic information on the type and
distribution of subsurface materials; and hydrogeologic parameters such as hydraulic conductivity,
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hydraulic gradients, and potential contaminant migration pathways to human or ecological receptor
exposure points.
The data collected during site characterization can be used to simulate the fate and transport of
contaminants in the subsurface. Such simulation allows prediction of the future extent and
concentrations of the dissolved contaminant plume. Several types of models can be used to simulate
dissolved contaminant transport and attenuation.
The natural attenuation modeling effort has five primary objectives:
• To evaluate whether MNA will be likely to attain site-specific remediation objectives in a
time period that is reasonable compared to other alternatives;
• To predict the future extent and concentration of a dissolved contaminant plume by
simulating the combined effects of contaminant loading, advection, dispersion, sorption,
and biodegradation;
• To predict the most useful locations for ground-water monitoring;
• To assess the potential for downgradient receptors to be exposed to contaminant
concentrations that exceed regulatory or risk-based levels intended to be protective of
human health and the environment; and
• To provide technical support for remedial options using MNA during screening and detailed
evaluation of remedial alternatives in a CERCLA Feasibility Study or RCRA Corrective
Measures Study.
Upon completion of the fate and transport modeling effort, model predictions can be used to
evaluate whether MNA is a viable remedial alternative for a given site. If the transport and fate
models predict that natural attenuation is sufficient to attain site-specific remediation objectives
and will be protective of human health and the environment, natural attenuation may be an
appropriate remedy for the site. Model assumptions and results should be verified by data obtained
from site characterization. If model assumptions and results are not verified by site data, MNA is
not.]ikely to be a viable option and should not be proposed as the remedy.
1.5 MONITORING
The Monitoring Program OSWER Directive on Monitored Natural Attenuation (9200.4-17)
describes EPA expectations for performance monitoring.
Perfonnance monitoring to evaluate remedy effectiveness and to ensure
protection of human health and the environment is a critical element of all response
actions. Perfonnance monitoring is of even greater importance for monitored natural
attenuation than for other types of remedies due to the longer remediation time
frames, potential for ongoing contaminant migration, and other uncertainties
associated with using monitored natural attenuation. This emphasis is underscored
by EPA 's reference to "monitored natural attenuation".
The monitoring program developed for each site should specify the location,
frequency, and type of samples and measurements necessary to evaluate remedy
performance as well as define the anticipated performance objectives of the remedy.
In addition, all monitoring programs should be designed to accomplish the following:
• Demonstrate that natural attenuation is occurring according to expectations;
Identify any potentially toxic transformation products resulting from
biodegradation;
• Detennine if a plume is expanding ( eitherdowngradient, laterally or vertically);
• Ensure no impact to downgradient receptors;
• Detect new releases of contaminants to the environment that could impact the
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effectiveness of the natural attenuation remedy;
• Demonstrate the efficacy of institutional controls that were put in place to
protect potential receptors;
• Detect changes in environmental conditions ( e.g., hydro geologic, geochemical,
microbiological, or other changes) that may reduce the efficacy of any of the
natural attenuation processes; and
• Verify attainment of cleanup objectives.
Detection of changes will depend on the proper siting and construction of
monitoring wells/points. Although the siting of monitoring wells is a concern for
any remediation technology, it is of even greater concern with monitored natural
attenuation because of the lack of engineering controls to control contaminant
migration.
Performance monitoring should continue_ as long as contamination
remains above required cleanup levels. Typically, monitoring is continued for a
specified period ( e.g., one to three years) after cleanup levels have.been achieved to
ensure that concentration levels are stable and remain below target levels. The
institutional and financial mechanisms for maintaining the monitoring program
should be clearly established in the remedy decision or other site documents, as
appropriate.
Natural attenuation is achieved when naturally occurring attenuation mechanisms, such as
biodegradation, bring about a reduction in the total mass, toxicity, mobility, volume, or concentration
of a contaminant dissolved in ground water. In some cases, natural attenuation processes will be
capable of attaining site-specific remediation objectives in a time period that is reasonable compared
to other alternatives. However, at this time, the authors are not aware of any sites where natural
attenuation alone has succeeded in restoring ground water contaminated with chlorinated aliphatic
hydrocarbons to drinking water quality over the entire plume.
The material presented here was prepared through the joint effort between the Bioremediation
Research Team at the Subsurface Protection and Remediation Division of U.S. EPA's National
Risk Management Research Laboratory (NRMRL) in Ada, Oklahoma, and the U.S. Air Force
Center for Environmental Excellence, Technology Transfer Division, Brooks Air Force Base,
Texas, and Parsons Engineering Science, Inc. (Parsons ES). It is designed to facilitate proper
evaluation of remedial alternatives including natural attenuation at large chlorinated aliphatic
hydrocarbon-contaminated sites.
This information is the most current available at the time of this writing. The scientific
knowledge and experience with natural attenuation of chlorinated solvents is growing rapidly and
the authors expect that the process for evaluating natural attenuation of chlorinated solvents will
continue to evolve.
This document contains three sections, including this introduction. Section 2 presents the
protocol to be used to obtain scientific data to evaluate the natural attenuation option. Section 3
presents the references used in preparing this document. Appendix A describes the collection of
site characterization data necessary to evaluate natural attenuation, and provides soil and ground-
water sampling procedures and analytical protocols. Appendix B provides an in-depth discussion
of the destructive and nondestructive mechanisms of natural attenuation. Appendix C covers data
interpretation and pre-modeling calculations.
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SECTION 2
PROTOCOL FOR EVALUATING NATURAL ATTENUATION
The primary objective of the natural attenuation investigation is to determine whether natural
processes will be capable of attaining site-specific remediation objectives in a time period that is
reasonable compared to other alternatives. Further, natural attenuation should be evaluated to
determine if it can meet all appropriate Federal and State remediation objectives for a given site.
This requires that projections of the potential extent of the contaminant plume in time and space be
made. These projections should be based on historic variations in contaminant concentration, and_
the current extent and concentrations of contaminants in the plume in conjunction with measured
rates of contaminant attenuation. Because of the inherent uncertainty associated with such predictions,
it is the responsibility of the proponent of monitored natural attenuation to provide sufficient evidence
to demonstrate that the mechanisms of natural attenuation will meet the remediation objectives
appropriate for the site. This can be facilitated by using conservative parameters in solute fate and
transport models and numerous sensitivity analyses in order to better evaluate plausible contaminant
migration scenarios. When possible, both historical data and modeling should be used to provide
information that collectively and consistently confirms the natural reduction and removal of the
dissolved contaminant plume.
Figure 2.1 outlines the steps involved in a natural attenuation demonstration and shows the
important regulatory decision points for implementing natural attenuation. For example, a Superfund
Feasibility Study is a two-step process that involves initial screening of potential remedial alternatives
followed by more detailed evaluation of alternatives that pass the screening step. A similar process
is followed in a RCRA Corrective Measures Study and for sites regulated by State remediation
programs. The key steps for evaluating natural attenuation are outlined in Figure 2.1 and include:
I) Review available site data and develop a preliminary conceptual model. Determine if
receptor pathways have already been completed. Respond as appropriate.
2) If sufficient existing data of appropriate quality exist, apply the screening process de-
scribed in Section 2.2 to assess the potential for natural attenuation.
3) If preliminary site data suggest natural attenuation is potentially appropriate, perform
additional site characterization to further evaluate natural attenuation. If all the recom-
mended screening parameters listed in Section 2.2 have been collected and the screening
processes suggest that natural attenuation is not appropriate based on the potential for
natural attenuation, evaluate whether other processes can meet the cleanup objectives for
the site (e.g., abiotic degradation or transformation, volatilization, or sorption) or select a
remedial option other than MNA.
4) Refine conceptual model based on site characterization data, complete pre-modeling
calculations, and document indicators of natural attenuation.
5) Simulate, if necessary, natural attenuation using analytical or numerical solute fate and
transport models that allow incorporation of a biodegradation term.
6) Identify potential receptors and exposure points and conduct an exposure pathways analy-
sts.
7) Evaluate the need for supplemental source control measures. Additional source control
may allow MNA to be a viable remedial option or decrease the time needed for natural
processes to attain remedial objectives.
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Review Available Sile Data
If Site Data are Adequate
Develop Preliminary Conceptual Model
Screen the Site using the Procedure
Presented in Figure 2.3
NO
Perform Site Characterization
to Evaluate Natural Attenuation
Refine Conceptual Model and
Complete Pre-Modeling
Calculations
Simulate Natural Attenuation
Using Solute Fate and
Transport Models
Verify Model Assumptions and
Results with Site
Characterization Data
Use Results of Modeling and
Site-Specific Information in an
Exposure Pathways Analysis
NO
Gather any Additional Data
Necessary to Complete
the Screening of the Site
NO
~.:Y,:;E;:;S:.._ __ ..,:~ Engineered Remediation Required,
Implement Other Protocols
Evaluate Use of
Selected Additional
Perform Site Characterization
to Support Remedy Decision Making
Remedial Options IE-----,
Including Source
Removal or Source .-----;►I Control Along with
Natural Attenuation
Vacuum Reactive Dewateting Barrier
NO
Enhanced
Bioremediation
Develop Draft Plan for
Performance Evaluation
Monitoring Wells and Lon · ·
Present Findings
and Proposed
Remedy in
Feasibility Study
NO
Assess Potential For
Natural Attenuation
With Remediation
System Installed
Refine Conceptual Model and
Complete Pre-Modeling
Calculations
Simulate Natural Attenuation
Combined with Remedial
Option Selected Above
Using Solute Transport Models
Verify Model Assumptions and
Results with Site
Characterization Data
Use Results of Modeling and
Site-Specific Information in
an Exposure Assessment
Figure 2.1 Natural attenuation of chlorinated solvents flow chart.
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8) Prepare a long-term monitoring and verification plan for the selected alternative. In some
cases, this includes monitored natural attenuation alone, or in other cases in concert with
supplemental remediation systems.
9) Present findings of natural attenuation studies in an appropriate remedy selection docu-
ment, such as a CERCLA Feasibility or RCRA Corrective Measures Study. The appropri-
ate regulatory agencies should be consulted early in the remedy selection process to clarify
the remedial objectives that are appropriate for the site and any other requirements that the
remedy will be expected to meet. However, it should be noted that remedy requirements
are not finalized until a decision is signed, such as a CERCLA Record of Decision or a
RCRA Statement of Basis.
The following sections describe each of these steps in more detail..
2.1 REVIEW AVAILABLE SITE DA TA AND DEVELOP PRELIMINARY
CONCEPTUAL MODEL
The first step in the_ natural attenuation investigation is to review available site-specific data.
Once this is done, it is possible to use the initial site screening processes presented in Section 2.2 to
determine if natural attenuation is a viable remedial option. A thorough review of these data also
allows development of a preliminary conceptual model. The preliminary conceptual model will
help identify any shortcomings in the data and will facilitate placement of additional data collection
points in the most scientifically advantageous and cost-effective manner possible.
The following site information should be obtained during the review of available data.
Information that is not available for this initial review should be collected during subsequent site
investigations when refining the site conceptual model, as described in Section 2.3.
• Nature, extent, and magnitude of contamination:
Nature and history of the contaminant release:
--Catastrophic or gradual release of NAPL ?
--More than one source area possible or present?
--Divergent or coalescing plumes?
Three-dimensional distribution of dissolved contaminants and mobile and residual
NAPLs. Often high concentrations of chlorinated solvents in ground water are the result
of landfill leachates, rinse waters, or ruptures of water conveyance pipes. For LNAPLs
the distribution of mobile and residual NAPL will be used to define the dissolved plume
source area. For DNAPLs the distribution of the dissolved plume concentrations, in addition
to any DNAPL will be used to define the plume source area.
Ground water and soil chemical data.
Historical water quality data showing variations in contaminant concentrations both
vertically and horizontally.
Chemical and physical characteristics of the contaminants.
Potential for biodegradation of the contaminants.
Potential for natural attenuation to increase toxity and/or mobility of natural occurring
metals.
• Geologic and hydrogeologic data in three dimensions (If these data are not available, they
should be collected for the natural attenuation demonstration and for any other remedial
investigation or feasibility study):
Lithology and stratigraphic relationships.
Grain-size distribution (gravels vs. sand vs. silt vs. clay).
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Aquifer hydraulic conductivity (vertical and horizontal, effectiveness of aquitards,
calculation of vertical gradients).
Ground-water flow gradients and potentiometric or water table surface maps ( over
several seasons, if possible).
Preferential flow paths.
Interactions between ground water and surface water and rates of infiltration/recharge.
• Locations of potential receptor exposure points:
Ground water production and supply wells, and areas that can be deemed a potential source
of drinking water.
Downgradient and crossgradient discharge points including any discharges to surface waters
or other ecosystems.
Vapor discharge to basements and other confined spaces.
In some cases, site-specific data are limited. If this is the case, all future site characterization
activities should include collecting the data necessary to screen the site for the use.of monitored
natural attenuation as a potential site remedy. Much of the data required to evaluate natural attenuation
can be used to design and evaluate other remedial measures.
Available site characterization data should be used to develop a conceptual model for the site.
This conceptual model is a three-dimensional representation of the source area as a NAPL or·
region of highly contaminated ground water, of the surrounding uncontaminated area, of ground
water flow properties, and of the solute transport system based on available geological, biological,
geochemical, hydrological, climatological, and analytical data for the site. Data on the contaminant
levels and aquifer characteristics should be obtained from wells and boreholes which will provide
a clear three-dimensional picture of the hydrologic and geochemical characteristics of the site.
High concentrations of dissolved. contaminants can be the result of leachates, rinse waters and
rupture of water conveyance lines, and are not necessarily associated with NAPLs.
-This type of conceptual model differs from the conceptual site models commonly used by risk
assessors that qualitatively consider the location of contaminant sources, release mechanisms,
transport pathways, exposure points, and receptors. However, the conceptual model of the ground
water system facilitates identification of these risk-assessment elements for the exposure pathways
analysis. After development, the conceptual model can be used to help determine optimal placement
of additional data collection points, as necessary, to aid in the natural attenuation investigation and
to develop the solute fate and transport model. Contracting and management-controls must be
flexible enough to allow for the potential for revisions to the conceptual model and thus the data
collection effort.
Successful conceptual model development involves:
• Definition of the problem to be solved (generally the three dimensional nature, magnitude,
and extent of existing and future contamination).
• Identification of the core or cores of the plume in three dimensions. The core or cores contain
the highest concentration of contaminants.
• Integration and presentation of available data, including:
-Local geologic and topographic maps,
-Geologic data,
-Hydraulic data,
-Biological data,
-Geochemical data, and
-Contaminant concentration and distribution data.
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• Determination of additional data requirements, including:
-Vertical profiling locations, boring locations and monitoring well spacing in three dimensions,
- A sampling and analysis plan (SAP), and
-Any data requirements listed in Section 2.1 that have not been adequately addressed.
Table 2.1 contains the recommended soil and ground water analytical methods for evaluating
the potential for natural attenuation of chlorinated aliphatic hydrocarbons and/or fuel hydrocarbons.
Any plan to collect additional ground water and soil quality data should include the analytes listed
in this table. Table 2.2 lists the availability of these analyses and the recommended data quality
requirements. Since required procedures for field sampling, analytical methods and data quality
objectives vary somewhat among regulatory programs, the methods to be used at a particular site
should be developed in collaboration with the appropriate regulatory agencies. There are many
documents which may aid in developing data quality objectives (e.g.,U.S. EPA Order 5360.1 and
U.S. EPA QA/G-4 Guidance for the Data Quality Objectives Process).
2.2 INITIAL SITE SCREENING
After reviewing available site data and developing a preliminary conceptual model, an
assessment of the potential for natural attenuation must be made. As stated previously, existing
data can be useful to determine if natural attenuation is capable of attaining site-specific remediation
objectives in a time period that is reasonable compared to other alternatives. This is achieved by
first determining whether the plume is currently stable or migrating and the future extent of the
plume based on (I) contaminant properties, including volatility, sorptive properties, and
biodegradability; (2) aquifer properties, including hydraulic gradient, hydraulic conductivity, porosity
and concentrations of native organic material in the sediment (TOC), and (3) the location of the
plume and contaminant source relative to potential receptor exposure points (i.e., the distance between
the leading edge of the plume and the potential receptor exposure points). These parameters
(estimated or actual) are used in this section to make a preliminary assessment of the effectiveness
of natural attenuation in reducing contaminant concentrations.
If, after completing the steps outlined in this section, it appears that natural attenuation will be
a significant factor in contaminant removal and a viable remedial alternative, detailed site
characterization activities that will allow evaluation of this remedial option should be performed.
If exposure pathways have already been completed and contaminant concentrations exceed protective
levels, or if such completion is likely, an engineered remedy is needed to prevent such exposures
and should be implemented as an early action. For this case, MNA may still be appropriate to attain
long-term remediation objectives for the site. Even so, the collection of data to evaluate natural
attenuation can be integrated into a comprehensive remedial strategy and may help reduce the cost
and duration of engineered remedial measures such as intensive source removal operations or pump-
and-treat technologies.
2.2.1 Overview of Chlorinated Aliphatic Hydrocarbon Biodegradation
Because biodegradation is usually the most important destructive process acting to reduce
contaminant concentrations in ground water, an accurate estimate of the potential for natural
biodegradation is important to consider when determining whether ground water contamination
presents a substantial threat to human health and the environment. This information also will be
useful when selecting the remedial alternative that will be most cost effective at eliminating or
abating these threats should natural attenuation alone not prove to be sufficient.
15
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Table 2.1 Soil, Soil Gas, and Ground-water Analytical Methods to Evaluate the Potential for Natural Attenuation of Chlorinated Solvents o r Fuel Hydrocarbons in Ground Water. Analyses other than those listed in this table may be required for regulatory compliance.
Recommended Sample Volume, Sample Field or
Frequency of Container, Sample •Fixed~Base Matrix Analvsis Method/Reference Comments Data Use Analysis Preservation Laboratory Soil Aromatic and SW8260A Data are used to Each soil sampling Sample volume Fixed.base Chlorinated determine the extent of round approximately JOO m1; hydrocarbons soil contamination, the subsample and extract in (benzene, contamination mass the field using methanol toluene, present, and the or appropriate solvent; ethylbenzene, and potential for source cool to 4°C. xykne [BTEX]; removal.
Chlorinated
Compounds
Soil Biologically Under development HCI extraction Optional method that One round of Minimum I inch Laboratory Available Iron followed by should be used when sampling in five diameter core samples (Ill) quantification of fuel hydrocarbons or borings, five cores collected into plastic released iron (III) vinyl chloride are from each boring liner. Cap and prevent present in the ground aeration. water to predict the
possible extent of
removal of fuel
hydrocarbons and
vinyl chloride via iron
reduction.
Soil Total organic SW9060 modified for Procedure must The rate of migration At initial sampling Collect 100 g of soil in a Fixed•base carbon (TOC) soil samples be accurate over of petroleum glass container with the range of 0.1 contaminants in Teflon•lined cap; cool to to 5 percent TQC ground water is 4°c.
dependent upon the
amount of TOC in the
aouifer matrix. Soil Gas Fuel and EPA Method T0-14 Useful for determining At initial sampling I ·liter Summa Canister Fixed•base Chlorinated chlorinated and BTEX voe, comnounds in soil Soil Gas Methane, Field Soil Gas Useful for determining At initial sampling 3·liters in a Tedlar bag, Field Oxygen, Carbon Analyzer bioactivity in vadose and respiration bags are reusable for dioxide zone. testing analysis of methane,
oxygen, or carbon
dioxide.
.. !!!!I == liliiil liii iiii ----- - -
liiiil liiliil
Table 2.1 (Continued)
---
Recommended Sample Volume, Field or
Frequency or Sample Container, Fixed-Dase
Matrix Analvsls Method/Reference Comments Data Use Analvsis Sanmle Preservation Laboratorv
Water Alkalinity Hach Alkalinity te1::t kit Phenolphthalein General water quality Each sampling Collect 100 mL of Field
model AL AP MG-L method parameter used (l) &5 a round water in glass container.
marker to verify that all site
samples arc obtained from
lhe same ground-water
system and (2) to measure
the buffering capacity of
e:round water.
Water Aromatic and SW8260A Analysis may be Method of analysis for Each sampling Collect water samples Fixed-base
chlorinated extended to higher BTEX and chlorinated round in a 40 mL VOA vial;
hydrocarbons molecular weight 1.olvents/byproduct-., which cool to 4°C; add
(BTEX, alkyl benzenes are the primary target hydrochloric acid 10
trimethylbenzene analytes for monitoring pH 2.
isomers, narural attenuation; method
chlorinated can be extended to higher
compounds) molecular weight alkyl -__, benzenes; trimethylben-
zenes are used to monitor
plume dilution if
degradation is primarily
anaerobic.
Water Arsenic EPA 200.7 or EPA -To detennine if anaerobic One round of Collect l 00 ml in a Laboratory
200.9 biological activity is sampling glass or pla-.tic
solubilizing arsenic from conlainer that is rinsed
the aquifer matrix material. in the field with the
ground water to be
sampled. Unfiltered
s~~ples obtained using
low flow sampling
methods are preferred
for analysis of dissolved
metals. Adjust pH to 2
with nitric acid. Do not
insert pH paper or an
electrode into the
samole.
Water Chloride Hach Chloride test kit Silver nitrate As above, and to guide Each sampling Collect 100 mL of Field
(optional, see model 8-P tiuation selection of additional data round water in a glass
dala use) point~ in real time while in container.
the field.
!!!!
-00
1!!119
Table 2.1 (Continued)
Matrix Analvsis
Water Chloride
Water Chloride
(optional, see
data use)
Water Conductivity
Water Iron (IIJ (fe+2)
Water Hydrogen (H,)
Waler Manganese
iiiil iiii
r-..tcthod/Rcrerence Comments
Mercuric niUale Ion chromatography
titration A4500-ct· C (IC) method EJ00
or met.hod S W9050
may aJso be used
Hach Chloride te,c;t k.it Silver nitrate
model 8-P titration
El20.l/SW9050, direct
reading meter
Colorimetric Filter if turbid.
Hach Method # 8146
Equilibration with gac; Optional
in lhe field. specialized analysis
Determined with a
reducing gas deteclor.
EPA 200.7 or EPA
200.9
--- ---- -
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Recommended Sample Volume, Field or
Frequency of Sample Container, Fixed-Dase
Data Use Analvsis Sample Preservation Laboratorv
General water quality Each sampling Collect 250 mL of Fixed-base
parameter u.~ed as a marker round water in a glass
lo verify that si1e samples container.
arc obtained from the same
ground-water system. Final
product of chlorinated
solvent reduction.
As abo\·e, and to guide Each sampling Collect I 00 mL of Field
selection of additional data round wa1cr in a glass
points in real time while in container.
1he field.
General water quality Each sampling Collect 100 lo 250 ml. Field
parameter used as a marker round or water in a glass or
lo verify that site samples pl.astic container.
are obtained from the same
_ground-water .system.
May indicate an anaerobic Each sampling Collect from a flow-Field
degradation process due to round through or over-flow
depletion of oxygen, cell / analyze at the well
nitrate, and manganese. head.
Determined terminal One round of Sampled at well head Field
electron accepting process. sampling on requires the production
Predicts the possiblity for selected wells. of 300 mL per minute
reductive dechlorination. of water for 30 minutes.
To determine if anaerobic One round of Collect 100 ml in a Laboratory
biological activity is sampling glass or plastic
solubilizing manganese container that is rinsed
from the aquifer matrix in the field with I.he
material. ground water to be
sampled. Unfiltered
samples obtained using
low flow sampling
met hods are preferred
for analysis of dissolved
metals. Adjust pH to 2
with nilric acid. Do not
insert pH paper or an
electrode into the
sample.
--.. !!!!!I !!!!I a=; liiiiii lilii iiii - ---- --liiil liiiil
Table 2.1 (Continued)
..
P, Recommended Sampl~ Volume~ Fle!d Oi"
Frequency or Sample Containff, 1Fh!ecl-Base
Matrix Anal\•sis Method/Reference Comments Data Use Annlvsls Ssmni.e W'resensi.lon Laborato...,
Water Methane, ethane, Kampbell el al., 1989 Method published The presence of CH.i Each sampling Collect water samples Fixed-base
and ethene and 1998 01 SW38IO by researcher.; at the suggest,; BlEX degrndation round in 50 mL gla5..,; serum
Modified U.S. Environmental via methanogenesis. bottles with gray butyl
Prolcction Agency. Ethane and ethene dnta are /Teflon-faced septa and
Limited to few used where chlorinated crimp caps; add HiSO4
commerciaJ labs. solvents are suspected of to pH less than 2, cool
undergoing biologicaJ to 4°C.
transformation.
Water Nitrate IC method EJCXJ Substrate for microbial Each sampling Collect up to 40 mL of Fixed-base
respiration if oxygen is round water in a glass or
depleted. plac:;tic container; add
Hi-504 to pH less than
2, cool to 4°C.
Water Oxidation-A2580B Measurements made The ORP of ground water Each sampling Measure in a flow Field
reduction with eleclrodes; influences and is influenced round through cell or an over-
potential results are displaye.d hy the nature of the flowing container fille.d
on a meter; protect biologically mediated from the botlom to
samples from degradation of prevent exposure of the
exposure to oxygen. contaminantc:;; the ORP ground water to the
Report result<; (expressed a<; Eh) of atmosphere.
against a ground waler may range
silver/silver chloride from more than 800 m V to
reference electrode. less than -400 mV.
(Eh) is calculated by
adding a correction
factor sped fie to the
electrode used.
Water Oxygen Dissolved oxygen meter Refer to The oxygen concentni.tion Each sampling Measure dissolved Field
calibrated between each method A4500 is a data input to the round oxygen on site using a
well according to the for a comparable Bioplume model; flow-through cell or
supplier's specifications laboratory concentrations le.c:;s than over-flow cell.
procedure. I mg/L generally indicate
an anaerobic oathwav.
Water pH Field probe with direct Field Aerobic and anaerobic Each sampling Measure dissolved Field
reading meter calibrated biological processe.c:; are round oxygen on site using a
in the field according to pH-sensitive. flow-through cell or
the supplier's over-flow cell.
soecifications.
--
"' 0
l!!!l!!!!I == llii liiiil liiiil -- - - -
Table 2.1 (Continued)
Recommended Sample Volume,
Frequency of , Sample Container,
Malm Analvsis Method/Reference Comments Data Use Analvsls Samnle Preservation
Water Sulfate (SO4·2) IC method 8300 If this method is Suhstrate for anaerobic Each sampling Collect up to 40 mL of
used for sulfate microbial respiration. round water in a glass or
analysis, do not U!>e pla-.tic container; cool
the field method. to 4"C.
Water Sulfate (SO,42) Hach method # 8051 Colorimetric, if this Same as above. Each sampling Collect up to 40 mL of
method is used for round water in a glass or
sulfate anaJysis, do pJa.,;tic container; cool
not me the fixed-to 4"C.
ba~ laboratory
method.
Water Temperature Field probe with direct field only To detemti ne if a well is Each sampling Read from oxygen
reading meter. adequately purged for round meter.
sampling.
Water Total Organic SW9060 Laboratory Used to clac;sify plume and Each sampling Measure using a flow-
Catbon also to determine if reductive round through cell or over-
called DOC dechlorination is possible flow cell.
in the absence of
anthrooo2enic carbon.
NOTES:
I. "Hach" refers to tl1e Hach Company catalog, 1990.
2. "A" refers to Standard Methods/or the Examination of Water and Wmtewater, 18th edition, 1992.
3. "E" refers to Metlwdf for Chemical Analysis of Water and Wastes, U.S. EPA, 1983.
4. "SW" refers to the Test Methods for Evaluating Solid Waste, Physical, and Chemical Methods, SW-846, U.S. EPA, 3rd edition, 1986.
-lilil
Field or
Flxed•Base
Laborato.-u
Pixed-base
Field
Field
Laboratory
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Table 2.2 Objectives for Sensitivity and Precision to Implement the Natural Attenuation Protocol. Analyses other than those listed in this table may be
required for regulatory compliance.
Matrix Analysis Method/Reference Minimum Limit of Precision A vallahlllty Potential Data Quality
Ouantificalion Problems
Soil Aromatic and SW8260A I mg/Kg Coefficient of Varialion of Common laboratory Volatile.~ Jost during shipment
chlorinated 20 percent. ana1ysis. lo laboratory; prefer extraction
hydrocarbons in the field.
(benzene,
toluene,
ethylhenz.ene, and
xylene [BTEXJ;
chlorinated
co,.,.,_""ounds \
Soil Biologically Under development 50 mg/Kg Coefficient of Variation of Specialized laboratory Sample must not be allowed
Available Iron 40 percent. anaJysis. to oxidize.
nm
Soil Total organic SW9060 modified for 0.1 percent Coefficient of Variation of Common laboratory Samples must be collecled
carbon (TOC) soil samples 20 percent analysis. from contaminant-
transporting (i.e.,
transmissiv~) intervals.
Soil G.is Fuel and EPA Method T0-14 1 ppm Coefficient of Variation of Common laboratory Potential for atmospheric
Chlorinated (volume/volume) 20 percent. analysis. dilution during sampling.
voes
Soil Gas Methane, Oi, CO2 Fit?ld Soil Gae; Analyzer l percent Coefficient of Variation of Readily available field Instrument must be properly
(volume/volume) 20 nercent. instrument. calibrated.
Water Alkalinity Hach alkalinity test kit 50 mg/L Standard deviation of 20 Com_mon field analysis. Analyze sample within I hour
model AL AP MG-L me/L of collection.
Water Aromatic and SW8260A MCLs Coefficient of Variation of Common laboratory Volatilization during shipment
chlorinated IO percent. analysis. and biodegradation due to
·hydrocarbons improper preservation.
(BTEX.
trimethylbenzene
isomers,
chlorinated
comnounds'
Water Otloride IC method E300 I mg/L Coefficient of Variation of Conimon laboratory ----
20 oercenl. analvsis.
Water Chloride Hach Chloride test kit 1 mg/L Coefficient of Variation of Common field analysis. Possible interference from
(optional, see model 8-P 20 percent. turbidity.
data use)
Water Conductivity El20.I/SW9050. direct 50 µS/cm2 Standard deviation of 50 Common field probe. Improperly calibrated
reading meter uS/cm2• instrument.
I!!!!! 1111m == liiiiil lliiil -- -- ----liiil liiiil
Table 2.2 (Continued)
Matrix Analysis l\ I eU1ocVRef erence l\lininmm Limit of Precision Availability Potential Data Quality
Quantification Problems
Water Hydrogen (H,)" See Appendix A 0.lnM S1andani deviation of Specialized field Numerous, sec Appendix A.
0.lnM. analysis.
Water Iron (II) (Fe2♦) Colorimetric 0.5 mg/L Coefficient of Variation of Common field analysis. Possible interference from xx Hach Mel.hod # 8146 20 percent. turbidity (must filter if turbid).
Keep out of sunlight and
analyze wilhin minutes of
collection.
Water Major Cations SW6010 I mg/L Coefficient of Variation of Common Laboratory Possible colloidal
20 percent. analysis. interferences.
Water Methane, elhane, Kampbell el al., 1989 or I µg/L Coefficient of Variation of Specialized laboratory Sample must be preserved
and ethene SW3810 Modified 20 percent. analysis. against biodegradalion and
collected without headspace
(to minimize volatilization).
Water Nitrate IC met.hod EJOO 0.1 mg/L StandanJ deviation of 0.1 Common laboratory Must be preserved.
mg/L analysis.
Water O.xidation-A2580B plus or minus plus or minus 50 mV. Common field probe. Improperly calibrated
reduction J00mV electrodes or introduction of
potential (ORP) atmospheric oxygen during
sampling.
Water Oxygen Dissolved o.xygen meter 0.2 mg!L StandanJ deviation of 0.2 Common field Improperly calibrated
mg/L instrument. electrodes or bubbles behind
the membrane or a fouled
membrane or introduction of
atmospheric oxygen during
sampling.
Water Sulfate (SOl°) IC method EJOO 5 mg/L Coefficient of Variation of Common laboratory. Fixed-base.
20 percent.
Water Sulfate (SO/") Hach melhod # 8051 5 mg/L Coefficient of Variation of Common field analysis. Possible interference from
xx 20 percent. turbidity (must filter if turbid).
Keep sample cool.
Water pH Field probe with direct 0.1 standard units 0.1 standani units. Common field meter. Improperly calibrated
reading meter. instrument; time sensitive.
Water Temperature Field probe with direct 0 degrees Celsius Standard deviation of l Common field probe. Improperly calibrated
reading meter. degrees Celsius. instrument; lime sensitive.
Water Total Organic SW9060 0.1 mg/L Coefficient of Variation of Common laboratory
Carbon 20 percent. analysis.
Notes:
** Filter if turbidity gives a response from the photometer before additio11 of the reagents that is as large or larger than the specified minimum qua11tificatio11 limit.
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Over the past two decades, numerous laboratory and field studies have demonstrated that
subsurface microorganisms can degrade a variety of chlorinated solvents (e.g., Bouwer et al., 1981;
Miller and Guengerich, 1982; Wilson and Wtlson, 1985; Nelson et al., 1986; Bouwer and Wright,
1988; Lee, 1988; Little et al., 1988; Mayer et al., 1988; Arciero et al., 1989; Cline and Delfino,
1989; Freedman and Gossett, 1989; Folsom et al., 1990; Harker and Kim, 1990; Alvarez-Cohen
and McCarty, 1991a, 1991b; DeStefano et al., 1991; Henry, 1991; McCarty et al., 1992; Hartmans
and de Bent, 1992; McCarty and Semprini, 1994; Vogel, 1994). Whereas fuel hydrocarbons are
biodegraded through use as a primary substrate (electron donor), chlorinated aliphatic hydrocarbons
. may undergo biodegradation under three different circumstances: intentional use as an electron
acceptor; intentional use as an electron donor; or, through cometabolism where degradation of the
chloririated organic is fortuitous and there is no benefit to the microorganism. At a given site, one
or all of these circumstances may pertain, although at many sites the use of chlorinated aliphatic
hydrocarbons as electron acceptors appears to be most important under natural conditions. 1n this
case, biodegradation of chlorinated aliphatic hydrocarbons will be an electron-donor-limited process.
Conversely, biodegradation of fuel hydrocarbons is an electron-acceptor-limited process.
In an uncontaminated aquifer, native organic carbon is used as an electron donor, and dissolved
oxygen (DO) is used first as the prime electron acceptor. Where anthropogenic carbon (e.g., as fuel
hydrocarbons) is present, it also will be used as an electron donor. After the DO is consumed,
anaerobic microorganisms typically use additional electron acceptors (as available) in the following
order of preference: nitrate, ferric iron oxyhydroxide, sulfate, and finally carbon dioxide. Evaluation
of the distribution of these electron acceptors can provide evidence of where and how chlorinated
aliphatic hydrocarbon biodegradation is occurring. In addition, because chlorinated aliphatic
hydrocarbons may be used as electron acceptors or electron donors (in competition with other
acceptors or donors), isopleth maps showing the distribution of these compounds and their daughter
products can provide evidence of the mechanisms of biodegradation working at a site. As with
BTEX, the driving force behind oxidation-reduction reactions resulting in chlorinated aliphatic
hydrocarbon degradation is electron transfer. Although thermodynamically favorable, most of the
reactions involved in chlorinated aliphatic hydrocarbon reduction and oxidation do not proceed
abiotically. Microorganisms are capable of carrying out the reactions, but they will facilitate only
those oxidation-reduction reactions that have a net yield of ener!)Y-
2.2.1.1 Mechanisms of Chlorinated Aliphatic Hydrocarbon Biodegradation
The following sections describe the biodegradation of those compounds that are most prevalent
and whose behavior is best understood.
2.2.1.1.1 Electron Acceptor Reactions (Reductive Dehalogenation)
The most important process for the natural biodegradation of the more highly chlorinated
solvents is reductive dechlorination. During this process, the chlorinated hydrocarbon is used as an
electron acceptor, not as a source of carbon, and a chlorine atom is removed and replaced with a
hydrogen atom. Figure 2.2 illustrates the transformation of chlorinated ethenes via reductive
dechlorination. In general, reductive dechlorination occurs by sequential dechlorination from PCE
to TCE to DCE to VC to ethene. Depending upon environmental conditions, this sequence may be
interrupted, with other processes then acting upon the products. During reductive dechlorination,
all three isomers of DCE can theoretically be produced. However, Bouwer (1994) reports that
under the influence ofbiodegradation, cis-1,2-DCE is a more common intermediate than trans-l,2-
DCE, and that 1, 1-DCE is the least prevalent of the three DCE isomers when they are present as
daughter products. Reductive dechlorination of chlorinated solvent compounds is associated with
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PCE
TCE
rP
1, 1 -DCE cis -1,2, -DCE
Vinyl Chloride
Ethene
Ethane
H H
@ Chlorine Atom
© Carbon Atom
@ Hydrogen Atom
Single Chemical
Bond
Double Chemical
Bond
trans-1, 2-DCE
rP
~
Complete Mineralization V~@
Figure 2.2 Reductive dehalogenation of chlorinated ethenes.
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the accumulation of daughter products and an increase in the concentration of chloride ions. Reductive
dechlorination affects each of the chlorinated ethenes differently. Of these compounds, PCE is the
most susceptible to reductive dechlorination because it is the most oxidized. Conversely, VC is the
least susceptible to reductive dechlorination because it is the least oxidized of these compounds.
As a result, the rate of reductive dechlorination decreases as the degree of chlorination decreases
(Vogel and McCarty, 1985; Bouwer, 1994). Murray and Richardson (1993) have postulated that
this rate decrease may explain the accumulation ofVC in PCE and TCE plumes that are undergoing·
reductive dechlorination. Reductive dechlorination has been demonstrated under nitrate-and iron-
reducing conditions, but the most rapid biodegradation rates, affecting the widest range of chlorinated
aliphatic hydrocarbons, occurunder sulfate-reducing and methanogenic conditions (Bouwer, 1994).
Because chlorinated aliphatic hydrocarbon compounds are used as electron acceptors during reductive
dechlorination, there must be an appropriate source of carbon for microbial growth in order for this
process to occur (Bouwer, 1994). Potential carbon sources include natural organic matter, fuel
hydrocarbons, or other anthropogenic organic compounds such as those found in landfill leachate.
2.2.J.l.2 Electron Donor Reactions
Murray and Richardson ( 1993) write that microorganisms are generally believed to be incapable
of growth using PCE and TCE as a primary substrate (i.e., electron donor). However, under aerobic
and some anaerobic conditions, the less oxidized chlorinated aliphatic hydrocarbons (e.g., VC) can
be used as the primary substrate in biologically mediated oxidation-reduction reactions (McCarty
and Semprini, 1994). In this type of reaction, the facilitating microorganism obtains energy and
organic carbon from the degraded chlorinated aliphatic hydrocarbon. In contrast to reactions in
which the chlorinated aliphatic hydrocarbon is used as an electron acceptor, only the least oxidized
chlorinated aliphatic hydrocarbons can be used as electron donors in biologically mediated oxidation-
reduction reactions. McCarty and Semprini (1994) describe investigations in which VC and 1,2-
dichloroethane (DCA) were shown to serve as primary substrates under aerobic conditions. These
authors also document that dichloromethane has the potential to function as a primary substrate
under either aerobic or anaerobic environments. In addition, Bradley and Chapelle (I 996) show
evidence of mineralization of VC under iron-reducing conditions so long as there is sufficient
bioavailable iron (III). Aerobic metabolism ofVC may be characterized by a loss ofVC mass and
a decreasing molar ratio ofVC to other chlorinated aliphatic hydrocarbon compounds. In addition,
Klier et al. ( 1998) and Bradley and Chapelle (I 997) show mineralization of DCE to carbon dioxide
under aerobic, Fe(III) reducing, and methanogenic conditions, respectively.
2.2.J.J.3 Cometabolism
When a chlorinated aliphatic hydrocarbon is biodegraded via cometabolism, the degradation
is catalyzed by an enzyme or cofactor that is fortuitously produced by the organisms for other
purposes. The organism receives no known benefit from the degradation of the chlorinated aliphatic
hydrocarbon. Rather, the cometabolic degradation o.f the chlorinated aliphatic hydrocarbon may in
fact be harmful to the microorganism responsible for the production of the enzyme or cofactor
(McCarty and Semprini, 1994). Cometabolism is best documented in aerobic environments, although
it potentially could occur under anaerobic conditions. It has been reported that under aerobic
conditions chlorinated ethenes, with the exception of PCE, are susceptible to cometabolic degradation
(Murray and Richardson, 1993; Vogel, 1994; McCarty and Semprini, 1994). Vogel (1994) further
elaborates that the rate of cometabolism increases as the degree of dechlorination decreases. During
cometabolism, the chlorinated alkene is indirectly transformed by bacteria as they use BTEX or
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another substrate to meet their energy requirements. Therefore, the chlorinated alkene does not
enhance the degradation ofBTEX or other carbon sources, nor will its cometabolism interfere with
the use of electron acceptors involved in the oxidation of those carbon sources.
2.2.1.2 Behavior of Chlorinated Solvent Plumes
Chlorinated solvent plumes can exhibit three types of behavior depending on the amount of
solvent, the amount of biologically available organic carbon in the aquifer, the distribution and
concentration of natural electron acceptors, and the types of electron acceptors being used. Individual
plumes may exhibit all three types of behavior in different portions of the plume. The different
types of plume behavior are summarized below.
2.2. 1 .2.1 Type 1 Behavior
Type I behavior occurs where the primary substrate is anthropogenic carbon (e.g., BTEX or
landfill leachate), and microbial degradation of this anthropogenic carbon drives reductive
dechlorination. When evaluating natural attenuation of a plume exhibiting Type I behavior, the
following questions must be answered:
I) ls the electron donor supply adequate to allow microbial reduction of the chlorinated
organic compounds? In other words, will the microorganisms "strangle" before they
"starve" (i.e., will they run out of chlorinated aliphatic hydrocarbons used as electron
acceptors before they run out of anthropogenic carbon used as the primary substrate)?
2) What is the role of competing electron acceptors (e.g., dissolved oxygen, nitrate, iron (III)
and sulfate)?
3) ls VC oxidized, or is it reduced?
Appendices B and C discuss what these questions mean and how they are answered. Type I
behavior results in the rapid and extensive degradation of the more highly-chlorinated solvents
such as PCE, TCE, and DCE.
2.2.1:2.2 Type 2 Behavior
Type 2 behavior dominates in areas that are characterized by relatively high concentrations of
biologically available native organic carbon. Microbial utilization of this natural carbon source
drives reductive dechlorination (i.e., it is the primary substrate for microorganism growth). When
evaluating natural attenuation of a Type 2 chlorinated solvent plume, the same questions as those
posed in the description of Type I behavior must be answered. Type 2 behavior generally results in
slower biodegradation of the highly chlorinated· solvents than Type !°behavior, but under the right
conditions (e.g., areas with high natural organic carbon contents), this type of behavior also can
result in rapid degradation of these compounds.
2.2.1.2.3 Type 3 Behavior
Type 3 behavior dominates in areas that are characterized by inadequate concentrations of
native and/or anthropogenic carbon, and concentrations of dissolved oxygen that are greater than
1.0 mg/L. Under these aerobic conditions, reductive dechlorination will not occur. The most
significant natural attenuation mechanisms for PCE, TCE, and DCE will be advection, dispersion,
and sorption. However, VC can be rapidly oxidized under these conditions. Type 3 behavior also
occurs in ground water that does not contain microbes capable of biodegradation of chlorinated
solvents.
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2.2.1.2.4 Mixed Behavior
As mentioned above, a single chlorinated solvent plume can exhibit all three types of behavior
in different portions of the plume. This can be beneficial for natural biodegradation of chlorinated
aliphatic hydrocarbon plumes. For example, Wiedemeier et al. (1996a) describe a plume at
Plattsburgh AFB, New York, that exhibits Type 1 behavior in the source area and Type 3 behavior
downgradient from the source. The most fortuitous scenario involves a plume in which PCE, TCE,
and DCE are reductively dechlorinated with accumulation of VC near the source area (Type 1 or
Type 2 behavior), then VC is oxidized (Type 3 behavior), either aerobically or via iron reduction
further downgradient. Vinyl chloride is oxidized to carbon dioxide in this type of plume and does
not accumulate. The following sequence of reactions occurs in a plume that exhibits this type of
mixed behavior.
PCE➔TCE➔DCE➔ VC➔Carbon Dioxide
In general, TCE, DCE, and VC may attenuate at approximately the same rate, and thus these
reactions may be confused with simple dilution. Note that no ethene is produced during this reaction.
Vinyl chloride is removed from the system much faster under these conditions than it is under VC-
reducing conditions.
A less desirable scenario, but one in which all contaminants may be entirely biodegraded,
involves a plume in which all chlorinated aliphatic hydrocarbons are reductively dechlorinated via
Type 1 or Type 2 behavior. Vinyl chloride is reduced to ethene, which may be further reduced to
ethane or methane. The following sequence of reactions occurs in this type of plume.
PCE➔ TCE➔ DCE➔ VC➔Ethene➔Ethane
This sequence has been investigated by Freedman and Gossett(] 989). In this type of plume,
VC degrades more slowly than TCE, and thus tends to accumulate.
2.2.2 Bioattenuation Screening Process
An accurate assessment of the potential for natural biodegradation of chlorinated compounds
should be made before investing in a detailed study of natural attenuation. The screening process
presented in this section is outlined in Figure 2.3. This approach should allow the investigator to
determine if natural bioattenuation of PCE, TCE, DCE, TCA, and chlorobenzenes is likely to be a
viable remedial alternative before additional time and money are expended. If the site is regulated
under CERCLA, much of the data required to make the preliminary assessment of natural attenuation
will be used to evaluate alternative engineered remedial solutions· as required by the NCP. Table 2.3
presents the analytical screening criteria.
For most of the chlorinated solvents, the initial biotransformation in the environment is a
reductive dechlorination. The initial screening process is designed to recognize geochemical
environments where reductive dechlorination is plausible. It is recognized, however, that
bioodegradation of certain halogenated compounds ca·n also proceed via oxidative pathways.
Examples include DCE, VC, the dichloroethanes, chloroethane, dichlorobenzenes,
monochlorobenzene, methylene chloride, and ethylene dibromide.
The following information is required for the screening process:
• The chemical and geochemical data presented in Table 2.3 for background and target
areas of the plume as depicted in Figure 2.4. Figure 2.4 shows the schematic locations of
these data collection points. Note: If other contaminants are suspected, then data on the
concentrations and distribution of these compounds also should be obtained.
• Locations of source(s) and potential points of exposure. If subsurface NAPLs are
sources, estimate extent of residual and free-phase NAPL.
• An estimate of the transport velocity and direction of ground-water flow.
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Analyze Available Site Data
Along Core of Plume
~'----1 Collect More Screening Data
to Determine if Blodegradation
is Occurring
No or
Insufficient
Data
Locate source(s)and
potential points of
exposure. Estimate
extent of NPAL,
residual and free-phase
Oetennine Groundwater Flow and
Solute Transport Parameters Along
Core of Plume using
Sile-Specific Data; Porosity and
Dispersivity May be Estimated
Estimate Biodegradation
Rate Constant
Compare the Rate of Transport
to the Rate of Attenuation using
Analytical Solute Transport Model
Yes
Perform Site Characterization
to Evaluate Natural Attenuation
Proceed to
Figure 2.1
No
No
Yes
Evaluate use of Selected
Additional Remedial Options
along with Natural Attenuation
Figure 2.3 Initial screening process flow chart.
28
Engineered
Remediation Required,
Implement Other
Protocols
Proceed to
Figure 2.1
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Table 2.3 Analytical Parameters and Weighting for Preliminary Screening for Anaerobic
Biodegradation Processesa1
Concentration In
Most Contaminated
~nalysis Zone Interpretation
pxygen• <0.5 mg/l Tolerated, suppresses the reductive pathway at higher
oncentrations
:)wnen* >5 mall Not tolerated; however, VC mav be oxidized aerobicallv
Nitrate* <1 moll At hinher concentrations ma" comoete with reductive nathwav
Iron 11• >1 mg/l Reductive pathway possible; VC may be oxidized under Fe(III
reducinn conditions
Sulfate· <20 mail At hiaher concentrations mav comoete with reductive oathwa\l
Sulfide• >1 mn/L Reductive nathwav oossible
Methane* <0.5 mg/l VC oxidizes
>0.5 mn/l Jltimate reductive dauahter nroduct, VC Accumulates
Jxidation Reduction !<50 millivolts (mV) Reductive pathway possible
Potential• (OAP) <-100mV Reductive pathway likely
against Ag/AgCI
electrode
pH' 5<pH<9 Optimal range for reductive pathway
; > oH >9 Ol.ltside ootimal ranne for reductive oathwav
oc > 20 mg/l Carbon and energy source; drives dechlorination; can be
natural or anthroooaenic
,.emoerature* >2rf'C \t T >20°C biochemical nrocess is accelerated
:arbon Dioxide >2x backaround Ultimate oxidative daunhter nroduct
,lkalinitv >2x backaround Results from interaction between CQ and aauifer minerals
vhloride* >2x backnround Daunhter nroduct of oraanic chlorine
Hvdroaen >1 nM Reductive nathwav nossible VC mav accumulate
Hvdroaen <1 nM VC oxidized
Volatile Fatty Acids > 0.1 mg/l Intermediates resulting from biodegradation of more complex
bomoounds; carbon and enerm, source
BTEX• I> 0.1 mail Carbon and enernu source; drives dechlorination
.,.etrachloroethene Material released
tfrichloroethene· Material released
Daunhter nroduct of PCE
DCE• Material released
Daughter product of TCE
f cis is > 80% of total DCE it is likely a daughter product
1, 1-DCE can be chemical reaction oroduct of TCA
~c· Material released
Dauohter oroduct of DCE
1, 1, 1-Trichloroethane• Material released
DCA Daunhter oroduct of TCA under reducinq conditions
:arbon Tetrachloride Material released
:hloroethane* Daunhter nroduct of. DCA or VC under reducina conditions
Ethane/Ethane >0.01mg/l Daughter product of VG/ethane
>0.1 mall
K:hlorofonm Material released
Daunhter nroduct of Carbon Tetrachloride
Dichloromethane Material released
Dauohter oroduct of Chloroform
Value
3
-3
2
3
2
3
0
3
1
2
0
-2
2
1
1
1
2
3
0
2
2
0
0 2o/
0
2o1
0 2o/
0
2
0
2
2
3
0
2
0
2
• Required analysis. a/ Points awarded only 1f II can be shown that the compound 1s a daughter product (1.e., not a const1ttueof the source
NAPL).
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Once these data have been collected, the screening process can be undertaken. The following
steps summarize the screening processes:
I) Determine ifbiodegradation is occurring using geochemical data. Ifbiodegradation is
occurring, proceed to step 2. If it is not, assess the amount and types of data available. If
data are insufficient to determine if biodegradation is occurring, collect supplemental data.
If all the recommended screening parameters listed in section 2.2 have been collected and
the screening processes suggest that natural attenuation is not appropriate, the screening
processes are finished. Perform site characterization to evaluate other remediation alterna-
tives.
2) Determine ground-water flow and solute transport parameters from representative field
data. Dispersivity and porosity may be estimated from literature but the hydraulic conduc-
tivity and the ground-water gradient and flow direction must be determined from field
data. The investigator should use the highest valid hydraulic conductivity measured at the
site during the preliminary screening because solute plumes tend to follow the path of
least resistance (i.e., highest hydraulic conductivity). This will give the "worst-case"
estimate of the solute migration distance over a given period of time. Compare this
"worst-case" estimate with the rate of plume migration determined from site characteriza-
tion data. Determine what degree of plume migration is accepable or unacceptable with
respect to site-specific remediation objectives.
3) Locate source(s) and potential points of exposure. If subsurface NAPLs are sources,
estimate extent of residual and free-phase NAPL.
4) Estimate the biodegradation rate constant. Biodegradation rate constants can be estimated
using a conservative tracer found commingled with the contaminant plume, as described
in Appendix C and by Wiedemeier et al. (1996b). When dealing with a plume that con-
tains chlorinated solvents, this procedure can be modified to use chloride as a tracer. Rate
constants derived from microcosm studies can also be used when site specific field data
are inadequate or inconclusive. If it is not possible to estimate the biodegradation rate
using these procedures, then use a range of accepted literature values for biodegradation of
the contaminants of concern. Appendix C presents a range of bi ode gradation rate con-
stants for various compounds. Although literature values may be used to estimate
biogradation rates in the bioattenuation screening process described in Section 2.2, litera-
ture values should not be used in the later more detailed analysis of natural attenuation,
described in Section 2.3.
5) Compare the rate of transport to the rate of attenuation.
Use analytical solutions or a screening model such as BIOSCREEN.
6) Determine if screening criteria are met.
Step 1: Determine if Biodegradation is Occurring
The first step in the screening process is to sample or use existing data for the areas represented
in Figure 2.4 and analyze them for the parameters listed in Table 2.3 (see also Section 2.3.2). These
areas should include (1) the most contaminated portion of the aquifer (generally in the "source"
area with NAPL or high concentrations of contaminants in ground water; (2) downgradient from
the source area but still in the dissolved contaminant plume; (3) downgradient from the dissolved
contaminant plume; and (4) upgradient and lateral locations that are not impacted by the plume.
Although this figure is a simplified two-dimensional representation of the features of a contaminant
plume, real plumes are three-dimensional objects. The sampling should be conducted in accordance
with Appendix A.
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0
Source Area Dissolved Contaminant Plume
0
Direction of Plume Migration
O Representative Sampling Location
0
Figure 2.4 Target areas for collecting screening data. Note that the number and location of monitoring
wells will vary with the three dimensional complexity of the plume(s),
The sample collected in the NAPL source area provides information as to the predominant
terminal electron-accepting process at the source area. In conjunction with the sample collected in
the NAPL source zone, samples collected in the dissolved plume downgradient from the NAPL
source zone allow the investigator (1) to determine if the plume is degrading with distance along
the flow path and (2) to determine the distribution of electron acceptors and donors and metabolic
by-products along the flow path. The sample collected downgradient from the dissolved plume
aids in plume delineation and allows the investigator to determine if metabolic byproducts are
present in an area of ground water that has been remediated. The upgradient and lateral samples
allow delineation of the plume and determination of background concentrations of the electron
acceptors and donors.
After these samples have been analyzed for the parameters listed in Table 2.3, the investigator
should analyze the data to determine if biodegradation is occurring. The right-hand column of
Table 2.3 contains scoring values that can be used as a test to assess the likelihood that biodegradation
is occurring. This method relies on the fact that biodegradation will cause predictable changes in
ground water chemistry. For example, if the dissolved oxygen concentration in the area of the
plume with the highest contaminant concentration is less than 0.5 milligrams per liter (mg/L), 3
points are awarded. Table 2.4 summarizes the range of possible scores and gives an interpretation
for each score. If the score totals 15 or more points, it is likely that biodegradation is occurring, and
the investigator should proceed to Step 2.
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Table 2.4 Interpretation of Points Awarded During Screening Step I
Score Interpretation
0 to 5 Inadequate evidence for anaerobic biodegradation* of chlorinated organics
6 to 14 Limited evidence for anaerobic biodegradation* of chlorinated organics
15 to 20 Adequate evidence for anaerobic biodegradation * of chlorinated organics
> 20 Strong evidence for anaerobic biodegradation* of chlorinated organics
*reductive dechlorination
The following two examples illustrate how Step I of the screening process is implemented.
The site used in the first example is a form.er fire training area contaminated with chlorinated
solvents mixed with fuel hydrocarbons. The presence of the fuel hydrocarbons appears to reduce
the ORP of the ground water to the extent that reductive dechlorination is favorable. The second
example contains data from a dry cleaning site contaminated only with chlorinated solvents. This
site was contaminated with spent cleaning solvents that were dumped into a shallow dry well situated
just above a well-oxygenated, unconfined aquifer with low organic carbon concentrations of dissolved
organic carbon.
Example 1: Strong Evidence for Anaerobic Biodegradation (Reductive Dechlorination) of
Chlorinated Organics
Analyte Concentration in Most Contaminated Zone Points A warded
Dissolved Oxygen 0.1 mg/L 3
Nitrate 0.3 mg/L 2
Iron (II) IO 3
u ate m
Methane Sm 3
ORP -190 inV 2
Chloride 3 times background 2
PCE (released) 1,000 µg/L 0
TCE (none released) 1,200 µg/L 2
cis-DCE (none released) 500 µg/L 2
VC (none released) 50 µg/L 2
Total Points Awarded 23 Points
In this example, the investigator can infer that biodegradation is likely occurring at the time of
sampling and may proceed to Step 2.
Example 2: Anaerobic Biodegradation (Reductive Dechlorination) Unlikely
Analyte Concentration in Most Contaminated Zone Points A warded
Dissolved Oxygen 3 mg/L -3
Nitrate 0.3 mg/L 2
Iron (II) Not Detected (ND) 0
Sulfate 10 m /L 2
Methane ND 0
ORP + l00 mV 0
Chloride background 0
TCE (released) 1,200 µg/L 0
cis-DCE (none released) ND 0
VC (none released) ND 0
Total Points Awarded I Point
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In this example, the investigator can infer that biodegradation is probably not occurring or is
occurring too slowly to contribute to natural attenuation at the time of the sampling. In this case,
the investigator should evaluate whether other natural attenuation processes can meet the cleanup
objectives for the site (e.g., abiotic degradation or transformation, volatilization or sorption) or
select a remedial option other than MN A.
Step 2: Determine Ground-water Flow and Solute Transport Parameters
After it has been shown that biodegradation is occurring, it is important to quantify ground-
water flow and solute transport parameters. This will make it possible to use a solute transport
model to quantitatively estimate the concentration of the plume and its direction and rate of travel.
To use an analytical model, it is necessary to know the hydraulic gradient and hydraulic conductivity
for the site and to have estimates of porosity and dispersivity. It also is helpful to know the coefficient
of retardation. Quantification of these parameters is discussed in detail in Appendix B.
In order to make the modeling as accurate as possible, the investigator must have site-specific
hydraulic gradient and hydraulic conductivity data. To determine the ground-water flow and solute
transport direction, it is necessary to have at least three accurately surveyed wells in each
hydrogeologic unit of interest at the site. The porosity and dispersivity are generally estimated
using accepted literature values for the aquifer matrix materials containing the plume at the site. If
the investigator has total organic carbon data for soil, it is possible to estimate the coefficient of
retardation; otherwise, it is conservative to assume that the solute transport and ground-water
velocities are the same. Techniques to collect these data are discussed in the appendices.
Step 3: Locate Sources and Receptor Exposure Points
To determine the length of flow for the predictive modeling to be conducted in Step 5, it is
important to know the distance between the source of contamination, the leading edge along the
core of the dissolved plume, and any potential downgradient or cross-gradient receptor exposure
points.
Step 4: Estimate the Biodegradation Rate
Biodegradation is the most important process that degrades contaminants in the subsurface;
therefore, the biodegradation rate is one of the most important model input parameters.
Biodegradation of chlorinated aliphatic hydrocarbons can be represented as a first-order rate constant.
Whenever possible, use site-specific bi ode gradation rates estimated from field data collected along
the core of the plume. Calculation of site-specific biodegradation rates is discussed in Appendix C.
If it is not possible to determine site-specific biodegradation rates, then literature values may be
used in a sensitivity analysis (Table C.3.5). A useful approach is to start with average values, and
then to vary the model input to predict "best-case" and "worst-case" scenarios. Estimated
biodegradation rates can be used only after it has been shown that biodegradation is occurring (see
Step!). Although literature values may be used to estimate biodegradation rates in the bioattenuation
screening process described in Section 2.2, additional site information should be collected to
determine biodegradation rates for the site when refining the site conceptual model, as described in
Section 2.3. Literature values should not be used during the more detailed analysis.
Step 5: Compare the Rate of Transport to the Rate of Attenuation
At this early stage in the natural attenuation demonstration, comparison of the rate of solute
transport to the rate of attenuation is best accomplished using an analytical model. Several models
are available. It is suggested that the decay option be first order for use in any of the models.
The primary purpose of comparing the rate of transport to the rate of natural attenuation is to
determine if natural attenuation processes will be capable of attaining site-specific remediation
objectives in a time period that is reasonable compared to other alternatives (i.e., to quantitatively
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estimate if site contaminants are attenuating at a rate fast enough to prevent further plume migration
and restore the plume to appropriate cleanup levels). The analytical model BIOSCREEN can be
used to determine whether natural attenuation processes will be capable of meeting site-specific
remediation objectives at some distance downgradiant of a source. The numerical model BIOPLUME
III can be used to estimate whether site contaminants are attenuating at a rate fast enough to restore
the plume to appropriate cleanup levels It is important to perform a sensitivity analysis to help
evaluate the confidence in the preliminary screening modeling effort. For the purposes of the
screening effort. if modeling shows that the screening criteria are met, the investigator can proceed
with the natural attenuation evaluation.
Step 6: Determine if Screening Criteria arc Met
Before proceeding with the full-scale natural attenuation evaluation, the investigator should
ensure that the answers to both of the following questions are "yes":
• Has the plume moved a shorter distance than would be expected based on the known (or
estimated) time since the contaminant release and the contaminant velocity in ground
water, as calculated from site-specific measurements of hydraulic conductivity and
hydraulic gradient, and estimates of effective porosity and contaminant retardation?
• Is it likely that site contaminants are attenuating at rates sufficient to meet remediation
objectives for the site in a time period that is reasonable compared to other alternatives?
If the answers to these questions are "yes," then the investigator is encouraged to proceed with
the full-scale natural attenuation demonstration.
2.3 COLLECT ADDITIONAL SITE CHARACTERIZATION DATA TO EVALUATE
NATURAL ATTENUATION AS REQUIRED
It is the responsibility of the proponent to "make the case" for natural attenuation. Thus, a
credible and thorough site assessment is necessary to document the potential for natural attenuation
to meet cleanup objectives. As discussed in Section 2.1, review of existing site characterization
data is particularly useful before initiating site characterization activities. Such review should
allow identification of data gaps and guide the most effective placement of additional data collection
points.
There are two goals during the site characterization phase of a natural attenuation investigation.
The first is to collect the data needed to determine if natural mechanisms of contaminant attenuation
are occurring at rates sufficient to attain site-specific remediation objectives in a time period that is
reasonable compared to other alternatives: The second is to provide sufficient site-specific data to
allow prediction of the future extent and concentrations of a contaminant plume through solute fate
and transport modeling. Thus, detailed site characterization is required to achieve these goals and
to support this remedial option. Adequate site characterization in support of natural attenuation
requires that the following site-specific parameters be determined:
• Location, nature, and extent of contaminant source area(s) (i.e., areas containing mobile
or residual NAPL or highly contaminated ground water).
• Chemical properties (e.g., composition, solubility, volatility, etc.) of contaminant source
materials.
• The potential for a continuing source due to sewers, leaking tanks, or pipelines, or other
site activity.
• Extent and types of soil and ground-water contamination.
• Aquifer geochemical parameters (Table 2.1).
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• Regional hydrogeology, including:
-Drinking water aquifers, and
-Regional confining units.
• Local and site-specific hydrogeology, including:
-Local drinking water aquifers;
-Location of industrial, agricultural, and domestic water wells;
-Patterns of aquifer use (current and future);
-Lithology;
-Site stratigraphy, including identification of transmissive and nontransmissive units;
-Potential pathways for NAPL migration (e.g., surface topography and dip of confining
layers);
-Grain-size distribution (sand vs. silt vs. clay);
-Aquifer hydraulic conductivity;
-Ground water hydraulic information;
-Preferential flow paths;
-Locations and types of surface water bodies; and
-Areas of local ground water recharge and discharge.
• Identification of current and future potential exposure pathways, receptors, and exposure
points.
Many chlorinated solvent plumes have enough three-dimensional expression to make it
impossible for a single well to adequately describe the plume at a particular location on a map of
the site.
Figure 2.5 depicts a cross section of a hypothetical site with three-dimensional expression of
the plume. A documented source exists in the capillary fringe just above the water table. Such
sources are usually found by recovering, extracting, and analyzing core ma.terial. This material can
be (1) a release of LNAPL containing chlorinated solvents; (2) a release of pure chlorinated solvents
that has been entrapped by capillary interactions in the capillary fringe; or (3) material that has
experienced high concentrations of solvents in solution in ground water, has sorbed the solvents,
and now is slowly desorbing the chlorinated solvents. Recharge of precipitation through this source
produces a plume that appears to dive into the aquifer as it moves away from the source. This effect
can be caused by recharge of clean ground water above the plume as it moves downgradient of the
source, by collection of the plume into more hydraulically conductive material at the bottom of
aquifer, or by density differences between the plume and the unimpacted ground water.
Below the first hydro logic unit there is a second unit that has fine-textured material at the top
and coarse-textured material at the bottom of the unit. In the hypothetical site, the fine-textured
material at the top of the second unit has inhibited downward migration of a DNAPL, causing it to
spread laterally at the bottom of the first unit and forrn a second source of ground-water contamination
in the first unit. Because DNAPL below the water table tends to exist as diffuse and widely extended
ganglia rather than of pools filling all the pore space, it is statistically improbable that the material
sampled by conventional core sampling will contain DNAPL. Because these sources are so difficult
to sample, these sources are cryptic to conventional sampling techniques.
At the hypothetical site, DNAPL has found a pathway past the fine-textured material and has
forrned a second cryptic source area at the bottom of the second hydrologic unit. Compare Figure 2.6.
The second hydrological unit at the hypothetical site has a different hydraulic gradient than the first
unit. As a result, the plume in the second unit is moving in a different direction than the plume in
the first unit. Biological processes occurring in one hydrological unit may not occur in another; a
plume may show Type 2 behavior in one unit and Type 3 beh.avior in another.
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Figure 2.5 A cross section through a hypothetical release, illustrating the three-dimensional character
of the plumes that may develop from a release of chlorinated solvents.
· Documented NAPL
/ L ___ -,----,-_:=:::,
Figure 2.6 A stacked plan representation of the plumes that may develop from the hypothetical release
depicted in Figure 2.5. Each plan representation depicts a separate plume that can
originate from discrete source areas produced from the same release of chlorinated solvents.
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As a consequence, it is critical to sample and evaluate the three-dimensional character of the
site with respect to (I) interaction of contaminant releases with the aquifer matrix material, (2)
local hydological features that control development and migration of plumes, and (3) the geochemical
interactions that favor bioattenuation of chlorinated solvents.
The following sections describe the methodologies that should be implemented to allow
successful site characterization in support of natural attenuation.
2.3.1 Characterization of Soils and Aquifer Matrix Materials
In order to adequately define the subsurface hydrogeologic system and to determine the three-
dimensional distribution of mobile and residual NAPL that can act as a continuing source of ground-
water contamination, credible and thorough soil characterization must be completed. As appropriate,
soil gas data may be collected and analyzed to better characterize soil contamination in the vadose
zone. Depending on the status of the site, this work may have been completed during previous
remedial investigation work. The results of soils characterization will be used as input into a solute
fate and transport model to help define a contaminant source term and to support the natural
attenuation investigation.
The purpose of sampling soil and aquifer matrix material is to determine the subsurface
distribution of hydrostratigraphic units and the distribution of mobile and residual NAPL, as well
as pore water that contains high concentrations of the contaminants in the dissolved phase. These
objectives can be achieved through the use of conventional soil borings or direct-push methods
(e.g., Geoprobe® or cone penetrometer testing), and through collection of soil gas samples. All
samples should be collected, described, analyzed, and disposed of in accordance with local, State,
and Federal guidance. Appendix A contains suggested procedures for sample collection. These
procedures may require modification to comply with local, State, and Federal regulations or to
accommodate site-specific conditions.
The analytical methods to be used for soil, aquifer matrix material, and soil gas sample analyses
is presented in Table 2.1. This table includes all of the parameters necessary to document natural
attenuation, including the effects of sorption, volatilization, and biodegradation. Each analyte is
discussed separately below.
• Volatile Organic Compounds: Knowledge of the location, distribution, concentration,
and total mass of contaminants sorbed to soils or present as mobile or immobile NAPL is
required to calculate contaminant partitioning from NAPL into ground water. This
information is useful to predict the long-term persistence of source areas. Knowledge of
the diffusive flux of volatile organic compounds from NAPLs or ground water to the
atmosphere or other identified receptor for vapors is required to estimate exposure of the
human population or ecological receptors to contaminant vapors. If the flux of vapors
can be compared to the discharge of the contaminants in ground water, the contribution of
volatilization to natural attenuation of contamination in ground water can be documented.
• Total Organic Carbon: Knowledge of the TOC content of the aquifer matrix is ·
important for sorption and solute-retardation calculations. TOC samples should be
collected from a background location in the stratigraphic horizon(s) where most
contaminant transport is expected to occur.
• Oxygen and Carbon Dioxide: Oxygen and carbon dioxide soil gas measurements can be
used to identify areas in the unsaturated zone where biodegradation is occurring. This
can be a useful and relatively inexpensive way to identify NAPL source areas, particularly
when solvents are codisposed with fuels or greases (AFCEE, 1994).
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• Fuel and Chlorinated Volatile Organic Compounds: Knowledge of the distribution of
contaminants in soil gas can be used as a cost-effective way to estimate the extent of soil
contamination.
2.3.2 Ground-water Characterization
To adequately determine the amount and three-dimensional distribution of dissolved
contamination and to document the occurrence of natural attenuation, ground-water samples must
be collected and analyzed. Biodegradation of organic compounds, whether natural or anthropogenic,
brings about measurable changes in the chemistry of ground water in the affected area. By measuring
these changes, it is possible to document and quantitatively evaluate the importance of natural
attenuation at a site.
Ground-water sampling is conducted to determine the concentrations and _distribution of
contaminants, daughter products, and ground-water geochemical parameters. Ground-water samples
may be obtained from monitoring wells or with point-source sampling devices such as a Geoprobe®,
Hydropunch®, or cone penetrometer. All ground-water samples should be collected, handled, and
disposed of in accordance with local, State, and Federal guidelines. Appendix A contains suggested
procedures for ground-water sample collection. These procedures may need to be modified to
comply with local, State, and Federal regulations or to accommodate site-specific conditions.
The analytical protocol for ground-water sample analysis is presented in Table 2.1. This
analytical protocol includes all of the parameters necessary to delineate dissolved contamination
and to document natural attenuation, including the effects of sorption and biodegradation. Data
obtained from the analysis of ground water for these analytes is used to scientifically document
natural attenuation and can be used as input into a solute fate and transport model. The following
paragraphs describe each ground-water analytical parameter and the use of each analyte in the
natural attenuation demonstration.
2.3.2_. l Volatile and Semivolatile Organic Compounds
These analytes are used to determine the type, concentration, and distribution of contaminants
and daughter products in the aquifer. In many cases, chlorinated solvents are found commingled
with fuels or other hydrocarbons. At a minimum, the volatile organic compound (VOC) analysis
(Method SW8260A) should be used, with the addition of the trimethylbenzene isomers if fuel
hydrocarbons are present or suspected. The combined dissolved concentrations of BTEX and
trimethylbenzenes should not be greater than about 30 mg/L for a JP-4 spill (Smith et al., 1981) or
about 135 mg/L for a gasoline spill (Cline et al., 1991; American Petroleum Institute, 1985). If
these compounds are found in higher concentrations, sampling errors such as emclsification of
LNAPL in the ground-water sample likely have occurred and should be investigated.
Maximum concentrations of chlorinated solvents dissolved in ground water from neat solvents
should not exceed their solubilities in water. Appendix B contains solubilities for common
contaminants. If contaminants are found in concentrations greater than their solubilities, then
sampling errors such as emulsification of NAPL in the ground-water sample have likely occurred
and should be investigated.
2.3.2.2 Dissolved Oxygen
Dissolved oxygen is the most thermodynamically favored electron acceptor used by microbes
for the biodegradation of organic carbon, whether natural or anthropogenic. Anaerobic bacteria
generally cannot function at dissolved oxygen concentrations greater than about 0.5 mg/L and,
hence, reductive dechlorination will not occur. This is why it is important to have a source of
carbon in the aquifer that can be used by aerobic microorganisms as a primary substrate. During
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aerobic respiration, dissolved oxygen concentrations decrease. After depletion of dissolved oxygen,
anaerobic microbes will use nitrate as an electron acceptor, followed by iron (III), then sulfate, and
finally carbon dioxide (methanogenesis). Each sequential reaction drives the ORP of the ground
water downward into the range within which reductive dechlorination can occur. Reductive
dechlorination is most effective in the ORP range corresponding to sulfate reduction and
methanogenesis, but dechlorination of PCE and TCE also may occur in the ORP range associated
with denitrification or iron (III) reduction. Dehalogenation ofDCE and VC generally are restricted
to sulfate reducing and methanogenic conditions.
Dissolved oxygen measurements should be taken during well purging and immediately before
and after sample acquisition using a direct-reading meter. Because most well purging techniques
can allow aeration of collected ground-water samples, it is important to minimize the potential for
aeration as described in Appendix A.
2.3.2.3 Nitrate
After dissolved oxygen has been depleted in the microbiological treatment zone, nitrate may
be used as an electron acceptor for anaerobic biodegradation of organic carbon via denitrification.
In order for reductive dechlorination to occur, nitrate concentrations in the contaminated portion of
the aquifer must be less than 1.0 mg/L.
2.3.2.4 Iron (II)
In some cases, iron (III) is used as an electron acceptor during anaerobic biodegradation of
organic carbon. During this process, iron (III) is reduced to iron (II), which may be soluble in water.
Iron (II) concentrations can thus be used as an indicator of anaerobic degradation of fuel compounds,
and vinyl chloride (see Section 2.2.1.1.2). Native organic matter may also support reduction of iron
(II). Care must be taken when interpreting iron (II) concentrations because they may be biased low
by reprecipitation as sulfides or carbonates.
2.3.2.5 Sulfate
After dissolved oxygen and nitrate have been depleted in the microbiological treatment zone,
sulfate may be used as an electron acceptor for anaerobic biodegradation. This process is termed
"sulfate reduction" and results in the production of sulfide. Concentrations of sulfate greater than
20 mg/L may cause competitive exclusion of dechlorination. However, in many plumes with high
concentrations of sulfate, reductive dechlorination still occurs.
2.3.2.6 Methane
During methanogenesis acetate is split to form carbon dioxide and methane, or carbon dioxide
is used as an electron acceptor, and is reduced to methane. Methanogenesis generally occurs after
oxygen, nitrate, and sulfate have been depleted in the treatment zone. The presence of methane in
ground water is indicative of strongly reducing conditions. Because methane is not present in fuel,
the presence of methane above background concentrations in ground water in contact with fuels is
indicative of microbial degradation of hydrocarbons. Methane also is associated with spills of pure
chlorinated solvents (Weaver et al., 1996). It is not known if the methane comes from chlorinated
solvent carbon or from native dissolved organic carbon.
2.3.2.7 Alkalinity
There is a positive correlation between zones of microbial activity and increased alkalinity.
Increases in alkalinity result from the dissolution of rock driven by the production of carbon dioxide
produced by the metabolism of microorganisms. Alkalinity is important in the maintenance of
ground-water pH because it buffers the ground water system against acids generated during both
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aerobic and anaerobic bi ode gradation. In the experience of the authors, biodegradation of organic
compounds rarely, if ever, generates enough acid to impact the pH of the ground water.
2.3.2.8 Oxidation-Reduction Potential
The ORP of ground water is a measure of electron activity and is an indicator of the relative
tendency of a solution to accept or transfer electrons. Oxidation-reduction reactions in ground
water containing organic compounds (natural or anthropogenic) are usually biologically mediated,
and, therefore, the ORP of a ground water system depends upon and influences rates of
biodegradation. Knowledge of the ORP of ground water also is important because some biological
processes operate only within a prescribed range of ORP conditions.
ORP measurements can be used to provide real-time data on the location of the contaminant
plume, especially in areas undergoing anaerobic biodegradation. Mapping the ORP of the ground
water while in the field helps the field scientist to determine the approximate location of the
contaminant plume. To map the ORP of the ground water while in the field, it is important to have
at least one ORP measurement (preferably more) from a well located upgradient from the plume.
ORP measurements should be taken during well purging and immediately before and after sample
acquisition using a direct-reading meter. Because most well purging techniques can allow aeration
of collected ground-water samples (which can affect ORP measurements), it is important to minimize
potential aeration by using a flow-through cell as outlined in Appendix A.
Most discussion of oxidation reduction potential expresses the potential as if it were measured
against the standard hydrogen electrode. Most electrodes and meters to measure oxidation-reduction
potential use the silver/silver chloride electrode (Ag/ AgCl) as the reference electrode. This protocol
uses the potential against the Ag/ AgCl electrode as the screening potential, not Eh as would be
measured against the standard hydrogen electrode.
2.3.2.9 Dissolved Hydrogen
In some ground waters, PCE and TCE appear to attenuate, although significant concentrations
of DCE and VC do not accumulate. In this situation, it is difficult to distinguish between Type 3
behavior where the daughter products are not produced, and Type I or Type 2 behavior where the
daughter products are removed very rapidly. In cases like this, the concentration of hydrogen can
be used to identify ground waters where reductive dechlorination is occurring. If hydrogen
concentrations are very low, reductive dechlorination is not efficient and Type 3 behavior is indicated.
If hydrogen concentrations are greater than approximately I nM, rates of reductive dechlorination
should have environmental significance and Type I or Type 2 behavior would be expected.
Concentrations of dissolved hydrogen have been used to evaluate redox processes, and thus
the efficiency of reductive dechlorination, in ground-water systems (Lovley and Goodwin, 1988;
Lovley et al., 1994; Chapelle et al., 1995). Dissolved hydrogen is continuously produced in anoxic
ground-water systems by fermentative microorganisms that decompose natural and anthropogenic
organic matter. This H2 is then consumed by respiratory microorganisms that use nitrate, Fe(III),
sulfate, or CO2 as terminal electron acceptors. This continuous cycling of H2 is called interspecies
hydrogen transfer. Significantly, nitrate-, Fe(III)-, sulfate-and CO2-reducing (methanogenic)
microorganisms exhibit different efficiencies in utilizing the H2 that is being continually produced.
Nitrate reducers are highly efficient H2 utilizers and maintain very low steady-state H2 concentrations.
Fe(III) reducers are slightly less efficient and thus maintain somewhat higher H, concentrations.
Sulfate reducers and methanogenic bacteria are progressively less efficient and maintain even higher
H2 concentrations. Because each terminal electron accepting process has a characteristic H2
concentration associated with it, H, concentrations can be an indicator of predominant redox
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processes. These characteristic ranges are given in Table 2.5. An analytical protocol for quantifying
H2 concentrations in ground water is given in Appendix A.
Table 2.5 Range of Hydrogen Concentrations for a Given Terminal Electron-Accepting Process
Terminal Electron
Accepting Process
Denitrification
Iron (III) Reduction
Sulfate Reduction
Reductive Dechlorination
Methano enesis
Hydrogen (H ,)
Concentration (nanomoles per liter)
< 0.1
0.2 to 0.8
I to 4
>I
5-20
Oxidation-reduction potential (ORP) measurements are based on the concept of thermodynamic
equilibrium and, within the constraints of that assumption, can be used to evaluate redox processes
in ground water systems. The H2 method is based on the ecological concept of interspecies hydrogen
transfer by microorganisms and, within the constraints of that assumption, can also be used to
evaluate redox processes. These methods, therefore, are fundamentally different. A direct comparison
of these methods (Chapelle et al., 1996) has shown that ORP measurements were effective in
delineating oxic from anoxic ground water, but that ORP measurements could not distinguish between
nitrate-reducing, Fe(III)-reducing, sulfate-reducing, or methanogenic zones in an aquifer. In contrast,
the H2 method could readily distinguish between different anaerobic zones. For those sites where
distinguishing between different anaerobic processes is important, H2 measurements are an available
technology for making such distinctions. At sites where concentrations of redox sensitive parameters
such as dissolved oxygen, iron (II), sulfide, and methane are sufficient to identify operative redox
processes, H, concentrations are not always required to identify redox zonation and predict
contaminant behavior.
In practice, it is preferable to interpret H2 concentrations in the context of electron acceptor
availability and the presence of the final products of microbial metabolism (Chapelle et al., 1995).
For example, if sulfate concentrations in ground water are less than 0.5 mg/L, methane concentrations
are greater than 0.5 mg/L, and H2 concentrations are in the 5 to 20 nM range, it can be concluded
with a high degree of certainty that methanogenesis is the predominant redox process in the aquifer.
Similar logic can be applied to identifying denitrificatiori (presence of nitrate, H2<0.1 nM), Fe(III)
reduction (production ofFe(II), H2 concentrations ranging from 0.2 to 0.8 nM), and sulfate reduction
(presence of sulfate, production of sulfide, H2 concentrations ranging from I to 4 nM). Reductive
dechlorination in the field has been documented at hydrogen concentrations that support sulfate
reduction or methanogenesis. If hydrogen concentrations are high enough to support sulfate reduction
or methanogenesis, then reductive dechlorination is probably occurring, even if other geochemical
indicators as scored in Table 2.3 do not indicate that reductive dechlorination is possible.
2.3.2. 10 pH, Temperature, and Conductivity
Because the pH, temperature, and conductivity of a ground-water sample can change
significantly within a short time following sample acquisition, these parameters must be measured
in the field in unfiltered, unpreserved, "fresh" water collected by the same technique as the samples
taken for dissolved oxygen and ORP analyses. The measurements should be made in a clean
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container separate from those intended for laboratory analysis, and the measured values should be
recorded in the ground-water sampling record.
The pH of ground water has an effect on the presence and activity of microbial populations in
ground water. This is especially true for methanogens. Microbes capable of degrading chlorinated
aliphatic hydrocarbons and petroleum hydrocarbon compounds generally prefer pH values varying
from 6 to 8 standard units.
Ground-water temperature directly affects the solubility of dissolved gasses and other
geochemical species. Ground-water temperature also affects the metabolic activity of bacteria.
Conductivity is a measure of the ability of a solution to conduct electricity. The conductivity
of ground water is directly related to the concentration of ions in solution; conductivity increases as
ion concentration increases.
2.3.2.11 Chloride
· Chlorine is the most abundant of the halogens. Although chlorine can occur in oxidation
states ranging from Cl· to Cl•', the chloride form (Cl') is the only form of major significance in
natural waters (Hem, I 985). Chloride forms ion pairs or complex ions with some of the cations
present in natural waters, but these complexes are not strong enough to be of significance in the
chemistry of fresh water (Hem, 1985). Chloride ions generally do not enter into oxidation-reduction
reactions, form no important solute complexes with other ions unless the chloride concentration is
extremely high, do not form salts oflow solubility, are not significantly adsorbed on mineral surfaces,
and play few vital biochemical roles (Hem, 1985). Thus, physical processes control the migration
of chloride ions in the subsurface. Kaufman and Orlob (1956) conducted tracer experiments in
ground water, and found that chloride moved through most of the soils tested more conservatively
(i.e., with less retardation and loss) than any of the other tracers tested.
During biodegradation of chlorinated hydrocarbons dissolved in ground water, chloride is
released into the ground water. This results in chloride concentrations in ground water in the
contaminant plume that are elevated relative to background concentrations. Because of the neutral
chemical behavior of chloride, it can be used as a conservative tracer to estimate biodegradation
rates, as discussed in Appendix C.
2.3.3 Aquifer Parameter Estimation
Estimates of aquifer parameters are necessary to accurately evaluate contaminant fate and
transport.
2.3.3.1 Hydraulic Conductivity
Hydraulic conductivity is a measure of an aquifer's ability to transmit water, and is perhaps the
most important aquifer parameter governing fluid flow in the subsurface. The velocity of ground
water and dissolved contamination is directly related to the hydraulic conductivity of the saturated
zone. In addition, subsurface variations in hydraulic conductivity directly influence contaminant
fate and transport by providing preferential paths for contaminant migration. Estimates of hydraulic
conductivity are used to determine residence times for contaminants and tracers, and to determine
the seepage velocity of ground water.
The most common methods used to quantify hydraulic conductivity are aquifer pumping tests
and slug tests (Appendix A). Another method that may be used to determine hydraulic conductivity
is the borehole dilution test. One drawback to these methods is that they average hydraulic properties
over the screened interval. To help alleviate this potential problem, the screened interval of the test
wells should be selected after consideration is given to subsurface stratigraphy.
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Information about subsurface stratigraphy should come from geologic logs of continuous cores
or from cone penetrometer tests. The rate of filling of a Hydropunch® can be used to obtain a
rough estimate of the local hydraulic conductivity at the same time the water sample is collected.
The results of pressure dissipation data from cone penetrometer tests can be used to supplement the
results obtained from pumping tests and slug tests. It is important that the location of the aquifer
tests be designed to collect information to delineate the range of hydraulic conductivity both vertically
and horizontally at the site.
2.3.3.1.1 Pumping Tests in Wells
Pumping tests done in wells provide information on the average hydraulic conductivity of the
screened interval, but not the most transmissive horizon included in the screened interval. In
contaminated areas, the extracted ground water generally must be collected and treated, increasing
the difficulty of such testing. In addition, a minimum 4-inch-diameter well is typically required to
complete pumping tests in highly transmissive aquifers because the 2-inch submersible pumps
available today are not capable of producing a flow rate high enough for meaningful pumping tests.
In areas with fairly uniform aquifer materials, pumping tests can be completed in uncontaminated
areas, and the results can be used to estimate hydraulic conductivity in the contaminated area.
Pumping tests should be conducted in wells that are screened in the most transmissive zones in the
aquifer. If pumping tests are conducted in wells with more than fifteen feet of screen, a down-hole
flowmeter test can be used to determine the interval actually contributing to flow.
2.3.3.1.2 Slug Tests in Wells
Slug tests are a commonly used alternative to pumping tests. One commonly cited drawback to
slug testing is that this method generally gives hydraulic conductivity information only for the area
immediately surrounding the monitoring well. Slug tests do, however, have two distinct advantages
over pumping tests: they can be conducted in 2-inch monitoring wells, and they produce no water.
If slug tests are going to be relied upon to provide information on the three-dimensional distribution
of hydraulic conductivity in an aquifer, multiple slug tests must be performed. It is not advisable to
rely on data from one slug test in one monitoring well. Because of this, slug tests should be
conducted at several zones across the site, including a test in at least two wells which are narrowly
screened in the most transmissive zone. There should also be tests in the less transmissive zones to
provide an estimate of the range of values present on the site.
2.3.3.1.3 Downhole Flowmeter
Borehole flowmeter tests are conducted to investigate the relative vertical distribution of
horizontal hydraulic conductivity in the screened interval of a well or the uncased portion of a
borehole. These tests can be done to identify any preferential flow pathways within the portion of
an aquifer intersecting the test well screen or the open·borehole. The work of Molz and Young
(1993), Molz et al. (1994), Young and Pearson (1995), and Young (1995) describes the means by
which these tests may be conducted and interpreted.
In general, measurements of ambient ground-water flow rates are collected at several regularly
spaced locations along the screened interval of a well. Next, the well is pumped at a steady rate,
and the measurements are repeated. The test data may be analyzed using the methods described by
Molz and Young (l 993) and Molz et al. (l 994) to define the relative distribution of horizontal
hydraulic conductivity within the screened interval of the test well. Estimates of bulk hydraulic
conductivity from previous aquifer tests can be used to estimate the absolute hydraulic conductivity
distribution at the test well.
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Using flowmeter test data, one may be able to more thoroughly quantify the three-dimensional
hydraulic conductivity distribution at a site. This is important for defining contaminant migration
pathways and understanding solute transport at sites with heterogeneous aquifers. Even at sites
where the hydrogeology appears relatively homogeneous, such data may point out previously
undetected zones or layers of higher hydraulic conductivity that control contaminant migration. In
addition, ground-water velocities calculated from hydraulic head, porosity, and hydraulic conductivity
data may be used to evaluate site data or for simple transport calculations. In these cases, it is also
important to have the best estimate possible of hydraulic conductivity for those units in which the
contaminants are migrating.
2.3.3.2 Hydraulic Gradient
The horizontal hydraulic gradient is the change in hydraulic head (feet of water) divided by the
distance of ground-water flow between head measurement points. To accurately determine the
hydraulic gradient, it is necessary to measure ground-water levels in all monitoring wells and
piezometers at a site. Because hydraulic gradients can change over a short distance within an
·aquifer, it is essential to have as much site-specific ground-water elevation information as possible
so that accurate hydraulic gradient calculations can be made. In addition, seasonal variations in
ground-water flow direction can have a profound influence on contaminant transport. Sites in
upland areas are less likely to be affected by seasonal variations in ground-water flow direction than
low-elevation sites situated near surface water bodies such as rivers and lakes.
To determine the effect of seasonal variations in ground-water flow direction on contaminant
transport, quarterly ground-water level measurements should be taken over a period of at least one
year. For many sites, these data may already exist. If hydraulic gradient data over a one-year period
are not available, natural attenuation can still be implemented, pending an analysis of seasonal
variation in ground-water flow direction.
2.3.3.3 Processes Causing an Apparent Reduction in Total Contaminant Mass
Several processes cause reductions in contaminant concentrations and apparent reductions in
the total mass of contaminant in a system. Processes causing apparent reductions in contaminant
mass include dilution, sorption, and hydrodynamic dispersion. In order to determine the mass of
contaminant removed from the system, it is necessary to correct observed concentrations for the
effects of these processes. This is done by incorporating independent assessments of these processes
into the comprehensive solute transport model. The following sections give a brief overview of the
processes that result in apparent contaminant reduction. Appendix B describes these processes in
detail.
Dilution results in a reduction in contaminant concentrations and an apparent reduction in the
total mass of contaminant in a system due to the introduction of additional water to the system. The
two most common causes of dilution (real or apparent) are infiltration and sampling from monitoring
wells screened over large vertical intervals. Infiltration can cause an apparent reduction in
contaminant mass by mixing unaffected waters with the contaminant plume, thereby causing dilution.
Monitoring wells screened over large vertical distances may dilute ground-water samples by mixing
water from clean aquifer zones with contaminated water during sampling. To avoid potential dilution
during sampling, monitoring wells should be screened over relatively small vertical intervals (e.g.
5 feet). Nested wells should be used to define the vertical extent of contamination in the saturated
zone. Appendix C contains example calculations showing how to correct for the effects of dilution.
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The retardation of organic solutes caused by sorption is an important consideration when
simulating the effects of natural attenuation over time. Sorption of a contaminant to the aquifer .
matrix results in an apparent decrease in contaminant mass because dissolved contamination is
removed from the aqueous phase. The processes of contaminant sorption and retardation are
discussed in Appendix B.
The dispersion of organic solutes in an aquifer is another important consideration when
simulating natural attenuation. The dispersion of a contaminant into relatively pristine portions of
the aquifer allows the solute plume to mix with uncontaminated ground water containing higher
concentrations of electron acceptors. Dispersion occurs vertically as well as parallel and perpendicular
to the direction of ground-water flow.
To accurately determine the mass of contaminant transformed to innocuous by-products, it is
important to correct measured contaminant concentrations for those processes that cause an apparent
reduction in contaminant mass. This is accomplished by normalizing the measured concentration
of each of the contaminants to the concentration of a tracer that is biologically recalcitrant. Because
chloride is produced during the biodegradation of chlorinated solvents, this analyte can be used as
a tracer. For chlorinated solvents undergoing reductive dechlorination, it is also possible to use the
organic carbon in the original chlorinated solvent and daughter products as a tracer. Trimethylbenzene
and tetramethylbenzene are two chemicals found in fuel hydrocarbon plumes that also may be
useful as tracers. These compounds are difficult to biologically degrade under anaerobic conditions,
and frequently persist in ground water longer than BTEX. Depending on the composition of the
fuel that was released, other tracers may be used.
2.3.4 Optional Confirmation of Biological Activity
Extensive evidence can be found in the literature showing that biodegradation of chlorinated
solvents and fuel hydrocarbons frequently occurs under natural conditions. Many references from
the large body of literature in support of natural attenuation are listed in Section 3 and discussed in
Appendix B. The most common technique used to show explicitly that microorganisms capable of
degrading contaminants are present at a site is the microcosm study.
If additional evidence (beyond contaminant and geochemical data and supporting calculations)
supporting natural attenuation is required, a microcosm study using site-specific aquifer materials
and contaminants can be undertaken.
If properly designed, implemented, and interpreted, microcosm studies c_an provide very
convincing documentation of the occurrence of biodegradation, Results of such studies are strongly
influenced by the nature of the geological material submitted for study, the physical properties of
the microcosm, the sampling strategy, and the duration of the study. Because microcosm studies
are time-consuming and expensive, they should be undertaken only at sites where there is considerable
uncertainty concerning the biodegradation of contaminants.
B iodegradation rate constants determined by microcosm studies often are higher than rates
achieved in the field. The collection of material for the microcosm study, the procedures used to set
up and analyze the microcosm, and the interpretation of the results of the microcosm study are
presented in Appendix C.
2.4 REFINE CONCEPTUAL MODEL, COMPLETE PRE-MODELING CALCULA-
TIONS, AND DOCUMENT INDICATORS OF NATURAL ATTENUATION
Site investigation data should first be used to refine the conceptual model and quantify ground-
water flow, sorption, dilution, and biodegradation. The results of these calculations are used to
scientifically document the occurrence and rates of natural attenuation and to help simulate natural
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attenuation over time. It is the responsibility of the proponent to "make the case" for natural
attenuation. This being the case, all available data must be integrated in such a way that the evidence
is sufficient to support the conclusion that natural attenuation is occurring.
2.4.1 Conceptual Model Refinement
Conceptual model refinement involves integrating newly gathered site characterization data to
refine the preliminary conceptual model that was developed on the basis of previously collected
site-specific data. During conceptual model refinement, all available site-specific data should be
integrated to develop an accurate three-dimensional representation of the hydrogeologic and
contaminant transport system. This refined conceptual model can then be used for contaminant
fate and transport modeling. Conceptual model refinement consists of several steps, including
preparation of geologic logs, hydrogeologic sections, potentiometric surface/water table maps,
contaminant and daughter product contour (isopleth) maps, and electron acceptor and metabolic
by-product contour (isopleth) maps.
2.4.1.1 Geologic Logs
Geologic logs of all subsurface materials encountered during the soil boring phase of the field
work should be constructed. Descriptions of the aquifer matrix should include relative density,
color, major and minor minerals, porosity, relative moisture content, plasticity of fines, cohesiveness,
grain size, structure or stratification, relative penneability, and any other significant observations
such as visible contaminants or contaminant odor. It is also important to correlate the results of
VOC screening using soil sample headspace vapor analysis with depth intervals of geologic materials.
The depth of lithologic contacts and/or significant textural changes should be recorded to the nearest
0.1 foot. This resolution is necessary because preferential flow and contaminant transport paths
may be limited to thin stratigraphic units.
2.4.1.2 Cone Penetrometer Logs
Cone Penetrometer Logs provide a valuable tool for the rapid collection of large amounts of
stratigraphic information. When combined with the necessary corroborative physical soil samples
from each stratigraphic unit occurring on the site, they can provide a three-dimensional model of
subsurface stratigraphy.
Cone penetrometer logs express stratigraphic information as the ratio of sleeve friction to tip
pressure. Cone penetrometer logs also may contain fluid resistivity data and estimates of aquifer
hydraulic conductivity. To provide meaningful data, the cone penetrometer must be capable of
providing stratigraphic resolution on the order of 3 inches. To provide accurate stratigraphic
information, cone penetrometer logs must be correlated with continuous subsurface cores. At a
minimum, there must be one correlation for every hydrostratigraphic unit found at the site. Cone
penetrometer logs, along with geologic boring logs, can be used to complete the hydrogeologic
sections discussed in Section 2.4.1.3.
2.4.1.3 Hydrogeologic Sections
Hydrogeologic sections should be prepared from boring logs and/or CPT data. A minimum of
two hydrogeologic sections are required; one parallel to the direction of ground-water flow and one
perpendicular to the direction of ground water flow. More complex sites may require more
hydrogeologic sections. Hydraulic head data including potentiometric surface and/or water table
elevation data should be plotted on the hydrogeologic section. These sections are useful in identifying
potential pathways of contaminant migration, including preferential pathways of NAPL migration
(e.g., surface topography and dip of confining layers) and of aqueous contaminants (e.g., highly
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transmissive layers). The potential distribution NAPL sources as well as preferential pathways for
solute transport should be considered when simulating contaminant transport using fate and transport
models.
2.4.1.4 Potentiometric Surface or Water Table Map(s)
A potentiometric surface or water table map is a two-dimensional graphic representation of
equipotential lines shown in plan view. These maps should be prepared from water level
measurements and surveyor's data. Because ground water flows from areas of higher hydraulic
head to areas of lower hydraulic head, such maps are used to estimate the probable direction of
plume migration and to calculate hydraulic gradients. These maps should be prepared using water
levels measured in wells screened in the same relative position within the same hydro geologic unit.
To determine vertical hydraulic gradients, separate potentiometric maps should be developed for
different horizons in the aquifer to document vertical variations in ground-water flow. Flow nets
should also be constructed to document vertical variations in ground-water flow. To document
seasonal variations in_ ground-water flow, separate potentiometric surface or water table maps
should be prepared for quarterly water level measurements taken over a period of at least one year.
In areas with mobile LNAPL, a correction must be made for the water table deflection caused by
accumlation of the LNAPL in the well. This correction and potentiometric surface map preparation
are discussed in Appendix C.
2.4.1.5 Contaminant and Daughter Product Contour Maps
Contaminant and daughter product contour maps should be prepared for all contaminants
present at the site for each discrete sampling event. Such maps allow interpretation of data on the
distribution and the relative transport and degradation rates of contaminants in the subsurface. In
addition, contaminant contour maps are necessary so that contaminant concentrations can be gridded
and used for input into a numerical model. Detection of daughter products not present in the
released NAPL (e.g., cis-1,2-DCE, VC, or ethene) provides evidence of reductive dechlorination.
Preparation of contaminant isopleth maps is discussed in Appendix C.
If mobile and residual NAPLs are present at the site, a contour map showing the thickness and
vertical and horizontal distribution of each should be prepared. These maps will allow interpretation
of the distribution and the relative transport rate of NAPLs in the subsurface. In addition, these
maps will aid in partitioning calculations and solute fate and transport model development. It is
important to note that, because of the differences between the magnitude of capillary suction in the
aquifer matrix and the different surface tension properties of NAPL and water, NAPL thickness
observations made at monitoring points may not provide an accurate estimate of the actual volume
of mobile and residual NAPL in the aquifer. To accurately determine the distribution of NAPLs, it
is necessary to take continuous soil cores or, if confidentthat chlorinated solvents present as NAPL
are commingled with fuels, to use cone penetrometer testing coupled with laser-induced fluorescence.
Appendix C discusses the relationship between actual and apparent NAPL thickness.
2.4.1.6 Electron Acceptor, Metabolic By-product, and Alkalinity Contour Maps
Contour maps should be prepared for electron acceptors consumed (dissolved oxygen, nitrate,
and sulfate) and metabolic by-products produced [iron (II), chloride, and methane] during
biodegradation. In addition, a contour map should be prepared for alkalinity and ORP. The electron
acceptor, metabolic by-product, alkalinity, and ORP contour maps provide evidence of the occurrence
of biodegradation at a site. If hydrogen data are available, they also should be contoured.
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During aerobic biodegradation, dissolved oxygen concentrations will decrease to levels below
background concentrations. Similarly, during anaerobic degradation, the concentrations of nitrate
and sulfate will be seen to decrease to levels below background. The electron acceptor contour
maps allow interpretation of data on the distribution of the electron acceptors and the relative transport
and degradation rates of contaminants in the subsurface. Thus, electron acceptor contour maps
provide visual evidence of biodegradation and a visual indication of the relationship between the
contaminant plume and the various electron acceptors:
Contour maps should be prepared for iron (II), chloride, and methane. During anaerobic
degradation, the concentrations of these parameters will be seen to increase to levels above
background. These maps allow interpretation of data on the distribution of metabolic by-products
resulting from the microbial degradation of fuel hydrocarbons and the relative transport and
degradation rates of contaminants in the subsurface. Thus, metabolic by-product contour maps
provide visual evidence of biodegradation and a visual indication of the relationship between the
contaminant plume and the various metabolic by-products.
A contour map should be prepared for total alkalinity (as CaCO3). Respiration of dissolved
oxygen, nitrate, iron (III), and sulfate tends to increase the total alkalinity of ground water. Thus,
the total alkalinity inside the contaminant plume generally increases to levels above background.
This map will allow visual interpretation of alkalinity data by showing the relationship between the
contaminant plume and elevated alkalinity.
2.4.2 Pre-Modeling Calculations
Several calculations must be made prior to implementation of the solute fate and transport
model. These calculations include sorption and retardation calculations, NAPL/water partitioning
calculations, ground-water flow velocity calculations, and biodegradation rate-constant calculations.
Each of these calculations is discussed in the following sections. The specifics of each calculation
are presented in the appendices referenced below.
2.4.2.1 Analysis of Contaminant, Daughter Product, Electron Acceptor, Metabolic By-product,
and Total Alkalinity Data
The extent and distribution (vertical and horizontal) of contamination, daughter product, and
electron acceptor and metabolic by-product concentrations are of paramount importance in
documenting the occurrence ofbiodegradation and in solute fate and transport model implementation.
Comparison of contaminant, electron acceptor, electron donor, and metabolic by-product
distributions can help identify significant trends in site biodegradation. Dissolved oxygen
concentrations below background in an area with organic contamination are indicative of aerobic
biodegradation of organic carbon. Similarly, nitrate and sulfate concentrations below background
in an area with contamination are indicative of anaerobic biodegradation of organic carbon. Likewise,
elevated concentrations of the metabolic by-products iron (II), chloride, and methane in areas with
contamination are indicative ofbiodegradation of organic carbon. In addition, elevated concentrations
of total alkalinity (as CaCO3) in areas with contamination are indicative ofbiodegradation of organic
compounds via aerobic respiration, denitrification, iron (III) reduction, and sulfate reduction. If
these trends can be documented, it is possible to quantify the relative importance of each
biodegradation mechanism, as described in Appendices B and C. The contour maps described in
Section 2.4.1 can be used to provide graphical evidence of these relationships. ·
Detection of daughter products not present in the released NAPL (e.g., cis-1,2-DCE, VC, or
ethene) provides evidence ofreductive dechlorination. The contour maps described in Section 2.4.1
in conjunction with NAPL analyses can be used to show that reductive dechlorination is occurring.
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2.4.2.2 Sorption and Retardation Calculations
Contaminant sorption and retardation calculations should be made based on the TOC content
of the aquifer matrix and the organic carbon partitioning coefficient (Koc) for each contaminant.
The average TOC concentration from the most transmissive zone in the aquifer should be used for
retardation calculations. A sensitivity analysis should also be performed during modeling using a
range ofTOC concentrations, including the lowest TOC concentration measured at the site. Sorption
and retardation calculations should be completed for all contaminants and any tracers. Sorption
and retardation calculations are described in Appendix C.
2.4.2.3 NAPL/Water Partitioning Calculations
If NAPL remains at the site, partitioning calculations should be made io account for the
partitioning from this phase into ground water. Several models for NAPL/water partitioning have
been proposed in recent years, including those by Hunt et al. (] 988), Bruce et al. (1991 ), Cline et al.
(1991 ), and Johnson and Pankow ( 1992). Because the models presented by Cline et al. (1991) and
Bruce et al. (1991) represent equilibrium partitioning, they are the most conservative models.
Equilibrium partitioning is conservative because it predicts the maximum dissolved concentration
when NAPL in contact with water is allowed to _reach equilibrium. The results of these equilibrium
partitioning calculations can be used in a solute fate and transport model to simulate a continuing
source of contamination. The theory behind fuel/water partitioning calculations is presented in
Appendix B, and example calculations are presented in Appendix C.
2.4.2.4 Ground-water Flow Velocity Calculations
The average linear ground-water flow velocity of the most transmissive aquifer zone containing
contamination should be calculated to check the accuracy of the solute fate and transport model and
to allow calculation of first-order biodegradation rate constants. An example of a ground-water
flow velocity calculation is given in Appendix C .
2.4.2,5 Apparent Biodegradation Rate-Constant Calculations
Biodegradation rate constants are necessary to accurately simulate the fate and transport of
contaminants dissolved in ground water. In many cases, biodegradation of contaminants can be
approximated using first-order kinetics. In order to calculate first-order biodegradation rate constants,
the apparent degradation rate must be normalized for the effects of dilution, sorption, and
volatilization. Two methods for determining first-order rate constants are described in Appendix C.
One method involves the use of a biologically recalcitrant compound found in-the dissolved
contaminant plume that can be used as a conservative tracer. The other method, proposed by Buscheck
and Alcantar (1995) is based on the one-dimensional steady-state analytical solution to the advection-
dispersion equation presented by Bear (1979). It is appropriate for plumes where contaminant
concentrations are in dynamic equilibrium between plume formation at the source and plume
attenuation downgradient. Because of the complexity of estimating biodegradation rates with these
methods, the results are more accurately referred to as "apparent" biodegradation rate constants.
Apparent degradation rates reflect the difference between contaminant degradation and production
which is important for some daughter products (e.g., TCE, DCE, and VC).
2.5 SIMULATE NATURAL ATTENUATION USING SOLUTE FATE AND TRANS-
PORT MODELS
Simulating natural attenuation allows prediction of the migration and attenuation of the
contaminant plume through time. Natural attenuation modeling is a tool that allows site-specific
data to be used to predict the fate and transport of solutes under governing physical, chemical, and
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biological processes. Hence, the results of the modeling effort are not in themselves sufficient
proof that natural attenuation is occurring at a given site. The results of the modeling effort are only
as good as the original data input into the model; therefore, an investment in thorough site
characterization will improve the validity of the modeling results. In some cases, straightforward
analytical models of solute transport are adequate to simulate natural attenuation.
Several well-documented and widely accepted solute fate and transport models are available
for simulating the fate and transport of contaminants under the influence of advection, dispersion,
sorption, and biodegradation.
2.6 CONDUCT A RECEPTOR EXPOSURE PATHWAYS ANALYSIS
After the rates of natural attenuation have been documented, and predictions from appropriate
fate and transport models indicate that MNA is a viable remedy, the proponent ofnatural attenuation
should combine all available data and information to provide support for this remedial option.
Supporting the natural attenuation option generally will involve performing a receptor exposure
pathways analysis. This analysis includes identifying potential human and ecological receptors and
points of exposure under current and future land and ground-water use scenarios. The results of
solute fate and transport modeling are central to the exposur~ pathways. analysis. If conservative
model input parameters are used, the solute fate and transport model should give conservative
estimates of contaminant plume migration. From this information, the potential for impacts on
human health and the environment from contamination present at the site can be assessed.
2.7 EVALUATE SUPPLEMENTAL SOURCE REMOVAL OPTIONS
Additional source removal, treatment, or containment measures, beyond those previously
implemented, may be necessary for MNA to be a viable remedial option or to decrease the time
needed for natural processes to attain site-specific remedial objectives. Several technologies suitable
for source reduction or removal are listed on Figure 2.1. Other technologies may be used as dictated
by site conditions and regulatory requirements. If a solute fate and transport model has been prepared
for a site, the impact of source removal can readily be evaluated by modifying the conta.minant
source term; this will allow for a reevaluation of the exposure pathways analysis.
In some cases (particularly if the site is regulated under CERCLA), the removal, treatment, or
containment of the source may be required to restore the aquifer as a source of drinking water, or to
prevent discharge of contaminants to ecologically sensitive areas. If a solute fate and transport
model has been prepared, it can also be used to forecast the benefits of source control by predicting
the time required to restore the aquifer to drinking water quality, and the reduction in contaminant
loadings to sensitive ecosystems.
2.8 PREPARE LONG-TERM MONITORING PLAN
This plan is used to monitor the plume over time and to verify that natural attenuation is
occurring at rates sufficient to attain site-specific remediation objectives and within the time frame
predicted at the time of remedy selection. In addition, the long-term monitoring plan should be
designed to evaluate long-term behavior of the plume, verify that exposure to contaminants does
not occur, verify that natural attenuation breakdown products do not pose additional risks, determine
actual (rather than predicted) attenuation rates for refining predictions of remediation time frame,
and to document when site-specific remediation objectives have been attained.
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The long-term monitoring plan should be developed based on site characterization data, analysis
of potential exposure pathways, and the results of solute fate and transport modeling. EPA is
developing additional guidance on long-term monitoring of MNA remedies, which should be
consulted when available.
The long-term monitoring plan includes two types of monitoring wells. Long-term monitoring
wells are intended to determine if the behavior of the plume is changing. Performance evaluation
wells are intended to confirm that contaminant concentrations meet regulatory acceptance levels,
and to trigger an action to manage potential expansion of the plume. Figure 2.7 depicts a schematic
that describes the various categories of wells in a comprehensive monitoring plan. Figure 2.7 is
intended to depict categories of wells, and does not depict monitoring well placement at a real site.
Included in the schematic representation are: 1) wells in the source area; 2) wells in unimpacted
ground water; 3) wells downgradient of the source area in a zone of natural attenuation; 4) wells
located downgradient from the plume where contaminant concentrations are below regulatory
acceptance levels but geochemical indicators are altered and soluble electron acceptors are depleted
with respect to unimpacted ground water; and 5) performance evaluation wells.
The final number and placement of long-term monitoring wells and performance evaluation
wells will vary from site to site, based on the behavior of the plume as revealed during the site
characterization and on the site-specific remediation objectives. In order to provide a valid monitoring
system, all monitoring wells must be screened in the same hydrogeologic unit as the contaminant
plume being monitored. This generally requires detailed stratigraphic correlation. To facilitate
accurate stratigraphic correlation, detailed visual descriptions of all subsurface materials encountered
during borehole drilling or cone penetrometer testing should be prepared prior to monitoring well
installation.
Dlssolved Contaminant Plume
Direction of Plume Migration
0 Long Term Monitoring Wells
e Performance Evaluation Wells
Figure 2.7 Hypothetical long-term monitoring strategy. Note that number and location of monitoring
wells will vary with the three-dimensional complexity of the plume(s) and site-specific
remediation objectives.
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Although the final number and placement of long-term monitoring wells and performance
evaluation wells should be determined through regulatory negotiation, the locations of long-term
monitoring wells should be based on the behavior of the plume as revealed during the site
characterization and on regulatory considerations. The final number and location of performance
evaluation wells will also depend on regulatory considerations.
A ground-water sampling and analysis plan should be prepared in conjunction with a plan for
placement of performance evaluation wells and long-term monitoring wells. For purposes of
monitoring natural attenuation of chlorinated solvents, ground water from the long-term monitoring
wells should be analyzed for the contaminants of concern, dissolved oxygen, nitrate, iron (II), sulfate,
and methane. For performance evaluation wells, ground-water analyses should be limited to
contaminants of concern. Any additional specific analytical requirements, such as sampling for
contaminants that are metals, should be addressed in the sampling and analysis plan to ensure that
all data required for regulatory decision making are collected. Water level and NAPL thickness
measurements should be made during each sampling event.
Except at sites with very low hydraulic conductivity and gradients, quarterly sampling of both
long-term monitoring wells and performance evaluation wells is recommended during the first year
to help determine whether the plume is stable or migrating, the direction of plume migration and to
establish a baseline for behavior of the plume. After the first year, an appropriate sampling frequency
should be established which considers seasonal variations in water table elevations, ground-water
flow direction and flow velocity at the site. If the hydraulic conductivity or hydraulic gradient are
low, the time required for ground water to move from upgradient monitoring wells to down gradient
monitoring wells should also be considered in determining the appropriate monitoring frequency.
Monitoring of long-term performance of an MNA remedy should continue as long as contamination
remains above required cleanup levels.
2.9 PRESENT FINDINGS
Results of natural attenuation studies should be presented in the remedy selection document
appropriate for the site, such as CERCLA Feasibility Study or RCRA Corrective Measures Study.
This will provide scientific documentation that allows an objective evaluation of whether MNA is
the most appropriate remedial option for a given site.
All available site-specific data and information developed during the site characterization,
conceptual model development, pre-modeling calculations, biodegradation rate calculation, ground-
water modeling, model documentation, and long-term monitoring plan preparation phases of the
natural attenuation investigation should be presented in a consistent and complementary manner in
the feasibility study or similar document. Of particular interest to the site decision makers will be
evidence that natural attenuation is occurring at rates sufficient to attain site-specific remediation
objectives in a time period that is reasonable compared to other alternatives, and that human health
and the environment will be protected over time. Since a weight-of-evidence argument will be
presented to support an MNA remedy, all model assuptions should be conservative and all available
evidence in support of MNA should be presented.
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SECTION 3
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Abriola, L.M., and Pinder, G.F., 1985a, A multiphase approach to the modeling of porous
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Abriola, L.M., 1996, Organic liquid contaminant entrapment and persistence in the subsurface:
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Acton, D.W., 1990, Enhanced in situ biodegradation of aromatic and chlorinated aliphatic
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Atlas, R.M, 1984, Petroleum Microbiology: Macmillan, New York.
Atlas, R.M., 1981, Microbial degradation of petroleum hydrocarbons -an Environmental
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Atlas, R.M., 1988, Microbiology -Fundamentals and Applications: Macmillan, New York.
ATSDR, 1990, Toxicological Profile for Hexachlorobenzene: Agency for Toxic Substances
and Disease Registry, USPHS/USEPA, December 1990.
Avon, L., and Bredehoeft, J.D., 1989, An analysis of trichloroethylene movement in ground-
water at Castle Air Force Base, California: J. Hydro!., 110:23-50.
Baedecker, M.J., and Back, W., 1979, Hydrogeological processes and chemical reactions at a
landfill: Ground Water, 17(5): 429-437.
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Water Works Assa., 76:52-59.
Wexler, E.J ., 1992, Analytical solutions for one-, two-, and three-dimensional solute transport
in ground-water systems with uniform flow: United States Geological Survey, Techniques
of Water-Resources Investigations of the United States Geological Survey, Book 3, Chap-
ter B7, 190p.
Wiedemeier, T.H., Benson, L.A., Wilson, J.T., Kampbell, D.H., Hansen, J.E., and Miknis, R.,
1996a, Patterns of natural attenuation of chlorinated aliphatic hydrocarbons at Plattsburgh
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of Chlorinated Solvents, Salt Lake City, UT, April 2, 1996 .
. Wiedemeier, T.H., Blicker, B., and Guest, P.R., 1994b, Risk-based approach to bioremediation
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Wiedemeier, T.H., Guest, P.R., Henry, R.L., and Keith, C.B., 1993, The use of Bioplume to
support regulatory negotiations at a fuel spill site near Denver, Colorado, In Proceedings
of the Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention,
Detection, and Restoration Conference: NWWA/API, p. 445 -459.
Wiedemeier, T.H., Miller, R.N., Wilson, J.T., and Kampbell, D.H., 1994a, Proposed Air Force
guidelines for successfully supporting the natural attenuation (natural attenuation) option
at fuel hydrocarbon contaminated sites: Presented at the 1994 NWWA/API Outdoor
Action Conference.
Wiedemeier, T.H., Swanson, M.A., Wilson, J.T., Kampbell, D.H., Miller, R.N., and Hansen,
J.E., 1995b, Patterns of intrinsic bioremediation at two United States Air Force Bases,
In Intrinsic Bioremediation: (Hinchee, R.E., Wilson, J.T. and Downey, D.C., Eds.),
Battelle Press, Columbus, OH.
Wiedemeier, T.H., Swanson, M.A., Wilson, J.T., Kampbell, D.H., Miller, R.N., and Hansen,
J.E., 1996b, Approximation of biodegradation rate constants for monoaromatic .hydro-
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of trichloroethylene in anoxic aquifer material from Pica tinny Arsenal, New Jersey,
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Monterey, CA: (Mallard, G.E., and Aronson, D.A., Eds.),Water Resources Investigation
Report 91-4034, p. 704-707.
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Biotransformation of monoaromatic and chlorinated hydrocarbons at an aviation gasoline
spill site: Geomicrobiology J., 8:225-240.
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(Averett, R.C. and Mcknight, D.M., Eds.), Lewis Publishers, Chelsea, MI, p. 125-137.
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76
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of Chlorinated Organics in Ground Water, Dallas, TX, September 11-13, 1996:
EPA/540/R-96/509.
Wilson, B.H., 1988, Biotransformation of Chlorinated Hydrocarbons and Alkylbenzenes in
Aquifer Material from the Pica tinny Arsenal, New Jersey. Proceedings of the Technical
Meeting, Phoenix, Arizona, September 26-30, 1988. U.S. GSWRIR 88-4220.389-394.
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American Society of Civil Engineers, J. Hydr. Div., 104(HY4):503-514.
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groundwater remediation technique: Ground Water Monit. Rev., Fall 1986, p. 56-64.
Wilson, J.T., Kampbell, D.H., and Armstrong, J., 1993, Natural bioreclamation of
alkylbenzenes (BTEX) from a gasoline spill in methanogenic groundwater: In Proceedings
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1985, Influence of microbial adaptation on the fate of organic pollutants in groundwater:
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Young, L.Y., 1984, Anaerobic degradation of aromatic compounds, In Microbial Degradation
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and application: Groundwater Monit. Remed., Fall 1995, p. 138-147.
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(Mitchell, R., Ed.), Wiley, New York, p. 349-376.
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I P:\PROJ\60313.011\drACCvr.doc
APPENDIXB
DESIGN CALCULATIONS
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Pilot Test Flow Test Data
and (SVE well) Condition Flow Vacuum
(scfm) (in.WC.)
Part 1 (SVE-1)
low 8.6 35
low 8.6 35
low 8.6 35
medium 15.8 48
medium 16.7 49
medium 16.6 50
high 26.6 74
high 25.6 74
high 25.6 74
Pneumatic Permeability (SVE-2)
low 9 42
low 9 46.5
medium 17.9 65
medium 17.9 65
high 21.9 75
high 20.9 75
~
SVE flow plots 050599.xls Sheet1 9/17/99
Averages
Flow
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8.6
16.4
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9.0
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Vacuum
(in.WC.)
35.0
49.0
74.0
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(!: I 5-/-J. elev,)
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♦
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C
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~
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Page 2 of 3
- - ----I!!!!! !!I!! == == ;;;a ;;;a fiiiiii liiil iiiii - ---
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y = 2.4508x + 21.959
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n2 -Bc9970 . . -~
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/
20 25 30
Page 3 of 3
I BROWN AND
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I CHART SHOWING FRICTION LOSS ANO VELOCITY IN PIPING
HANDLING AIR AT DENSITY OF 0.07 5 LBS. PER CU. FT.
JANDY DATA '' t00CI0
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0 29.92
500 29.36
.1000 . 28.80
1500 28.26
2000 27.72
2500 27.20
3000 26.68
3500 26.18
4000 25.68
4500 25.20
5000 24.72
5500 24.25
6000 23.79
6500 . 23.35
7000. 22.90
7500 22.48
1000 ' \ •1
\ I ! ...... V
3000
\Y \Yl \v ,"✓1~·. '( •ooo
100 ✓ ' I /
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FRICTION LOSS IN LBS. PE, SQ. IN. PER 100 FT. OF PIPING
flll(T!QN LOSS 15 0111ECHY i'.l!QPOIITIQNAl TO Alli' DENSITY .
f lb. per sq. in. = 2.036 in. mercury ot 32• F. = 2.311 ft. of.waler Of 70"F. = 27 .7 J. in. water ot 70° F.
. ·.·a,
V=.VK ·../P
· V = Velocity in feel per minute (rheonti~
cal mo.cimum)
VK = l J.,786 with P e:11preueci i,,
inches of mercury
VK = 4005 with P e:,iprened i,1
inche1o of water
P = Preuure
l in.Mercuryat32"F. = 1.136ft.waterot70"F.· = 13.63 in. water at 70° F.
VIC•=· Velocity co·nstont based on air den-
sity of :07 .. 95 lbs. per cu. ·11.
VK = 21,09.4 with P upreued in
lbs. per sq. in.
Coefficient· of discharge through roun ~
sharp edge orilice1 = .55 10 .65 wi1h .6)
aYeroge.
Designers and manufacturers of multistage centrifugal air and gas blowers/exhausters-portable and stationary industria:
vacuum cleaning systems.:..... pneumatic conveying equipment -dust collectors -Smooth-Flow tubing and fittings -
cOntinuous metal, rubber and plastic strip/sheet drye(S -steam and gas operat~ air heaters -valves and accessones.
AIR APPLIANCE DIVISION
~ .-~11 Si-Si 1\/] ½\\~ .
L__ ~ L.J ~_J L.__j vu ~ L...: \___:
INDUSTRIES; INC. A SUBSIDIARY OF HOFFMAN INTERNATIONAL
103 FOURTH AVENUE • NEW YORK 3, N. Y. • ORegon 7-3600 206 .
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AIR PURIFICATION ADSORBERS
1,000 -3,000 LB. ACTIVATED CARBON
MODELS
G-4
G-6
·.G-9
SPECIFICATIONS
MODEL G-4
CARBON: 1,000 lbs. DIMENSIONS: 45-1/2" 0 X 64" H SHIPPING WT: 1,500 lbs. Dry
MODEL G-6
CARBON: 1,800 lbs.' DIMENSIONS: 45-1/2" 0 x ea· H SHIPPING WT: 2,500 lbs. Dry
MODEL G-9
CARBON: 3,000 lbs.• DIMENSIONS: 60" 0 X 93" H SHIPPING WT: 3,500 lbs. Dry
FEATURES
• Low pressure drop.
• Epoxy lined mild steel construction .
• High activity carbon.
• Fork lift fittings for easy handling.
• 4"0 slotted inlet distributor.
• Acceptable for transport of hazardous
spent carbon.
OPTIONS
• Plastisol (PVC) lining.
• Interconnecting piping.
•0
p 36
A
E 32 s
s 28
D
R ,. ·o
p
20
I
N 16 C
H e 12
s
H • ,,
2 5' 0 4 ,"
0
G·9WITH
:9000 lbS
CARBON :a.
G·9WTTH
"'· ·2000 lb$
CARBON
• 2,000 lbs. option available
0 250 500 750 10UO USO 1500 1750 2000
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CORPORATION
51 Riverside Avenue 1 •800-242·115D • Fax # [phone] WeslDon CT D6880 Web AddrJ1Bs: http://www.1:arbtrol.eom. ~ Copynghl 1991 Carbtrol Corporation -11/15/96 · ·· · AT-411/#1
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· Figures 5-35 and 5-36 show that as in the other parts of the test program, both the
extracted air flow rate and the SVE wellhead vacuum remained relatively steady
. throughout the test with little to. no influence from the air sparging at AS-1. The vacua
measured in MP-lA through MP-SA show the influence that air sparging had at these
locations (see Figures 5-37 and 5-38). The measured vacuum decreased at each location
after the air sparging began (at 1.78 hours SVE run time). Negative vacua, i.e. positive
pressures, were measured in MP-3A, MP-4A, and MP-SA, to the north of the extraction
well. The highest pressure was measured at MP-3A and was greater than 25 inches W.e.
(the pressure gauge at that location had a maximum end range of25 inches W.e. and was
pegged for the last two data points shown in Figures 5-37 and 5-38). 'At 4.65 hours SVE
run time, the readings at MP-2A changed from vacuum to a slight pressure and remained
as pressure readings to the end of the test. Vacuum readings were maintained at MP-IA.
Analytical reports of the results for the voe analyses of the vapor samples collected
during test Part 4 are included in Appendix F. These reports were validated by the
independent validator, EDS. PeE was the only voe detected above the detection limit.
The PeE concentrations reported for the two vapor samples collected from the SVE well
prior to initiation of air sparging were 150 and 170 µg/L. The three samples collected
after initiation of air sparging had PeE concentrations of 140, 120, and 110 µg/L. Given
the inherent difficulties in collection and analysis of vapor samples, these five results are
not considered to be significanVy different. The sample collected from injected air into
AS-1 had a PeE result of 1.2:µg/L. This PeE result was flagged with al! analytical
qualifier as estimated.
5.5.6 Pneumatic Permeability Test Results
Results of extracted air flow rate and the measured wellhead vacuum are plotted in
Figures 5-39 and 5-40, respectively, as a function of SVE run time for the pneumatic
permeability test that was conducted using extraction well SVE-2, located inside the
textile plant. Each time the extracted air flow rate was manually changed, a
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Figure 5-41. VOC Concentration at SVE-2 Wellhead for Pneumatic Permeability Test (8/26/98)
q:\proj\0313.08\SVE-2 Pncum Pcrm.ids2/9/99 figOVA
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APPENDIXC
PRELIMINARY CONSTRUCTION COST ESTIMATE
P:\PROJ\60313.011\drACCvr.doc
I CONSTRUCTION OF PHASE I COST ESTIMATE
AIR SPARGING ANO SOIL VAPOR EXTRACTION REMEDIATION METHOD
I FCX SUPERFUND SITE OU3
STATESVILLE, NORTH CAROLINA
Ouanti!Y Units Unit Costs Line Item Costs
I Low High Low High Low High
DIRECT CAPITAL COST
I MOBILIZA TIQN/OEMOBILIZA TION LS $30,000 $70,000 $30,000 $70,000
AIR SPARGING WELLS2 2 EA $3,000 $10.000 $6,000 $20,000
SOIL VAPOR EXTRACTION WELLS2 10 10 EA $3,000 $7,000 $30,000 $70,000 n MONITORING WELLS2 12 12 EA $3,000 $7,000 $38,000 $84,000
WELL PROTECTION 12 24 EA $50 $75 $800 $1,800
D DEGON PAO EA $1,000 $2,000 $1,000 $2,000
DEGON WATER DlSPOSAL (OWNER) LS $2,000 $5,000 $2,000 $5.000
WELL CUTTINGS DISPOSAL (OWNER) LS $7,000 $10,000 $7,000 $10,000
m 3/4" AIR SPARGING PIPING AND APPURTENANCES S00 700 LF $0.44 $1 $820 $1,400
2~ SVE FEEDER PIPING AND APPURTENANCES 300 500 LF $3.80 $12 $2,840 $9,000
6M SVE HEADER PIPING AND APPURTENANCES 1000 1200 LF $15.00 $28 $18,000 $40,600 I (possible pipe welder rental fee) $24,000
PIPE INSULATION 30 50 LF $6.90 $8.25 $207 $413
PIPE HANGERS AND SUPPORTS LS $10,000 $18,500 $10,000 $16,500
I PACKAGED AIR SPARGE SYSTEM EA $14,200 $17,040 $14,200 $17,040
PACKAGED SOIL VAPOR EXTRACTION SYSTEM EA $17,000 $20,400 $17,000 $20,400
I INSTRUMENTATION LS $29,000 $37,000 $29,000 $37,000
CONTROL EQUIPMENT LS $100,000 $150,000 $100,000 $150,000
(includes PLC, PC, etc.)
EQUIPMENT ACCESS DOOR LS $3,300 $4,525 $3,300 $4,525 I EQUIPMENT FENCE 150 175 LF $26 $30 $4,125 $5,750
SURVEYING/CONSTRUCTION QC LS $5,000 $10,000 $5,000 $10,000
I CONCRETE RESTORATION LS $5,000 $10,000 $5,000 $10,000
(includes restoration of floors, disturbed soils, etc.)
VAPOR PHASE GRANULAR ACTIVATED CARBON 2 2 EA $6,000 $7,000 $12,000 $14,000
I DRIP TRAPS 5 10 EA $400 $600 $2.000 $6,000
PERFORMANCE AND WARRANTY BONDS LS $10,000 $40,000 $10,000 S-40,000
AIR PERMIT REQUIREMENTS EA $5,000 $15,000 $5,000 $15,000
I SUBTOTAL $351,000 $686,000
INDIRECT CAPITAL COSTS
I ADMIN. AND LEGAL @ 5% of Direct Costs Subtotal $18,000 $34,000
CONSTRUCTION SERVICES $75,000 $150,000
I CONTINGENCY @ 20% of Direct Costs Subtotal $70,000 $137,000
TOTAL CAPITAL COST $514,000 $1,007,000
I Notes: 1. Costs related to temporary Interruptions of production have not been included.
2. Cuttings to be disposed of by owner, containerized by con1ractor.
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ANNUAL OPERATION AND MAINTENANCE COSTS
AIR SPARGING ANO SOIL VAPOR EXTRACTION REMEDIATION METHOD
FCX SUPERFUND SITE OUJ
ANNUAL O&M cosr
GRANULAR ACTIVATED CARBON CHANGEOUT
A1R SPARGE SYSTEM ROUTINE MAJNTENANCE
SOIL VAPOR EXTRACTION ROUTINE MAINTENANCE
PROCESS OPERATION AND OVERSIGHT
NON-ROUTINE MAINTENANCE
ELECTRICITY
DISPOSAL OF PROCESS CONDENSATE
SOURCE AREA GROUNDWATER MONITORING
REPORTING (2 per year)
O&M CONTINGENCY @ 20% of Annual O&M Subtotal
OU3 NATURAL ATTENTUATION MONITORING
"O&M cost does not include Phase I performance testing.
PIP~OJ\80013011\,,..IMI.,.
STATESVILLE, NORTH CAROUNA
TOTAL ANNUAL O&M
Annual Costs
Low High
$64,000 $76,800
$2,000 $2,500
$2,000 $2,500
$10,000 $20,000
$25,000 $50,000
$8,000 $16,000
$500 $700
$40,000 $80,000
$8,000 $16,000
$32,000 $53,000
$192,000 $318,000
$60,000 $100,000