NCD095458527_19990407_FCX Inc. (Statesville)_FRBCERCLA RD_Preliminary Remedial Design Report OU-3-OCR

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Rt=r.FIVED
APR 09 1999
SIJPERFUNU St.C110N
PRELIMINARY 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
April 1999
27-W313.009
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I EC KEN FELDER~
• -AN INTEGRAL PART OF
BROWN AND
I CALDWELL
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227 French Landing Drive
Nashville, Tennessee 37228-1605
Tel: {615) 255-2288
Fax: [phone]
April 7, 1999
Mr. McKenzie Mallary
North Site Management Branch
EPA Region4
Atlanta Federal Center
61 Forsyth Street
Atlanta, GA 30303
p l=CEIVED60313.008
APR 09 1999
SUPERFUND SECTION
RE: Preliminary Remedial Design Report for Operable Unit Three (OU3)
FCX-Statesville Superfund iSite, Statesville, North Carolina
Dear Ken:
I Enclosed are four copies of the "Preliminary Remedial Design Report for Operable
Unit Three (OU3), FCX-Statesville Superfund Site, Statesville, North Carolina". For
your convenience, one of the rep~rt copies is provided in a three-ring binder.
If you have any questions regarding this document, please call Ms. Nancy Prince of
El Paso Energy Corporation at Cj13) 420-3306 or me at [phone].
Sincerely,
Brown and Caldwell
Kenton H. Oma, P.E.
Assistant Technical Director
Design and Solid Waste
cc: N. Testerman, NCDEHN~
N. Prince, El Paso
S. Miller, El Paso
J. Porter, The Porter Law Group ' H. Mitchell, Jr., Beaunit ,
J. Wright, Burlington
G. House, BPMH&L
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TABLE OF CONTENTS
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Letter of Transmittal
Table of Contents
List of Tables
List of Figures
1.0 INTRODUCTION
I.I
1.2
1.3
Site Background
1.1.1 Site Description
1.1.2 Site History
1.1.3 Site Conditions
Remedial Design Objectives
1.2.1 Soil Design Objectives :
1.2.2 Groundwater Design Objectives:
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Description of OU3 Remediation Technologies
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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
2.2
2.3
2.4
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Installation of Groundwater Monitoring Wells
Results of Groundwater Sampling and Analyses I
Evaluation of Natural Attenuation 1 Pilot Test Results
3.0 CONCEPTUAL DESIGN
3.1 Phase I Conceptual Design
3.1.1
3.1.2
3.1.3
3.1.4
3.1.5
3.1.6
Conceptual Design of Soil Vapor Extraction
Conceptual Design of Air Sparging
Conceptual Design of Monitoring Probes
Operational Testing of Soil Vapor Extraction I Operational Testing of Air Sparging
Source Area Monitoring '
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3.2
3.3
3.4
' TABLE OF CONTENTS (Continued)
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Phase II Conceptual Design
Conceptual Design for Monitored Natural Attenuation
Preliminary Design Documents 1
3.4.1 Preliminary Design Plans
3.4.2 Outline of Technical Specifications
4.0 REMEDIAL ACTION
4.1 Project Delivery Strategy
4.2
4.1.1 Remedial Action Work Plan
4.1.2 Remedial Action
Permit Requirements
4.2.1
4.2.2
4.2.3
4.2.4
Air Emissions from Soil Vapor Extraction
Soil Erosion and Sediment Control
Well Construction Permits I
Handling Potentially Contamimited Soil '
4.3 Preliminary Schedule
APPENDICES
Appendix A -Preliminary Design Plans
Outline of Technical Specifications ,
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Appendix B -
Appendix C-Technical Protocol for Evaluating Natural Attenuation of
Chlorinated Solvents in Groundwattir
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Table No.
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Figure No.
LIST OF TABLES
. I Title
Monitoring Wells Selected for Semi-Annual Groundwater
Sampling, Operable Unit Three (OU3), FCX-Statesville Superfund
Site
Summary of Chemical Analyses and Analytical Method References
for Semi-Annual Groundwater Sampling, Operable Unit Three
(OU3), FCX-Statesville Superfund Site
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' LIST OF FIGURES
Title I --,
1-1 Site Location Map
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Site Layout
Monitoring Well Location Map
' Conceptual Layout of Phase I SVE and Air Sparging Wells '
' Conceptual Layout of Phase II SVE Wells in Relation to Phase I
SVE Wells '
Preliminary Schedule for Remedial Action for Operable Unit Three
(OU3), FCX-Statesville Superfund Site
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1.0 INTRODUCTION
' This Preliminary Remedial Design (RD) Report provides a conceptual design and a
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preliminary schedule for performing the Remedi,al 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 accordanc~ with the "Remedial Design Work Plan
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for OU3 FCX-Statesville Superfund Site, North Carolina," dated July 1998 by
' ECKENFELDER INC. (now Brown and Caldwell). The OU3 RD addresses the
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remediation of the soils and groundwater associated with the property currently owned
i and operated by Burlington Industries, Inc. (Burlington). Operable Units One (OUJ) and
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Two (OU2) address soil and groundwater cqntamination 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
I I Corporation (El Paso). Subsequent phases, if required, will be conducted by the FCX-
' Statesville Superfund Site OU3 Respondent Gr9up (Group), which consists of El Paso
and Burlington.
Section 1.1 of this introduction provides background information and a brief overview of I
the Site conditions; Section 1.2 presents the RD objectives; and Section 1.3 provides a I description of the remedial technologies included in the selected remedy for OU3.
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Included in Section 2.0 of this report is a Jummary description of the pre-design
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investigation (PD!), which was performed in support of the RD ("Pre-Design
Investigation Report for Operable Unit T~ree, . FCX-Statesville Superfund Site,
' Statesville, North Carolina" dated March 1999 by Brown and Caldwell). The data from
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the PDI are summarized in the PD! report and' are not reproduced in this RD Report. I
Section 3.0 presents the conceptual design (also: considered to be the design criteria) for
OU3. Section 4.0 discusses the RA, includind the project delivery strategy, potential
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permit requirements, and the preliminary schedule.
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1.1 SITE BACKGROUND
The background information includes a Site: description, a Site history, and a brief
overview of the Site conditions.
1.1.1 Site Description
The OU3 Site is located in Iredell County approximately 1.5 miles west of downtown
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Statesville, North Carolina (see Figure 1-1 ). The Site consists of the soil, groundwater,
sediment, and surface water contamination e~anating from the textile. plant property
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currently owned by Burlington. The propertx is approximately 15 acres in size. Two
large buildings consisting of a warehouse (approximately 60,000 square feet in size) and
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the textile plant building (approximately 275,000 square feet in.size) are present on-Site I
(see Figure 1-2).
Land immediately surrounding the Site is predominantly industrial with a variety of other
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uses ranging from commercial to residential with associated school and church facilities.
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Farther from the Site, rural land in the States~ille 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 g~ntly rolling slopes. The Blue Ridge-Inner
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Piedmont Belt consists of metamorphic rocks including gneisses and schists. These rocks
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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.
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Saprolite forms the uppermost hydrogeologic: unit. Groundwater occurs within the pore
spaces of the saprolite under water table con:ditions. Groundwater within the fractured
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bedrock unit occurs under unconfined or s~mi-confined conditions. Site information
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FIGURE 1-1
SITE LOCATION MAP
FCX-STATESVILLE SUPERFUND SITE
STATESVILLE, NORTH CAROLINA
60313.009 3/99 ~ I ~ SOURCE: U.S.G.S. TOPOGRAPHIC J.lAP. STATESVILLE WEST OUAORANGL£, NC k ~
u... ------------r-Ncstwille. Tenna:r-
o .._ _______________ ~---..1...;;;ECKENFELD=;;,;;;:::E:::R~IN:;;.C:;;_.' _____ _;;"';.,,_.;.,;"-,..Now;..,..J~ ... ";:..J
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SITE LAYOUT
SUPERFUND SITE, 0U3 FCX-~i~i~~~f NORTH CAROLINA 3199 60313.009
BROWN AND
CALDWELL Na,hvllle, Tenne:nee
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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
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flowing both to the north and to the south from the textile plant.
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 knob 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 Beaun.it II, Inc. As a part of that transaction,
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Beaunit changed its name to BEM Holding 8orporation (BEM), and Beaunit II, Inc.
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changed its name to the Beaunit Corporation. In July 1978, the textile plant was sold by
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the Beaunit Corporation (formerly Beaunit II, Inc.) to Beaunit Fabrics Corporation
(Beaunit Fabrics). In 1981,
plant, from Beaunit Fabrics.
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Burlington purchased certain assets, including the textile
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Burlington presently owns the textile plant and is in the
process of closing the facility and offering the plant for sale.
In June 1993, the United States Environmental Protection Agency (USEPA) Region 4 I
· signed an Administrative Order on Consent for OU3 with Burlington, as well as the
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former property owner, El Paso; hence the OU3 Respondent Group consists of El Paso
' and Burlington. The USEP A Region IV issued the Final Record of Decision (ROD) for
OU3 in September 1996. The Consent J;lecree (CD) for OU3 was lodged on
' December 18, 1997, and became final on April 1, 1998. The USEPA issued an
' ' Explanation of Significant Difference (ESD) for OU3 on March 24, 1998.
1.1.3 Site Conditions
Several media and constituents of interest are associated with OU3. The pnmary
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constituents of interest present within OU3 include perchloroethylene (PCE), also called
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tetrachloroethene, and other chlorinated hydrocarbons. The groundwater contains
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primarily PCE and other volatile organic compounds (VOCs). On-Site soil contains
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primarily VOCs and to a lesser extent, inorganics and polynuclear aromatic hydrocarbons
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(P AHs). Surface water and sediment associated with an intermittent stream originating
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from the seep to the north of the Burlington textile plant also contains some inorganic
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constituents, polychlorinated biphenyls (PCBs),land VOCs.
1.2 REMEDIAL DESIGN OBJECTIVES
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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 ofVOC-
' 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 POI was performed and included
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installation of additional monitoring wells, groundwater sampling, an evaluation of
' natural attenuation, and a pilot-test of SVE and air sparging. The POI has provided
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additional data needed for the Preliminary RD development. The additional data that was
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obtained provides information critical to preparing the design of the selected remedial
components.
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The design objectives for soils and groundwater in OU3 are provided below.
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1.2.1 Soil Design Objectives
Elevated levels of several constituents, primarily VOCs, are present in the soil of OU3.
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No cleanup levels have been established for oniSite impacted soil; however, the objective
of the soil RD is to minimize the potential fo~ vapor transport and infiltration of VOCs
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from the soil into the groundwater using SVE technology.
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1.2.2 Groundwater Design Objectives
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Groundwater containing VOCs has been identified m the shallow saprolite and
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intermediate bedrock aquifers. Air sparging was selected in the ROD to treat
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grotmdwater constituents of concern by removing VOC mass and controlling migration
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in order to meet Federal Maximum Contaminant Levels (MCLs) or the North Carolina
Groundwater Standards, whichever are more prdtective. These will be referred to as the
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' ROD MCLs in this report. The objective of the RD for groundwater is to design an air
' sparging remediation for impacted groundwater pased upon the results of the air sparging
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and SVE (AS/SVE) pilot test that was conductec\ as part of the PD!.
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Sampling and analysis of the groundwater wil1 be performed to monitor the OU3 RA
performance as well as to monitor the extent and effectiveness of natural attenuation.
' The ESD to the OU3 RA ( dated March I 998)1 incorporates the institutional control of
restrictive covenants in the OU3 remedy. The
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purpose of the institutional control is to
prohibit the consumption of impacted groundw4ter (associated with OU3) from drinking
water wells.
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1.3 DESCRIPTION OF OU3 REMEDIATION TECHNOLOGIES
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Various technologies were reviewed in the ROD for remediation of OU3 including air
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sparging, SVE, and groundwater extraction andltreatment. The remediation technologies
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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
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volatilize and remove VOCs. In the most commonly practiced method of application, a
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blower (e.g., a vacuum source) is attached to Jn SVE well which is screened across the
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impacted interval of the vadose zone. The b,lower creates a partial vacuum (reduced
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pressure) within the well and induces air flow. from the surrounding soils towards the
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SVE well. As the air moves through the impa~ted soils, the portion of the VOCs that is
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present in the vapor phase flows towards the SVE well and is removed through the well
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along with the extracted air. The VOCs associated with the soils and present as free
phase liquids ( either between the soil particlbs or present as a layer on top of the
' groundwater) will gradually partition (volatilize) into the surrounding soil gas and will be
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extracted with the recovered air. When appropriate, based on regulations and VOC
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concentrations in the extracted air, an off-gas treatment system is incorporated as part of
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the SVE process.
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The SVE technology has been used widely at !Comprehensive Environmental Response
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Compensation and Liability Act (CERCLA)'., Resource Conservation Recovery Act
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mandated sites. 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 SVOCs and
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non-volatile compounds is limited to the extent that these compounds are biodegradable
by aerobic microorganisms.
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
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lots, and other man-made structures. Furthermore, the process removes contaminant
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mass with minimal potential to spread the contamination. In some cases SVE can also I .
serve to prevent migration of vapors into basements and utility trenches. Many VOCs are
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relatively easily vaporized from the absorbe'd 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 systems. Installation and
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operation of SVE systems are relatively straightforward making the technology cost-,
effective for many site conditions especially for very large soil treatment areas.
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The SVE technology has been implemented as part of multicomponent remedial systems
in conjunction with air sparging and monitor~d natural attenuation as well as other
technologies. When used as an integral part of air sparging, SVE can remove existing '
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 Extractioh
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' Air sparging introduces air into groundwater to remove voes. This is accomplished
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through injection of air under pressure through small diameter wells that are screened
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within or below the contaminated interval and/or near the base of the aquifer. The . ' I 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 r~ach the vadose zone, they are typically
removed by SVE. The introduction of air necessarily introduces oxygen into
groundwater. · This increases the oxidation/redhction potential (Eh) of the groundwater
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and promotes aerobic degradation of aerobically degradable compounds such as benzene
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and toluene. Added oxygen can also promote c9metabolism of some chlorinated solvents
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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 ryeed to be maintained.
Air sparging is most effective when operated in a pulse mode (air injection in individual
wells operated on 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
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I occurs when air flow is initiated, serves to mix the groundwater and enhance the removal
ofVOCs. I
i The effectiveness of air sparging is dependent upon many factors including the Henry's
law· constant of the specific constituents as Jell as both the initial concentration of
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 zo~e. As a result, individual wells may
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remediate relatively small or large areas based on differences in permeability that are not
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necessarily evident from well installation logs.' Pilot testing, as was performed in the
PDI, is typically required to determine the ard of influence of individual wells. There
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can be large variability across even a small site( which will not necessarily be identified
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by the pilot testing.
1.3.3 Monitored Natural Attenuation
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Natural processes that reduce the mass and ccincentrations of the chlorinated organics . I
present in OU3 have been observed in Site groundwater samples. For this reason,
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monitored natural attenuation is being evaluated to determine the extent to which it may
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complement the active remediation technologies of SVE and air sparging being evaluated
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at the Site. Monitored natural attenuation, also 'referred to as intrinsic remediation, relies I
on the natural restorative capacity of aquifers ttj control migration and reduce the masses
' of constituents of concern. The mechanisms that contribute to natural attenuation include I
adsorption, diffusion, dispersion, volatilization, and degradation. I
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The hydraulic conductivity, gradient, and poro'sity of the aquifer determine the rate of I
groundwater flow. As the groundwater moves through the aquifer, mixing of affected
groundwater with clean groundwater occurs a~ a result of dispersion and, to a lesser I
extent, diffusion. These processes result in somewhat lower constituent concentrations
I and marginally broader plumes. As the constituents move through the aquifer they
I adsorb to the aquifer materials, especially when appreciable organic content is present,
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and subsequently desorb (dissolve). The adsorption/desorption process retards the rate at
' . which constituents move through the aquifer. Af a result, constituent migration is slower
than otherwise would occur as a consequence qf groundwater flow through the aquifer.
' The processes of diffusion, dispersion, ~nd retardation moderate constituent
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concentrations but do not cause a reduction in cc\nstituent mass. Chemical and biological I
degradation reactions reduce both mass and cpncentrations of the degradable organic
constituents. For chlorinated aliphatic hydrocarbons (e.g., PCE and TCE), the process of
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degradation in groundwater occurs largely thro~gh anaerobic (in the·absence of oxygen)
biodegradation. The specific process is referred to as reductive dechlorination. In this
process, the chlorine atoms on chlorinated 6thenes are sequentially replaced with '
hydrogen atoms. This anaerobic process is as follows:
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PCE ➔ TCE ➔ DCE ➔ Vinyl Chloride ➔ Ethene
' In addition to the anaerobic process, DCE, vinyl chloride, and ethene can also be
biodegraded aerobically, ultimately yielding chloride ions, carbon dioxide, and water.
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The reductive dechlorination process requires the presence of other degradable organic I compounds and species referred to as electron, acceptors, and appropriate geochemical
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conditions. According to the protocol, these parameters, as well as the presence and
distribution of the chlorinated solvents and degradation products, should be measured
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from appropriate monitoring wells and interpreted as part of the natural attenuation
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evaluation. The USEPA has issued appropriate guidance in the document entitled
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"Technical Protocol for Natural Attenuation of1Chlorinated Solvents in Ground Water"
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which was developed in conjunction with the U.S. Air Force Center for Environmental
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Excellence (AFCEE). USEP A Region 4 has incorporated that document into the recently
issued "Draft Region 4 Approach to Natural Attbuation of Chlorinated Solvents." Both
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of the documents provide useful guidance to evaluate specific sites for the potential for
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monitored natural attenuation to be incorporated into the Site remedy. Another USE~A ' .
document, OSWER Directive 9200.4-17 entitled "Use of Monitored Natural Attenuation
I at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites" clarifies
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USEPA's policy regarding the use of"monitored natural attenuation" for the remediation
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of contaminated soil and groundwater. The OSWER directive was appended to the PD! I
report (Brown and Caldwell 1999) and th~ AFCEE document is reproduced in
Appendix A.
It is necessary to evaluate the extent to which t~e 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
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required for monitored natural attenuation. , The primary objective of long term
monitoring is to observe whether the natural attfnuation processes along with any active
remediation are serving to reduce or limit expansion of the plume.
\\BCNSI I0J\PROJECTS\PROJ\OJ I J. 09\s0 I .doc 1-10 '
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2.0 PRE-DESIGN INVESTIGATION
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A Pre-Design Investigation '(PD!) Report was prepared and describes investigation work
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that was performed in support of preparation of'. the OU3 RD. This section provides a
' brief summary of the PDI. For more detailed information, refer to the PDI report "Pre-
Design Investigation Report for Operable Unit Three (OU3), FCX-Statesville Superfund
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Site, Statesville, North Carolina" dated March (999 by Brown and Caldwell. . The PDI
work included installation of additional grounqwater monitoring wells, sampling and
analysis of groundwater from selected monit~ring wells, an evaluation of natural
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attenuation at the Site, and a pilot test of air sparging and SVE.
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2.1 INSTALLATION OF GROUNDWATERiMONITORING WELLS
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Groundwater within the saprolite and intermediate bedrock aquifers associated with the
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Site generally flows both to the north and to t~e south creating two potential transport
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mechanisms from the Site. Additional shallow liud intermediate (saprolite and bedrock)
groundwater monitoring wells were required to I define the horizontal and vertical extent
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of the constituents of concern in the OU3 groundwater.
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Two new wells, W-31 s and W-31 i, were installed as a couplet to further delineate the
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downgradient extent of the groundwater plumel to the north (see Figure 2-1 ). The well
couplet consists of a shallow monitoring well screened within the saprolite and an
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intermediate monitoring well screened within the underlying bedrock unit. A third new
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well, W-32i, screened within the upper bedro6k (intermediate zone), was installed to
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further delineate the downgradient extent of the groundwater plume to the south. To
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further evaluate the vertical extent of the· groundwater plume to the north, a monitoring
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well, W-20d, was installed in the plume to the 1north adjacent to the existing monitoring
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well couplet W-20s and W-20i.
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Legend
300 0 300 600
SCALE FEc:T
(
Shallow Monitoring Well Location
Intermediate Monitoring Well Location
Deep Monitoring Well Location
z-----+-
FIGURE 2-1
MONITORING
LOCATION
WELL
MAP
FCX-STATESVILLE SUPERFUND SITE, OU3
STATESVILLE, NORTH CAROLINA
60313.009 3/99
BROWN AND
C~"l..LDWELL :ieshville, Tennel!l::iee
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' ' 2.2 RESULTS OF GROUNDWATER SAMPLING AND ANALYSES
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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
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constituents of concern, to evaluate metals concentr*ions, and to measure biodegradation
parameters within the groundwater plume. As part :of this sampling, potable water from
two residential groundwater drinking wells located ~owngradient of the Site was sampled
in order to establish a broader database of groundwater quality.
The groundwater samples were analyzed for the desi'gnated parameters, which sometimes
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differed between wells and sampling events depending on the purpose of the sample.
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' The chemical tests and analytical parameters (including the basis for selecting those
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parameters) are presented in the PD! report and incl,ude voes, metals, pesticides, and a
suite of natural attenuation parameters.
In general, the voe results from this PD! were consistent with the RI results from 1994
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' and I 995 with some exceptions. A summary of the groundwater results and observations
is as follows.
• No voes or pesticides were reported that dxceeded the ROD MeLs for 003 in
the three new downgradient monitoring wells (W-3ls and W-3li to the north
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and W-32i to the south). This indicates th~t the horizontal extent of the plume
has been defined in the downgradient directions to the north and south of 003.
• At monitoring well W-20d, which was installed to assess the vertical extent of
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the groundwater plume to the north of the :site, the concentrations of PeE and
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1,2-dichloropropane were elevated. Interval packer testing during installation of
W-20d indicated that there are at least 40 f~et of media with significantly lower
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permeability that separate the upper fractured unit from the deeper groundwater
' unit (the well was screened above the media with lower permeability). As a
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result, the groundwater quality data from W-20d are considered representative
' of the vertical extent of the groundwater plume at this location.
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• No VOCs were detected in the two residential water supply wells that were
sampled downgradient of the Site.
• The PD! sample results showed PCE conc~ntrations at downgradient monitoring
wells W-20s and W-29i to be higher tha1,1 the RI sample results. The higher
results in these wells may represent changes in groundwater elevations,
sampling techniques, laboratory procedures, or may represent processes
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occumng within the aquifer. Similar variations in sampling results were
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observed for W-30i, which was used as a ,control well. Alternatively, the PD!
' 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
I 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
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conclude, before any more sampling events have been performed, that these
variations have any significance.
° Concentrations of aluminum, iron, and manganese m the slow purge and I
unfiltered samples were elevated. Even thqugh slow purge sampling was used,
I fine mica flakes from the saprolite formation were observed in the samples. I
' 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 att~nuation 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 unrel.ated to the manufacturing process at
the facility and is not considered to be a Site-wide issue. I
2.3 EVALUATION OF NATURAL ATTENUATION
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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 an1d physical processes, as well as an
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attempt to quantify the contributions from the bio,degradation and physical processes.
The evaluation process was applied to what might bJ considered four plume areas. These
I consist of the shallow saprolite saturated interval ~o the north and to the south of the
I groundwater divide, and the intermediate bedrock ~aturated interval to the north and to
the south of the groundwater divide. Evidence that natural attenuation is occurring at the
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Site is as follows.
• The groundwater quality data from the RI and the· PD! were evaluated to
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identify the presence snd relative concentrations of constituents of concern I
(especially PCE) and reductive dechlorination products. Reductive I . . I dechlorination products are present across the plume. · In some areas the ratio of
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the reductive dechlorination products relat/ve to the parent compound, PCE, is
fairly high, suggesting extensive redJctive dechlorination. Trends in
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concentrations over time and along the, groundwater flow path provide a
' semi-quantitative understanding of the extent to which reductive dechlorination
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' is limiting the migration of groundwate'r constituents in the downgradient I
direction.
Another indication of natural attenuation is whether the plume has reached a
dynamic equilibrium or steady state conqition, i.e., are the mechanisms that
retard migration and destroy constituent mass in an approximate equilibrium
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with the mechanisms of dissolution and advection that result in migration? The
site-wide water quality data (with a fe~ exceptions that require additional
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sampling to determine if variations have any significance) suggest a fairly
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constant plume. This is based on a comparison of PCE concentrations reported
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during the RI sampling events (1994 throu1,sh 1996) to those reported during the
PD! sampling events (1998 and I 999). Some variation was observed and is
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anticipated due to normal variability associated with groundwater
characterization. The. relative stability df the plume is not surpnsmg since
chlorinated solvent plumes where biodeg~ adation is occurring typically reach
equilibrium over time.
• A USEP A 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
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migration ofVOCs.
Active remediation of the source area may alter the Site geochemistry. As a result,
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natural. attf:nuation -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 I
the source area at least over the near future if air sparging were implemented. Air
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sparging has the potential to slightly impact natural I attenuation in directions away from
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the source area.
2.4 PILOT TEST RES UL TS
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A pilot test was conducted in the apparent source area at OU3 to evaluate air sparging
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and SVE. Air sparging was performed within the saprolite at two depths, 50 feet and 66
I • feet. Five monitoring probe clusters were installed around the air sparging wells and the
SVE well. The pilot test objectives were to in':estigate and measure the physical
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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
I 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.
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
indicates that an observational ( or phased) approach to the design and implementation of
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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 th-::
•
SVE well. Thus, SVE performance in the '.vicinity of any well (whether outside
of the building or inside the building) is expected to be asymmetrical, e.g., air
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flow and the lateral distance of influence will not be the same in all directions
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and will not be predictable.
The pneumatic permeability range of the ~adose zone soil in the SVE well is
I almost identical to that of the well located inside the building. Therefore,
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performance of SVE with wells underneath the building can be expected to
behave similarly with regard to achievable' flow rates and wellhead vacuum for
similarly designed wells. Subsurface infrastructure at the Site is anticipated to
have at least some influence on the perfor~ance 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
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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
\\llCNSI I0J\PROJECTS\J>ROJ\0J 13. 09\502.doc 2-6
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some of the vr.dose zone monitoring probes. Air sparging reduced the radius of '
influence of a single SVE well from the rmige of22-to 59 feet to the range of 12
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to 54 feet., This may be less important where an array of SVE wells is installed.
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' ' • 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 la~ers within the -saturated zone which
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inhibit injected air movement to the vadose zone, especially at the deeper
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sparging depth of 66 feet. Further evidence of the heterogeneity of the saturated
I ' . zone is supplied by the variability of the V\)C results from the pre-and post-test
groundwater sampling and by the vatiability 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
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depth of 66 feet. Consequently, careful pl~cement of SVE wells using a phas,~d
approach should be considered to maximiz~ the capture of injected air.
• Air sparging has the potential to inhibit natural attenuation if the injected air
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tra·,•erses long distances in the saturated zone or if dissolved oxygen traverses
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downgradient.
' • The VOC data indicate that VOCs were being removed during SVE only and
dming air sparging with SVE. The variability of the data and the types of data I ' collected do not allow a quantitative calculation of the mass of VOCs removed
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from the vadose zone or the groundwater.
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3.0 CONCEPTUAL DESIGN
Based on the. heterogeneity .of the Site observed 4uring the PD!, an observational (or
phased) approach to design and construction of an AS/SVE system will be utilized. This
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type of phased approach consists of the installati.on of an initial system using well
separations that are based upon anticipated Site conditions. The initial installation would '
then be operated. and monitored prior to additional installation. The operation
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information from each phase of installation will be considered and incorporated into I .
subsequent phases as appropriate. The RD will be !b,ased on this observational (phased)
approach assuming that there will be one or more phases of installation ( e.g., Phase I and
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Phase II, etc.). The process equipment (i.e. compressors, blowers, air emissions control '
devises, etc.) will be sized to accommodate conti~gent air sparging and/or SVE well
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installation which may be necessary if a Phase
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II installation is required. Piping
manifolds will. also be designed with spare ports to !accommodate contingent wells. It is
' anticipated that the air sparging wells and SVE we,lls associated with the RD will be in ,
the PCE source area located underneath the existing:textile plant building,
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In addition to Site-wide monitored natural attenuation, the natural attenuation parameters
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in the source area will be monitored to determine if the implementation of air sparging .
alters or acts to hinder the natural attenuation obseived to be occurring at the Site. The
sampling frequency for monitored· natural attenuation will also be evaluated after an
initial period to determine if either more or less freq'uent sampling events are warranted.
3.1 PHASE l CONCEPTUAL DESIGN
The first phase will include installation of an SVE system, air-sparging system, and a
network of monitoring probes for checking the iinpact of the selected remedy on the
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source area. Figure 3-1 shows the conceptual locations of the SVE and air sparging
' wells. The proposed locations of the monitoring probes.will be provided in the Final RD.
I ' As part cf 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.
\\BCNSH0)\PROIECTS\PROI\Ol l 3. 0<:J\.03 .doe 3-1
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7 LEGEND ,o
/ (!J :::,
X
100
Potential SVE Well Location
(Illustrated with 50' Radius af Influence)
Potential Air Sparging Well Location
Tetrachloroe\hene (PCE) Shallow
Groundwater lsoconcentration (ppb) Contours
(Dashed where Inferred)
NOTE: 1. lsoconcentration contour information taken
from "Final Remedial Investigation Report,
FCX-S\a\esville Superfund Site Operable
Unit 3, Statesville, North Carolina"
Aquaterra, Inc., 1996
2. Air sparging and SVE equipment shall be
located inside, or adjacent to the existing
textile plan\ building.
200
FEET
FIGURE 3-1
CONCEPTUAL LAYOUT OF
PHASE I SVE AND
AIR SPARGING WELLS
FCX-STATESVILLE SUPERFUNO SITE, OU3
STATESVlLLE, NORTH CAROLINA
60313.009 3/99
BROWN AND
CALDWELL Nasnville. Tennessee
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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.
3.1.1 Conceptual Design of Soil Vapor Extraction
As stated above, the design and installation of the SVE system will utilize an
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observational approach. The Phase I installation of SVE will cover that area bounded by
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the 10,000 µg/L PCE isoconcentration line depicted in Figure 3-1, plus a buffer area to
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accommodate recovery of the injected air. It 'is proposed that ten SVE wells be installed
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in a triangular grid with an assumed radi:us of influence of 50 feet (i.e. I 00 foot
separation). The radius of influence was s6lected based on the calculated radius of
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influence data from the PD! report. The outer SVE wells will be placed 100 feet beyond
the air sparging wells. The exact locations of these wells will be dependent upon the
owners operations inside the plant. Therefore, the locations (and possibly the number of
' wells) may change from what is depicted in Figure 3-1. The detailed designs for the SVE
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wells will be included with the final RD report. An automated control system will be
' designed for operation of the SVE wells. T9e control system will have automatic data
acquisition and process control capabilities: as well as providing remote access to
authorized users.
In addition to the SVE wells, vapor transport and discharge systems will be designed.
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The blower will be designed to provide vacuum to the vadose zone. It is anticipated that
one or more blowers will provide vacuum to [the SVE wells through a piping manifold
' system. The extracted vapors will be piped t:, an· air emissions control device. Even
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though the extracted vapors are not expected to exceed any V OC discharge standards, thP.
' RD will include an air emissions control d~vice. Control technologies that will be
evaluated include carbon adsorption, thermal o~idation, and catalytic oxidation.
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3.1 .2 Conceptual Design of Air Sparging
As part of the RD, two air sparging wells will be installed during Phase I at the time of
the SVE well installation. The air sparging 'Yells will be installed in close proximity to
SVE wells in the central portion of the plume rn,000 µg/L PCE isoconcentration line (see
' Figure .3-J ). The air sparging wells wil.l be screened in the shallow aquifer
(approximately 50 feet below ground surface based on the PD! pilot test results). An air
compressor system will be designed to suppl~ clean air to the air sparging wells and to
' any pneumatic controls within the system. Piping, valves, instrumentation, and other
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ancillary equipment will also be designed to, support the air sparging portion of the
remedy. The automatic control system will be capable of providing process control and
data acquisition as mentioned previously for SVE. It is anticipated that air sparging will
be performed intermittently using pulsed flow.1 The control system will be programmed
to provide automatic control of the air flow rate: and the air pulsing sequence.
As stated previousiy, the air sparging and SVE:wellswill be installed in a triangular grid
pattern. If it is determined that additional wells are necessary, new wells will also be
installed in a grid arrangement between the original grid points.
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3.1.3 Conceptual Design of Monitoring Probes
Monitoring probes will be installed within the a,nticipated area of influence as part of the
' Phase I installation. The monitoring probes will be used to gather data to be used in
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evaluating the system performance and the need for additional wells in the Phase II
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installation, if determined necessary. The RDI, will provide the locations, design, and
' instrumentation for th~ monitoring probes.
3.l:4' Operational Testing of Soil Vapor Extraction
'
During the initial operation, each SVE well will',be operated independently and data will
I be collected to allow radius-of-influence calculations at each well location. Next, the
\\BCNSH0J\PROJECTS\PROJ\OJ I 3.09\s0J. doc 3-3
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SVE system will be operated without air sparging for a trial period (a minimum trial
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period of about six months is anticipated). The operational protocol will be adjusted as
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necessary to balance the vacuum between the SVE wells in the vadose zone. After
reaching. steady state, the SVE configurat_ion will be evaluated to determine the
effectiveness of voe removal. Provisions will be made to adjust the operational
' protocol during this trial period if necessary. , ·
3.1.5 Operational Testing of Air Sparging ,
After the initial trial period of the SVE syste(n, air sparging will be operated with SVE
for another trial period (a brief trial period' for air sparging of about two weeks is
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anticipated). The radius of influence for each air sparging well will be determined and
I the combined radius of influence for both wells will be measured. Helium tracer tests
and groundwater upwelling measurements will be used to calculate the air sparging
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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 I will include evaluation of the data and
l adjustments to improve the system performanye. Provisions will be made to adjust the
' operational protocol if necessary.
3.1.6 Source Area Monitoring
Groundwater data will be collected from the au sparging wells and from selected
monitoring probes before and after the air sparging trial period. The groundwater will be
tested for voes and natural attenuation parameters. The RD will .present a plan for the
source area sampling and analysis that will be p~rformed.
3.2 PHASE II CONCEPTUAL DESIGN
If implementation of Phase II is determined to be needed, then additional SVE wells, air
' sparging wells, and/or monitoring probes would be installed. The Phase II conceptual
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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 II ;-veils might be located in relationship to the
Phase I grid layout. The anticipated radius-of-influence of the Phase II SVE wells would
be approximately 29 feet (as compared to 5? feet for the Phase I SVE wells). If this
Phase II grid layout were to be completely implemented, then nine additional SVE wells
would be installed for a total of 19 SVE wells (from both Phase I and Pha3e II). The
exact locations of these wells will be dependent upon the owners operations inside the
plant. Therefore, the locations (and possibly the number of wells) may change. The need '
for. and location of additional air sparging i wells and monitoring probes would be
' determined based on the Phase I data and only those Phase II wells which were needed
would be installed. The Final RD will provide .a protocol for evaluating the Phase I data.
3.3 CONCEPTUAL DESIGN FOR MONITORED NATURAL ATTENUATION
The natural attenuation portion of the RD would be implemented by performing semi" .
annual sampling of selected monitoring wells
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,for natural attenuation parameters. The
selected monitoring wells, sample frequency,, and analysis would be reevaluated and
' adjusted as appropriate after the first two years of monitoring and again after five years of
monitoring. In addition, adjustments to the sampling would be needed to monitor the
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source control once the remedy is in place. Th~ 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 (QNQC) sampling and analysis in
accordance with a Quality Assurance Project Plan,
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\\DCNSI I0J\PROJECTS\PROJ\03 I J.0'>\103 doc 3-5
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LEGEND
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Contingent Phase II SVE
Wells to be located at
center of Phase I wells
1t" Phase .1 SVE Well Location
0 Phose II SVE Well Location
r SVE Radius of Influence
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FIGURE 3-2
CONCEPTUAL LAYOUT OF
PHASE II SVE WELLS IN RELATION
TO PHASE I SVE WELLS
FCX-STATESVILLE SUPERFUND SITE, OU3
STATESVILLE, NORTH CAROLINA
60313.009 3/99
BROWN AND
CALDWELL Nashville, Tennessee
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Groundwater Zone
Shallow:.
Intermediate:
Deep:
TABLE3-l
MONITORING WELLS SELECTED FOR
SEMI-ANNUAL GROUNDWATER SAMPLING,
OPERABLE UNIT THREE (OU3),
FCX-STATESVILLE1SUPERFUND SITE
Monitoring Well'
W-5s
W-12sb
W-l~s
W-18sb.
W-19s
W-20s
W-22s
' W-24s
W-3ls
'
W-5i
W-!Oib
W-12ib
' W-2pi
W-22i
W-28i
W-29i
W-30i
W-3:li
W-32i
W-20d
' I
'The analytical methods for the analyses of groundwater samples are given in Table 3-2.
"Natural attenuation background well.
P .lpmj\OJ I J.09\1orn1 .doc Page ! of!
--------
Sample Evaluation
Field Measurements:
TABLEJ-2
SUMMARY OF CHEMICAL ANALYSES AND ANALYTICAL METHOD REFERENCES FOR
SEMI-ANNUAL GROUNDWATER SAMPLING,
OPERABLE UNIT THREE (OUJ),
FCX-STATESVILLE SUPERFOND SITE
Chemical Test/ Analyte Parameter Analytical Reference Methoda
Carbon dioxide Hach KitC
Iron (II). Hach KitC
Manganese (II) Hach KitC
Sulfide Hach KitC
Conductivity · ASTM Method D-1125-82
Oxidation-reduction potential (ORP) ASTM Method D-1498-76
pH ASTM Method D-1293-84
Dissolved oxygen (DO) Hach or CHEMETRICS KitC
Temperature NAd
_ Laboratory Analyses:--Chloride -------USEPA Method 325.2
Iron (total) Aquaterra QAPP Table 3
Manganese (total) Aquaterra QAPP Table 3
Nitrate/nitrite USEPA Method 353.2
Sulfate USEPA Method 375.4/9038
Ethane, ethene, and methanee USEPA Method 8015-Modified
TCL voes Aquaterra QAPP Table 2
Alkalinity (carbonate/bicarbonate/ Standard Methods 2320B
Dissolved total organic carbon (TOC) USEPA Method4I5.I
Volatile fatty acids Standard Metho_ds_5560C
DQO Levetb
II
II
II
I
II
II
II
II
II
II/-
IV
IV
III
Ill
I II
IV
III
III
III
asample 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.
CAnalysis 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.
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• validation of the analytical reports for metals and VOCs, and I ,.
• interpretation and reporting of the analytical results to the USEP A.
. '
3.4 PRELIMINARY DESIGN DOCUMEl~TS
The design documents included with the prdiminary RD include preliminary design
I
plans and an outline of the technical specifications.
3.4.1 Preliminary Design Plans
The following drawings have been identified to be included as part of the RD:
• Cover Sheet with Site location maps.
• Existing Site Conditions Plan
• Process Flow Diagram
• Piping and Instrumentation Diagram (P&ID)
•
•
•
•
•
Construction Plan
Piping and Equipment Layout
Cross Sections
Details (2 sheets)
Piping Profiles
•
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Electrical Layout and Requirements:
I Preliminary plans have been developed as part of the Preliminary RD. Draft versions of
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the Cover Sheet and the Existing S~te Conditipns Plan are included in Appendix B of this
' Preliminary RD submittal. Detailed design of the system components will be included in
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the Pre-Final Design submission. When: the Pre-Final Design is submitted, the :
construction plans will provide detail sufficient for bidding and actual construction by a
qualified contractor.
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3.4.2 Outline of Technical Specifications
An outline of technical specifications which will address various aspects of the work and
supplement the drawings is.included in App~ndix B. The final specifications will include
general material, equipment, and procedure ~equirements and other related.items.-As part
of the general requirements, a Health and; Safety Plan Specification for the remedial
contractor will also be included.
It is expected that the· outline · of technical specifications will include the following·
Divisions:
•
•
•
•
•
•
•
Division I -General.Requirements
Division 2 ° Site Work
Division 3 -Concrete
Division 7 -Thermal and Moistur~ Protection
Division 13 -Special Construction,
Division I 5 -Mechanical EquipmJnt
Division 16 ° Electrical
The outline· will be further refined during development of the specifications as needed.
I
The technical specifications will be prepared in a typical, standardized Construction
Specifications Institute (CS!) Master Format. When the Pre-Final Design is submitted,
the · technical specifications will provide detail sufficient for bidding· and • actual
construction-by a qualified contractor.
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4.0 REMEDIAL ACTION
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The Remedial Action (RA) includes a 'project delivery strategy, evaluation of permitting
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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 either devices may require field adjustment, depending. upon the
owner's operations within the textile plant.
4.1 PROJECT DELIVERY STRATEGY '
It is expected that approval of the Final RD will be received from the USEPA and
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NCDEHNR on or about October 27, 1999! and that USEPA will provide approval to
proceed with the RA.
4.l.l Reinedial Action Work Plan
The RA Work Plan will be submitted to the:USEPA and the NCDEHNR in January 2000
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in accordance with the revised RA schedule. The RA Work Plan will include the
following elements:
0
•
•
•
•
•
0
•
•
RA Schedule
Permitting Plan
Construction Management Plan (CMP)
Construction Quality Assurance Plan (CQAP)
Field Sampling Plan
Contingency Plan
Project Delivery Strategy
Groundwat-~r and Surface Water Monitoring Plan
Operation and Maintenance (O&M) Plan
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4.1.2 Remedial Action
I. The Group will begin procurement o~ a· prime Contractor to implement the RA
during the review and approval of the Final Remedial Design. Potential contractors
will be pre-qualified prior to solicitation of bids.
2. Qualified contractors will be required to meet at the Site for a pre-bid meeting to I
review specifics of the Contract Documents and the Site features.
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3. Acquisition of the approvals outlined llater in this section of the Preliminary RD
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Report will be pursued concurrent with the contractor qualification and bid
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solicitation process. Work products developed to meet substantive requirements
' will be submitted to the appropriate regulatory agencies. The USEP A and
NCDEHNR will be copied on these submittals as required.
'
4. . Bids will be opened, reviewed, and a contract awarded to the prime Contractor
approximately J 20 days after solicitipg bids. It is anticipated . that the prime
i contractor will employ several subcontractors to complete portions of the PA as
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necessary. It is anticipated that the ;Group will separately contract the task of
construction observation and construction quality assurance (CQA). The CQA
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contractor will serve as the Group's Representative on Site during the RA activities.
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5. Once the contract has been awarded and the notice to proceed has been issued, a
6.
' pre-construction meeting will be held.: The purpose of the meeting will be to detail
' ' '
the project requirements including submittal requirements, project schedule, quality
' assurance/quality control, and project .close-out. It is anticipated that this meeting
wili occur at the project Site.
It is expected that the Contractor ~ill prepare a major portion of the • project .
' ' submittals for the materials to be installed as part of the RA prior to mobilization at
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the Site. Project submittals will be performed in accordance with the Contract
Documents.
7. It is expected that RA activities will be continuous (for Phase I) from the initial
mobilization by the prime Contractor through completion and demobilization from I
the project Site.
8. ·· The Phase I bid will include units costs for Phase II. Alternatively, the Group may
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electto.rebid the project if Phase II work is necessary.
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9. It is anticipated that the prime Contractor will be responsible for procurement of all
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major equipment. Prior to procurement, equipment and materials to be
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used/installed as part of the RA win be submitted to the Group's Representative for
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review and approval as specified in the 'Contract Documents.
I 0. The prime Contractor will be responsible for the health and .safety of all on Site
I pcr~onnel. Air emission and spill control requirements will be addressed in the
health and safety plan developed by the Contractor in accordance with the
C
specifications. The Contractor will be :responsible for developing and submitting a
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health and safety plan to the Group prior to commencement of the work.
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11. Construction quality control and quality assurance requirements will be outlined in
the Project Specifications and CQAP, respectively. The CQAP will be included as
part of the Final RD submittal.
12. · Weekly construction meetings and a monthly progress meeting win be· held at the
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Site. The Contractor is required to develop a detailed construction schedule that
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includes project meetings· and majo~ milestones as described . in tht> Contract
Documents.
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13. Within ninety (90) days after the RA has been completed, a pre-certification
inspection with representatives from psEP A and NCDEHNR 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 complt;ted 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 jwork and shall document major deviations
'
from the approved RD. The Remedial Action Documentation Report shall be
certified by a Professional Engineer licensed in the State of North Carolina as well
as an authorized representative of the Group.
14. It is anticipated that the O&M Plan d~veloped as part of the RD will be revised
once the RA has been completed and the RA Documentation 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.
15. The RD for Phase I will include requirements for Phase II. It is anticipated that the
contingent Phase II design will be consistent with the Phase I design and that no
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design submission to USEPA and NCDEHNR will be required after the Final RD.
4.2 PERMIT REQUIREMENTS
Construction and operation of the AS/SVE system requires, or potentially requues,
permits or agreemems from various agencies; or parties. The following potential permit
requirements or approvals were identified and 1evaluated:
\\DCNSII0)\PROJECTS\PROJ\0J I J.0Q\s04 doc 4-4
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• Operations
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Air emissions from the treatment system . ' . ·11'.) .. . -~
• Construction
Soil erosion and sediment control
Well Construction Permits
Handling potentially contaminated soil
Local Building Permits
Agencies and parties contacted to establish j~risdictions and permit requirements :,c,'ere:
• North Carolina Department of Environment Health and Nat11ral Resoun;es
(NCDEHNR)
• Iredell County, North Carolina
A discussion of each issue follows, includibg whether or not a permit or agreement is
required and which agency or party requires the permit or agreement. The Contractor
will be re5ponsible for local building permid as part of construction.
' 4.2.1 Air Emissions from Soil Vapor Extraction
The NCDEHNR regulates emissions to the atmosphere in accordwce with regulations
' established pursuant to the Clean Air Act. '. Our review of the relevant regulations and
discussions with NCDEHNR personnel rev!!aled that as long a~ the VOC emissions are
less than 5 tm1s per year, there are no requirements applicable to the discharge of off gas
vapors containing VOCs at the Site. However, NCDEHNR procedure requires that a
'
ietter containing emissions calculations be ~ubmitted for its review and determination of
' the need, or lack thereof, to pem1it 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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4.2.2 Soil Erosion and Sediment Control
The NCDEHNR regulates .soil eros10n and sediment r_elease related to construction·
activities.· A review of the relevant regulations with NCDEHNR 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.
Nevertheless, these activities will be perfohned in accordance with appropriate storm
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water. management. practices · and a Soil Erosion and Sediment Control Plan to be
developed consistent with storm water regubtory specifications.
Iredell County confirmed that it has no ordi~ances that regulate soil erosion or sediment
release ...
4.2.3 Well'Construction Permits
Based on conversations with NCDEHNR pr(or to pre-design investigation activities, well
construction pem1its .are not required due to the fact that this Site is regulated under
CERCLA ..
4.2.4 · Handling Potentially Contaminated Soil
Potentially contaminated soil will likely b~ encountered during well construction. In
conversations with NCDEHNR, we have established that no pem1its or petitions are
required for. this activity. The impacted excavated soil should be placed into drums or
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roll-off containers to be characterized and disposed of appropriately, Based on .North
' Carolir,a Regulation I SA NCAC 2H .02 l 7(a)(9), "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.
Furthe1more, under !SA NCAC 2H .0217('))(7), the same "deemed permitted" status is
conferred upon the land application of less than or. equal to 50 cubic yards of non-
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hazardous soil when approved by the Division Regional Supervisor. However, if the drill
' cuttings/mud or purge well water have i been contaminated by hazardous waste
'
constituents, the Division of Waste Management, Hazardous Waste Section should be '
contacted to determine the regulatory status qf the contaminated material.
4.3 PRELIMINARY SCHEDULE
The Preliminary Remedial Action 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 I owner's operations within the facility. It
should be noted that only some portions of the system will be operational following
completion of Phase I. The dates included on the schedule are dependent upon regulatory
review and approval consistent with those on the schedule.
\\J3CNSH03\PROJECTS\J'ROJ\031J.O<l\s04 doc 4-7
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FIGURE 4-1
PRELil\1INARY SCHEDULE FOR REMEDIAL ACTION FOR OPERABLE UNIT THREE (OU3),
FCX-STATESVILLE SUPERFUND SITE
Task Name
US EPA & NCDEHNR ..\porol'al of Final rm Rcporr
Remedial Acriol1 Work Plan
Prcnarc R.:\ \Vork Plan
Submit RA Work Plan to USP.PA & NCDlcHNR
USEP.-\ & NCOF;HNR Review of RA Work Phm
USEPA & NCOEHNR Anoro,·:il of RA Work Pinn
Rcmetlinl Ae1ion Contractor Sdection
Pre1m.rc and Transmit Biel Packn:re to Ridders
Bid Preuarn hon
Bill Due Date
!'.~valuate Bid~
Submit Not.ice of' .-\,vard ro Succe~sful Bidder
N~~201.ia1e ContTHCl wil'h Succe~::-ful Bidder
Remedial Construction
Notice to Proceed
Contrnctor i\fobi.lization
-~ Remedial Con~t1·uction U:-ing Oh:--en·ational Annroach
Subsrantial Comnletion
Site Walk Throus,h with USEP:\ an<I NCDEHNR
Pnform Punr:h Li:'it Item:-
Proiccr Clo~eOur
Pre1>are Ce1·tificat.ion R1:11ort
Submit Cer1:ifi.c:1rion Rnt. to US EPA and NCDEHNR
Monthh' Progress Rcnorr~
P:/Pl{O.Jto:-113.0U/PDRSCl I ED. TLP
Start End
Oct/27 /D9 Ocr/27 /99
Nm·/0 l/99 Feh/07/00
Nm·/0l/(l9 ,Jan/04/00
Jan/0,1/00 ,Jan/04/00
,Jan/(H/00 Feb/07/00
l'eb/07/00 Feh/07/0U
Feh/07/00 ,Jul/0G/00
Fch/07/1)0 Mar/07/00
l\lar/07/00 i\l,1 v/02/00
l\la,·/02/00 1\lav/02/00
Ma,·/02/00 ,fun/14/00
,Jun/\ ,1/00 ,Jun/!4/00
.Ju ni 1-1/00 ,Jul/0G/00
,Jul/06/00 i\lav/0:l/O I
,Jul/06/()0 ,Jul/07 /00
.Jul/07/00 ,Jul/27/00
,Jul/27/00 Anr/1:l/01
.--\m·/\ :l/0 I Aor/\:l/01
Apr/ lcl/0 I Apr/l:1/0 I
.-''-IJr/t:l/01 Mn,·/03/0 l
!\la viO:J/0 I i\l a,'/0:J/0l
i\lav/(J:l/01 ,Jun/15/01
,Jun/l 5/0 I ,Jun/l;}/01
Aus,/l0/99 ,Jun.ll~/01
:-..;ote: The :-chedule is dependimr on the nC'tual duration of L'SEP,\ and :\'CDEH:,,.;7{ nwi1•w,; :rnd 11,1tuHI □pprovnl Oar.es.
1999 2000 I 2001
Sep Ocr I Nm· I O,,c Jan Feb ,\lar I Apr May ,Jun ,Jul Aug Sep Oct No,· Dee I -Jan Feb Mar I Apr I i\la)· .Jun
"~I I I I I I I I
'-,.·.,, c---~g-------
·-1'®1ilftH@flM_W1 I ,,
i\lile.5torw
,Jul
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P;\PRQJ\0313. 09\ACCVR. DOC
APPENDIX A
PRELIMINARY DESIGN PLANS
g
<.!> z I
RAND McNALLY, ROAD ATLAS, 1996
LOCATION MAP
SCALE: 1 IN.= 15± MILE•S
I
> 1 ~ I If I ) ..
LIST .OF DRAWINGS
SHEET NO .
60313 -001
60313-002
60313-003
60313-004
---._, ---r
60313-005
60313-006
60313-007
60313-008
60313-009
DESCRIPTION
EXISTING SITE PLAN
PROCESS FLOW DIAGRAM
PIPING AND INSTRUMENTATION DIAGRAM
CONSTRUCTION PLAN
PIPING AND EQUIPMENT LAYOUT
CROSS SECTIONS
DETAILS
DETAILS
PIPING PROFILES
ELECTRICAL PLAN SHEETS
PRELI INARY DESIGN PLANS FOR
OP:ERABLE UNIT THREE (OU3)
FCX.-ST A TES VILLE SWPERF.UND SITE
ST ATES VILLE, NORTH CAROLINA
PREPARED FOR
EL Pi ASO ENERGY CORPORATION
HOUSTON, TEXAS
PREPARED BY
BROWN AND
CALDWELL
Nash ville , Tennessee
APRIL 1999
RE FERENCE:
~t.fr1st9tLiG~fs~'.c N~A~tADRANGLE, 1993
VICINITY MAP
SCALE : 1 IN .= 2000± FT.
JOB NO. 27-60313.009
a .. -••,-•··· ...... ~ ~ -. ·-------
._ __________ =-"i·---,-■---------------------'-------------------------+-------------------------------------------------------------------~----------------------------------------' w
~ ,:11 ( ~ £.t.u. ;,t1,,;.,;:,':-i1t·:: l•·'.,: .... :·., 1, .: . ·., .:.-.·;:; ,:_.: :; ." ._::•,:: ... ",;:-.;•. ;.-, .. ;· .:, •. -:~ J.,1 ·:( ,\: . ,.'.: . -;.; ,,-.;. •. i:1 ;;i!ho\,, ,,_ 1, ,.,1. n .. -.J;k·-,-~ . ., 1,,.•,.t,~,_;_,, ,:~,1 .•.· ,•'. r
! l ' • ' d, !' '<' .') I ,, l"' '1·•,::: ·:•v~j2-,\1~•\'· I,,, .. '/.': ,:\.~ ;, !:: ·,:,; ··'J',' ::,· u_l;;J_': '-'-~it.: t:·t;k{ilt(~ ':'';, 1,·:· ;· ~It t '! f'• l''.:1_,. ;:,::: !t,f ;i:,!,':':f 1.~: ..
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81 -----.....J
Textile Plant
War eh Ol se
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W-16i
[b
W-9s t
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-2s
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-----o-..,,r ru:::-~•:::n·" 'll...,,.r.,. .,,..,. 'UI ,..,.,. •1 , l"l'llr9E P:lu!orr1:r1· ,,11D1 •n :cn:n::::'ff ...... ,..,...,......n, n 1:,;--,m-,.Trmr:11,w:;;r1n•r1xnr:::?
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W-4s
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-$-
W-22s
.
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-$-j
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MW-2
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MW-6d
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W-13i
W-13s
□ LJ
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W-18s
Piedmont Street
Legend
----
I
Shallow Monitoring Well Location
Intermediate Monitoring
Deep Monitoring Well LCDcation
I Property Line
50 I 0 ----SCALE
NO. REVISIONS
N
50 100
FEET
REV'D DATE APPRO'O
BY BY
BROWN AND
CALDWELL
FCX-STATESVILLE SUPERFUND SITE
OPERABLE UNIT THREE (OU3)
STATESVILLE, NORTH CAROLINA
I
Nashville, Tennessee EXISTING SITE PLAN
SCALE: AS SHOWN DATE: 3/17 /99
DRAWN BY DATE CH[CKED BY APPROVED BY JOB NUMBER DRAWING NUtJBER
....__ _______ , =' -,,.,, ,, _________________ UJ. __ _-1,.....J.J..._....t._ __ ...____.~ __ ....;i;;i,_,.:::. ___ ;;._ __ __.__ _________ ---+-----'---__;;_-'-'----------'---'----------------'------''-------l.---1---------l JLT 2/99 KHO 60313.008 0313-001
I )1JiliiLiiiiiillm::ni.lli :W::X:rn ii.lilt U ,WWWIC bA-mJN WIIM&i ;, , ,l,!!.:JLiJV. n ►
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P. \PROJ\0313. 09\ACCVR. DOC
APPENDIXB
OUTLINE OF TECHNICAL SPECIFICATIONS
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DRAFT
OUTLINE OF TECHNICAL SPE,CIFICATIONS FOR
OPERABLE UNIT THREE (OU3)
FCX-STATESVILLE SUPERFUND SITE
STATESVILLE, NORTH CAROLINA '
DIVISION 1 -GENERAL REQUIREMENTS
01010
01027
01035
01040
01050
01060
01120
01160
01200
01310
01300
01370
01380
01400
01410
01500
01540
01560
01580
01600
01650
01700
01730
01740
SUMMARY OF WORK
APPLICATIONS FOR PAYMENT
CHANGE ORDER PROCEDURES
PROJECT COORDINATION
CONTRACTOR FIELD ENGINEERING
REGULATORY REQUIREMENTS
SITE MAINTENANCE '
HEALTH AND SAFETY PLAN
PROJECT MEETINGS AND ADMINISTRATION
CONSTRUCTION PROGRESS ~CHEDULE
SUBMITTALS
SCHEDULE OF VALUES I
CONSTRUCTION PHOTOGRAP,HS
QUALITY CONTROL 1 TESTING AND TESTING LABORATORY SERVICES
CONSTRUCTION FACILITIE's AND TEMPORARY CONTROLS
SECURITY '
DUST AND NOISE CONTROL<
DECONTAMINATION PLAN :
MATERIAL AND EQUIPMENT:
STARTING OF SYSTEMS
PROJECT CLOSEOUT
OPERATION AND MAINTENArcE DATA
WARRANTIES AND BONDS
DIVISION 2 -SITE WORK
02010
02050
02100
02110
02200
02220
02270.
02444
02600
02920
SUBSURFACE INVESTIGATION
DEMOLITION
SITE PREPARATION
CLEARING
EARTHWORK
TRENCHING
SOIL EROSION AND SEDIMENT CONTROL
CHAIN LINK FENCING 1
SVE WELLS, AIR SPARGING WELLS, AND MONITORING
PROBES 1
SEEDING AND MULCHING; SODDING
P: \proj\0313. 09\SPS:CI'OC .DOC 1
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DIVISION 3 -CONCRETE
03100
03300
03400
03600
CONCRETE FRAMEWORK
CAST-IN-PLACE CONCRETE
PRECAST CONCRETE MANHOLES
GROUT '
DIVISION 7 -THERMAL AND MOISTURE PROTECTION
07900 FREEZE PROTECTION OF ABOVE GROUND PIPING
DIVISION 13 -SPECIAL CONSTRUCTION :
13120 PRE-ENGINEERED STRUCT~RES (IF REQUIRED)
13250 OFF-GAS TREATMENT SYSTEM
' ' DIVISION 15 -MECHANICAL EQUIPMENT :
15010
15060
15100
15140
15150
15991
15992
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MECHANICAL GENERAL PROVISIONS
PIPING SYSTEMS
VALVES AND APPURTENANCES
PIPE HANGERS AND SUPP0RTS
PENETRATION OF BUILDING ELEMENTS
CENTRIFUGAL BLOWERS
AIR COMPRESSORS
DIVISION 16 -ELECTRICAL
16010
16100
16120
16140
16452
16460
16470
16900
16901
REQUIREMENTS
AND yABINETS
BASIC ELECTRICAL
RACEWAYS, BOXES,
WIRES AND CABLES
WIRING DEVICES
GROUNDING
TRANSFORMERS
PANELBOARDS
INSTRUMENTATION AND
PROGRAMMABLE LOGIC
'
C0NTROLS
CONTROL SYSTEM '
P: \proj \0313, 09\SPECI'OC. DOC 2
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APPENDIXC
TECHNICAL PROTOCOL FOR
EVALUATING NATURAL ATTENUATION OF
CHLORINATED SOLVENTS IN GROUNDWATER
P. \PROJ\0313. 09\ACCVR 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 ~atural
Attenuation 1 of Chlorinated
Solvents in !Ground Water
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TECHNICAL PROTOCOL FOR EVALUATING NATURAL
ATTENUATION OF CHLORI~ATED SOLVENTS IN
i
GROUNDWATER
by
I Todd H. Wiedell]eier
Parsons Engineeri_ng Ssience, Inc.
Pasadena, California
Matthew A. Swanson, David E. Moutoux, and E. Kinzie Gordon ' Parsons Engineering Science, Inc. I Denver, Colora,do
John T. Wilson, Barbara H. Wilson, and Donald H. Kampbell
' United States Environmental Protection Agency
National Risk Management Research Laboratory
I 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 clro!ina
!AG #RW57936164
Project Officer
John T. Wilson
' National Risk Management Re~earch Laboratory
Subsurface Protection and.Re111ediation Division
Ada, Oklahoma
'
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIG 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 p.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 contdct 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 a~d analysis with support from ManTech
Environmental Research Services Corp., the in-house analytical support contractor for SPRD.
I
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
I 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, arc the
sole responsibility of the user. '
' Mention of trade names or commercial products does not constitute endorsement or
recommendation for use. 1
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FORE\VORD
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
I the environment. The focus of the Laboratory's research1program 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 reg~latory 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 grourid 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 1 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 allo»' 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. '
Clintonlw. Hall, Director
Subsurface Protection and Remediation Division
National Risk Management Research Laboratory
'
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TABLE OF CONTENTS
. ! . .
Notice ........................................................................................................................................... 11
Foreword ...................................................................... , .............................................................. iii
Acknowledgments ........................................................ '. ............................................................ viii
List of Acronyms and Abbreviations ........................... ' .............................................................. ix
Definitions .................................................................... ! ............................................................. xii
' I SECTION I INTRODUCTION .................................. , ............................................................... I
1.1 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 ...................... ' ............................................................. I 5
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 Reactionsl(Reductive Dehalogenation) ............... 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 ................... L ........................................................... 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 ATTENUATIO~ 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 Semi volatile Organic Compounds ..................................... 38
2.3.2.2 Dissolved Oxygen ..................... : ............................................................ 38
2.3.2.3 Nitrate········································'-··························································· 39
2.3.2.4 Iron (ll) ...................................... , ............................................................ 39
2.3.2.5 Sulfate ....................................... ! ............................................................ 39
2.3.2.6 Methane ..................................... 1 ............................................................ 39
2.3.2.7 Alkalinity ................................... 1
•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 39
2.3.2.8 Oxidation-Reduction Potential .. : ............................................................ 40
2.3.2.9 Dissolved Hydrogen .................. :. ........................................................... 40
2.3.2.10 pH, Temperature, and Conductiv'ity ....................................................... 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 Apparent1Reduction 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 ~DI CATO RS 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 Wate~ Table Map(s) ....................................... 47
2.4.1.5 Contaminant and Daughter Prod,uct 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 Calcul~tions .................................................... 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 c·alculations ............................................ 49
2.5 SIMULATE NATURAL ATTENUATION .USING SOLUTE FATE AND
TRANSPORT MODELS .......................... .t. ............................................................ 49
2.6 CONDUCT A RECEPTOR EXPOSURE PATHWAYS ANALYSIS ...................... 50
2.7 EVALUATE SUPPLEMENTAL SOURCEIREMOVAL 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 .............................................................. 1
••••••••••••••••••••••••••••••••••••••••••••••••••••••••• CI-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 flo\v chart .................................................. 12
Reductive dehalogenation of chlorinated etheries .......................................................... 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 . l , hypothet1ca release ......................................... ! .............................................................. 36
Hypothetical long-term monitoring strategy .... : .............................................................. 51
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TABLES I
No. Title : Page
1. Contaminants with Federal Regulatory Standar.ds ........................................................ xiv
2.1 Soil, Soil Gas, and Ground-water Analytical Protocol .................................................. I 6
2.2 Objectives for Sensitivity and P'.ecision to : .
Implement the Natural Attenuation Protocol ... , ............................................................. 21
2.3 Analytical Parameters and Weighting for Preliminary Screening for
An.aerobic Biodegradation Processes ............... : ............................................................. 29
2.4 Interpretation of Points Awarded During Screening Step l ... : ....................................... 32
2.5 Range of Hydrogen Concentrations for a Given Terminal
Electron-Accepting Process .............................. : ............................................................. 41
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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
I Alvarado Benson, R. Todd Herrington, Robert Nagel, Cindy Merrill, Peter Guest, Mark Vesseley,
' John Hicks, and Saskia Hoffer for their contributions to this project.
VIII
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I AAR
AFB
I AFCEE
ASTM
I bgs
BRA
BRAC
BTEX
I CAP
CERCLA
I cfm
CFR
I COPC
CPT
CSM
I OAF
DERP
DNAPL
I DO
DOD
DQO
I EE/CA
I FS
gpd
I G,
HOPE
HSSM
I HSWA
ID
I JDW
IRP
I L
LEL
LNAPL
LUFT
I MAP
MCL
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LIST OF ACRONYMS AND IABBREVIATIONS
' American Association of Rajlroads
Air Force Base I
Air Force Center for Environmental Excellence
American Society for Testing and Materials
below ground surface
baseline risk assessment
Base Realignment and Closuie
benzene, toluene, ethylbenzepe, 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 Ainendments of 1984'
'
inside-diameter
investigation derived waste
Installation Restoration Program
liter
lower explosive limit
light nonaqueous-phase liquid'
leaking underground fuel tank 1
management action plan
maximum contaminant level
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I MDL
µg
µg/kg
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µg/L
mg
mg/kg
mg/L
I mg/m3
mmHg
MOC
I MOGAS
NAPL
I NCP
NFRAP
NOAA
NOEL
I NPL
OD
I ORP
OSHA
OSWER
I PAH
PEL
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POA
POC
POL
ppmv
I psi
PVC
I QA
QC
I RAP
RBCA
RBSL
redox
I RFI
RI
RME
I RPM
SAP
I SARA
scfm
SPCC
g
0
method detection limit
microgram
microgram per kilogram
microgram per liter
milligram
milligrams per kilogram
milligrams per liter
milligrams per cubic meter
millimeters of mercury
method of characteristics
motor gasoline
nonaqueous-phase liquid
National Contingency Plan ,
no further response action plan
National Oceanographic and Atmospheric Administration
no-observed-effect level I
National Priorities List
outside-diameter
oxidation-reduction potential I
Occupational Safety and Health Administration
Office of Solid Waste and Em:ergency Response
polycyclic aromatic hydrocarbon
permissible exposure limit
point-of-action
point-of-compliance
petroleum, oil, and lubricant
parts per million per volume
pounds per square inch
polyvinyl chloride
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quality assurance
quality control
remedial action plan
risk-based corrective action
risk-based screening level
reduction/oxidation
RCRA facility investigation 1
remedial investigation
reasonable maximum exposure
remedial project manager 1
sampling and analysis plan
Superfund Amendments and ~cauthorization Act
standard cubic feet per minute
spill prevention, control, and Countermeasures
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I SSL soil screening level
SSTL site-specific target level
SVE soil vapor extraction
svoc semi volatile organic compound
I TC toxicity characteristic
TCLP toxicity-characteristic leachipg proccdllre
I TI technical impracticability
TMB tri meth y I benzene
TOC total organic carbon
I TPH total petroleum hydrocarbon~
TRPH total recoverable petroleum liydrocarbons
TVH total volatile hydrocarbons '
'
I TVPH total volatile petroleum hydrocarbons
TWA time-weighted-average 1
UCL upper confidence limit'
I us United States
USGS US Geological Survey
UST underground storage tank
I voes volatile organic compounds
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DEFINITIONS
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Aerobe: bacteria that use oxygen as an electron acceptor. 1
Anabolism: The process whereby energy is used to build organic compounds such as enzymes and
nucleic acids that arc necessary for life functions. ln dsscncc, energy is derived from catabolism,
stored in high-energy intermediate compounds such a~ adenosine triphosphate (ATP), guanosine
triphosphate (GTP) 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
I 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 propeny boundaries or potential receptors -it is the
plume area which the ground water must be returned tb beneficial use during the implementation of
a remedy.
Anthropogenic: Man-made. i
Autotrophs: Microorganisms that synthesize organic materials from carbon dioxide.
Catabolism: The process whereby energy is extracted from organic compounds by breaking them down
• • I , mto their component pans.
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 fonuitciusly degraded by an enzyme or cofactor
produced during microbial metabolism of another corripound.
Daughter Product: A compound that results directly from th'c biodcgradation of another. For example ' cis-1,2-dichloroethene (cis-1,2-DCE)is commonly a d~ughter product of trichlorocthene (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 rcgi1on 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 alkcne.
Dispersivity: A property that quantifies mechanical dispersibn in a medium.
Effective Porosity: The percentage of void volume that cont~ibutes to percolation; roughly equivalent to
the specific yield. '
Electron Acceptor: A compound capable of accepting electrbns during oxidation-reduction reactions.
Microorganisms obtain energy by transferring clcctroris from electron donors such as organic
compounds (or sometimes reduced inorganic compou~ds such as sulfide) to an electron acceptor. ' Electron acceptors are compounds that arc relatively o~idized and include oxygen, nitrate,
iron (III), manganese (IV), sulfate, carbon dioxide, or in some cases the chlorinated aliphatic
hydrocarbons such as perchloroethcne (PCE), TCE, DCE, and vinyl chloride.
Electron Donor: A compound capable or supplying (giving Jp) electrons during oxidation-reduction
reactions. Microorganisms obtain energy by transferrihg electrons from electron donors such as
' organic compounds (or sometimes reduced inorganic c,ompounds such as sulfide) to an electron
acceptor. Electron donors arc compounds that are relatively reduced and include fuel
hydrocarbons and native organic carbon. 1
Electrophile: A reactive species that accepts an electron pair.
Elimination: Reaction where two groups such as chlorine and hydrogen arc 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 anacfobic 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 o'f 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 bf elevation head and pressure head.
Hydraulic Gradient: The maximum change in head per unit 'distance. I Hydrogenolysis: A reductive reaction in which a carbon-halogen bond is broken, and hydrogen replaces
the halogen substituent. 1
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. I
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 i9 contrast to mixing due to diffusion.
Metabolic Byproduct: A product of the reaction between an electron donor and an electron acceptor.
Metabolic byproducts include volatile fatty acids, daughter products of chlorinated aliphatic
hydrocarbons, methane, and chloride. I
Monooxygenase: A microbial enzyme that catalyzes reactio~s in which one atom of the oxygen molecule
is incorporated into a product and the other atom appc~rs in water.
Nucleophile: A chemical reagent that reacts by fon11ing covalent bonds with electronegative atoms and
compounds. I
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).
Performance Evaluation Well: A ground-water monitoring well placed to monitor the effectiveness of
the chosen remedial action. I
Porosiry: The ratio of void volume to total volume of a rock ~r sediment.
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.
XIII
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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 perchloroethvlene; tetrachloroethvlene c,c1.
TCE trichloroethene 79-01-6 trichloroethvlene C2HCJ,
1,1-DCE I, 1-dichloroelhene 75-35-4 I, 1-dichloroethy]ene; vinvlidine chloride C,H,CI,
trans-1,2-DCE (E)-1,2-dichloroethene 156-60-5 trans-I ,2-dichloroethene;trans-1,2-dichloroethylene C,H,CI,
cis-1,2-DCE 156-59-2 cis-1,2-dichloroethene; cis-1,2-dichloroethylene C2H2Cl 2
vc chloroethene 75-01-4 vinyl chloride; chloroethylene C2H3CI
1,1,1-TCA I, I, I-trichloroethane 71-55-6 C,H3Cl3
1,1,2-TCA I, 1,2-trichloroelhane 79-00-5 C2H3CI,
1,1-DCA I, 1-dichloroethane 75-34-3 C,H.CI,
1,2-DCA 1,2-dichloroelhane 107-06-02 C2H4CJ,
CA chloroethane 75-00-3 C,H,CI
CF trichloromethane 67-66-3 chloroform CHCI,
CT tetrachloromethane 56-23-5 carbon tetrachloride CCL,
Methylene Chloriae·-aictiforometliane ----· ---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.l!4CJ,
1,3-DCB 1,3-dichlorobcnzene 541-73-1 m-dichlorobenzene C.l-I.CI,
1,4-DCB l ,4-dichlorobenzene 106-46-7 o-dichlorobenzene C.H,CI,
1,2,3-TCB 1,2,3-trichlorobenzene 87-61-6 C.H,CI,
1,2,4-TCB 1,2,4-lrichlorobenzene 120-82-1 C.H3Cl 3
l,3,5-TCB 1,3,5-lrichlorobenzene 108-70-3 C.H,Cl3
1,2,3,5-TECB l ,2,3,5-tetrachlorobenzene 634-90-2 1,2,3,5-TCB C.H,Cl,
1,2,4.5-TECB 1,2,4,5-tetrachlorobenzene 95-94-3 C.H,Cl,
HCB hexachlorobenzene 118-74-1 C.CI,
EDB 1.2-dibromoethane l 06-93-4 ethylene dibromide; dibromoelhane C,HJ3r2
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SECTION 1
INTRODUCTION
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' 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 1objectives 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 attdnuation processes as part of overall site
remediation. The United States Environmental Protectibn 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 re111edial
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 I 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 approp,riate 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 tl,at 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 ,!_,easures ( e.g., source control), or
as a follow-up to active re111ediation 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 biolbgical processes for remediating ground
I water contaminated with mixtures of fuels and chlorinated aliphatic hydrocarbons. This document I 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 oft 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
I contaminants from source materials; and 4) restore the jplume 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 "actiJe" remedial methods. For other cleanup
programs, remedial objectives may be focused on prevehting exposures above acceptable levels.
Therefore, it is imperative that users of this document be 1aware of and understand the Federal and
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State statutory and regulatory requirements, as well as p<;ilicy considerations that apply to a specific
site for which this protocol will be used to evaluate lv!NA 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 inyesting 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 Ta~k 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 docu'.mentation 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 nat~ral attenuation. This protocol should be
I used to evaluate whether MNA by itself or in conjunction with other remedial technologies is
sufficient to achieve site-specific remedial objectives. Jr\ evaluating the appropriateness of MNA,
the user of this protocol should consider both existing 1exposure pathways, as well as exposure
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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
I involving monitored natural attenuation is being consider~d. 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 includJ 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 accdptable degree of certainty (e.g., complex
fractured bedrock, karst aquifers). 1
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 I
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 an~ transport.
1.1 APPROPRIATE APPLICATION ON NATURAIJ ATTENUATION
The intended audience for this document includes Project Managers and their contractors, I scientists, consultants, regulatory personnel, and others ~harged with remediating ground water
contaminated with chlorinated aliphatic hydrocarbons or mixtures of fuel hydrocarbons and
chlorinated aliphatic hydrocarbons. This protocol is intdnded 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 attcriuation to be a default or presumptive I
remedy at any contaminated site (OSWER Directive 9200.4-17, 1997), as its applicability is highly
I 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 atte~uation is sufficiently protective of human
health and the environment. I
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 mechan{sm, although abiotic destruction of some
compounds does occur. Nondestructive attenuation mechanisms include sorption, dispersion, dilution
from recharge, and volatilization. The natural attenuatiiln of fuel hydrocarbons is described in the ' Technical Protocol for Implementing Intrinsic Remediation with Long-Term Monitoring for Natural
I Attenuation of Fuel Contamination Dissolved in Groundwater, published by the Air Force Center
for Environmental Excellence (AFCEE) (Wiedemeier ~t al., l 995d). This document differs from
the technical protocol for intrinsic remediation of fuel hydrocarbons because it focuses on the
individual processes of chlorinated aliphatic hydrocarbop biodegradation which are fundamentally
different from the processes involved in the biodegradation of fuel hydrocarbons.
For example, biodegradation of fuel hydrocarbons, especially benzene, toluene, ethylbenzene,
I and xylenes (BTEX), is mainly limited by electron acceptor availability, and generally will proceed
until all of the contaminants biochemically accessible to th
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e microbes are destroyed. In the experience
I of the authors, there appears to be an adequate supplyi of electron acceptors in most, if not all,
hydrogeologic environments. On the other hand, the /nore 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. 1Electron donors include fuel hydrocarbons
I or other types of anthropogenic carbon (e.g., landfill leachate) or natural orgaqic carbon. If the
subsurface environment is depleted of electron donors before the chlorinated aliphatic hydrocarbons
are removed, biological reductive dechlorination will cetise, 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 aliphritic hydrocarbon biodegradation.
For this reason, it is more difficult to predict the lohg-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 a'ttenuation. 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 requirkd 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 cohtinued 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 stat1e, 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 T~E at Picatinny Arsenal, in New Jersey.
The plume was at steady state or declining. Ten yearsi after surface spills ceased, leaching of
contaminants from subsurface DNAPLs and desorption 1from 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 biotransforrnation 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) 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, discharh 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
. d I organic compoun s. I -
Chlorinated solvents are released into the subsurface as either aqueous-phase or nonaqueous
phase liquids. Typical solvent releases include nonaquebus phase relatively pure solvents that are·
more dense than water and aqueous rinseates. Addition~lly, a release may occur as a mixture of
fuel hydrocarbons or sludges and chlorinated aliphatic' hydrocarbons which, depending on the
I 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 generai'ly 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 gteatest mass of contaminant is associated
with these NAPL source areas, not with the aqueous phase.
When released at the surface, NAPLs move downw~rd under the force of gravity and tend to
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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 DNAP
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L releases can sink to greater depths than
expected by following preferential flow paths. Thus, the r6lative volume of the release and potential
migration pathways should be considered when developing the conceptual model for the distribution
of NAPL in the subsurface. I
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 parti,tion 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 surfades. 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 ptocesses 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
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treatment of NAPL 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 iteri~. 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
, I
available.) I
The advantages of monitored natural attenuation (MNA) remedies are:
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• As with any in situ process, generation of lesser voluine of remediation wastes reduced potential
for cross-media transfer of contaminants comm01ily associated with ex situ treatment, and
reduced risk of human exposure to contaminated nledia;
• Less intrusion as few surface structures are requirdd;
• Potential for application to all or part ofa given site: depending on site conditions and cleanup
objectives; 1
• 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.
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The potential disadvantages of monitored natural attenuation (MNA) include:
• Longer time frames may be required to achieve re,nediation objectives, compared to active
remediation; \
• Site characterization may be more complex and co,itly;
• 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 lo'ng-term protectiveness;
• Potential exists for continued contamination migration, and/or cross-media transfer of
contaminants; I
• Hydrologic and geochemical conditions amenable to natural attenuation are likely to change
over time and could result in renewed mobility ofpre1•iously stabilized contaminants, adversely
impacting remedial effectiveness; and
• More extensive education and outreach efforts 111ay be required in order to gain public
acceptance of monitored natural attenuation.
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At some sites the same geochemical conditions and processes that lead to biodegradation of
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chlorinated solvents and petroleum hydrocarbons can chemically transform naturally occurring
manganese, arsenic and other metals in the aquifer mat~ix, 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 a~d analysis for these metals.
This document describes ( 1) 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
' subsu1face attenuation processes arc capable ofreducing 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 necessai-y to deterllline that a proposed
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treatment technology, if properly installed and operated, will reduce the collfaminant
concentrations in the soil and water to legally mandated limits. In this sense, the
use of biological treatment systems calls for·the sall!e level r,f 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 docume~t 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 I
The OSWER Directive 9200.4-17 ( 1997) identifies:three lines of evidence that can be used to
estimate natural attenuation of chlorinated aliphatic hyd~ocarbons, including:
( J) Historical ground water and/or soil chemistry ddta that demonstrate a clear and meaningful
trend of decreasing contaminant mass and/of concentration over time at appropriate
monitoring or sampling points. (in the cas'.e of a ground water plume, decreasing
concentrations should not be solely the result ofplwne migration. in the case of inorganic
contaminants, the primary attenuating mechanism should also be understood.)
(2) Hydro geologic and geochemical data that can b4 used to demonstrate indirectly the type(s)
of natural attenuation processes active at the site, and rhe rate at which such processes
will reduce contaminant concentrations to required levels. For example, characterization
data may be used to quanrify the rates of contaniinanr sorption, dilurion, or volarilization,
or to demons/rate and quantify the rares of biological degradation processes occurring ar
the site. 1
' ( 3) Data from field or microcosm studies ( conducted in or with acrual contaminated site
media) which directly demonstrate the occurrence of a particular natural attenuation
process at the site and its ability to degrade the c}ntaminants of concern (typically used to
demonstrate biological degradation processes ~nly ).
' ' 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 exp~cts that data characterizing the
nature and rates of natural attenuation procesies at the site (Number 2 above)
should be provided. Where the latter are also iizadequate or i11co11clusive, data ' . from microcosm studies (Number 3 above) may also be necessary. in general,
more supporting information may be required
1
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,
i110rganics), ar sites wirh contaminants rhat transform inro more toxic and/or mobile
forms Than the parent contaminallt, or at sites where monitoring has been pe,formed
for a relatively short period of time. The amount a)ul type of informarion needed for
such a demonstrarion will depend upon a numberofsire-specificfacrors, such as rhe
size and nature of rhe conraminarion problem, rhe proximiry of receptors and the
potential risk to those receprors, and orher physical characrerisrics of rhe
e11vironmenral setring ( e.g., lzydrogeology, ground cover, or climaric condirions). '
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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 adve1ction, 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, biodegrada\ion 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 balan~e 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/ate 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 down gradient performance
evaluation wells and within the area of the dissolved plu1me.
Microcosm studies may be necessary to physical!~ 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 studib 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 siles 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 accessible 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) describe_s EPA requirements for adequate site
characterization. I
Decisions to employ monitored natural attrnuatio11 as a re111edy or re111edy
component should be thoroughly and adequately supported with site-specific
characterizatio11 data and a11alysis. In general, the level of site characterization
necessary to support a comprehensive evaluationj 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 distribu'tion and partitioning between soil,
groundwater, and soil gas; rates of biological and non-biological transformation;
and an understanding of how all of these factoi-s are likely to vary with time. This
I information is generally necessary since conuiminant behavior is governed by
dynamic processes which must be well underst[!od before natural attenuation can
be appropriately applied at a site. Demonstrating the efficacy of this remediation
approach likely will require analytical or nhmerical simulation of complex ' . attenuation processes. Such analyses, which are critical to demonstrate natural
attenuation's ability to meet remedial action obje,ctives, generally require a detailed
conceptual site model as a foundation. 1
A conceptual site model is a three-dimensional representation that conveys C
what is known or suspected about contamination'sources, release mechanisms, and
the transport and fate of tlwse 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 infor!nation. 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 o/,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 col/ec1ting data to define (in three spatial
dimensions over time) the nature and distributionjof 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 characterizat(on may require more detail or
additional elements. For example, to assess the contributions ofsorption, dilution,
and dispersion to natural attenuation of contaminated ground water, a very detailed
understanding of aquifer hydraulics, recharge a,)d 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
I and acceptors present in the ground water, the concentrations of co-metabolites
and metabolic by-products, and perhaps specific c11wlyses 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 transpdrt developed for the site.
Development of an adequate database during the ite}ative 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 ~xtent of contaminant sources. Site ' characterization also should provide information on the location, extent, and concentrations of
I 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 . I exposure points. 1
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. I
The natural attenuation modeling effort has five piimary objectives:
' • To evaluate whether MNA will be likely to attain site-specific remediation objectives in a
time period that is reasonable compared to oth~r 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 rdceptors 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 CERCUA Feasibility Study or RCRA Corrective
Measures Study. I
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 r~sults should be verified by data obtained
' from site characterization. If model assumptions and results are not verified by site data, MNA is
not likely to be a viable option and should not be proposed as the remedy.
1.5 MONITORING \
The Monitoring Program OSWER Directive on Mo~itored Natural Attenuation (9200.4-17)
describes EPA expectations for performance monitoring. 1
Pe,formance monitoring to evaluate re,~edy effectiveness and to ensure
I protection of hwnan health and the environment is ·a critical element of all response
actions. Pe,fonnance monitoring is of even greater importance for monitored natural
attenuation than for other types of remedies duel to the longer remediation time
frames, potential for ongoing contaminant mig~ation, and other uncertainties
associated with using monitored natural attenuatici,n. 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;
1
• Determine if a plume is expanding ( either downgradient, 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., hydrogeologic, 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
I 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 deiision 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 ca~es, natural attenuation processes will be
capable of attaining site-specific remediation objectives in' a time period that is reasonable compared I 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 th~ 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 A,da, 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.
1
_
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. IAppendix 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 th~ 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 u~certainty 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
• I 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 !JlOdeling should be used to provide
information that collectively and consistently confirms the natural reduction and removal of the
dissolved contaminant plume. · i
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 s~reening 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:
1) Review avail~ble site data and develop a prelimimiry 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 attenuation1is 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 biodbgradation term. ' ,
6) Identify potential receptors and exposure points and conduct an exposure pathways analy-
sis. ' 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 Site 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 Ml)dels
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 oata
Necessary to Complete
the Screening of the Site
' Ives '),.!..;;:;:.. ___ ~ Engineered Remediation Required,
Implement Other Protocols
Perform Site Characterization
Evaluate Use 'of
Selected Additional
to Support Remedy Decision Making
Remedial Opti6ns IE-----,
Including Source
Removal or SoUrce
Control Along With
Natural Attenuation
NO
Develop Draft Plan for
Performance Evaluation
Monitoring Wells and
Lon -Term Monitorin
Present Findings
and Proposed
Remedy in
Feasibility Study
Excavation
Reactive
Berrier
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 Model
Verify Model Assumptions and
Results with Site
Characterization Data
Use Results of Modeling and
Site-Specific Information in
an Exposure Assessment
I
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 attenuatio1n 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 spould 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 ~ore· detail.
2.1 REVIEW AVAILABLE SITE DATA AND DEVELOP PRELIMINARY
CONCEPTUAL MODEL '
The first step in the natural attenuation investigati6n 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 opt_ion. 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 obtaiJed 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: I
--Catastrophic or gradual release of NAPL? i
--More than one source area possible or present ?
--Divergent or coalescing plumes?
I 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 'Yill 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 contaminantk.
Potential for natural attenuation to increase toxity and/or mobility of natural occurring
metals.
• Geologic and hydrogeologic data in three dimensions:(lf these data are not available, they
should be collected for the natural attenuation demonstra~ion 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 arid horizontal, effectiveness of aquitards, ' calculation of vertical gradients).
Ground-water flow gradients and potentiomet~ic 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
' . I of drinking water.
Downgradient and cross gradient 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 thi{ 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 bel 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 conJeptual site models commonly used by risk
assessors that qualitatively consider the location of c6ntaminant sources, release mechanisms,
transport pathways, exposure points, and receptors. Hoi,ever, 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 ih the natural attenuation investigation and
' to develop the solute fate and transport model. Contrqcting and management controls must be
flexible enough to allow for the potential for revisions to the conceptual model and thus the data
collection effort. 1
Successful conceptual model development involvei: ' • 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, includ(ng:
-Local geologic and topographic maps,
-Geologic data,
-Hydraulic data,
-Biological data,
-Geochemical data, and
-Contaminant concentration and distribution data. 1
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• Determination of additional data requirements, including:
I -Vertical profiling locations, boring locations and monitoring well spacing in three dimensions,
- A sampling and analysis plan (SAP), and 1 -Any data requirements listed in Section 2.1 that have not been adequately addressed.
Table 2.1 contains the recommended soil and grouhd water analytical methods for evaluating
I the potential for natural attenuation of chlorinated aliphafic 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 Objec\ives Process).
2.2 INITIAL SITE SCREENING
After reviewing available site data and developing a preliminary conceptual model, an
assessment of the poteniial 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 (1) contaminant properties, including volatility, sorptive properties, and
biodegradability; (2) aquifer properties, including hydrauli,c 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 recepto~ 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 p~eliminary 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 vi'able remedial alternative, detailed site
characterization activities that will allow evaluation of this remedial option should be performed. I If exposure pathways have already been completed and contaminant concentrations exceed protective
levels, or if such completion is likely, an engineered renledy 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, fhe collection of data to evaluate natural
attenuation can be integrated into a comprehensive reme4ial strategy and may help reduce the cost
and duration of engineered remedial measures such as intensive source removal operations or pump-
and-treat technologies. 1
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 accur~te 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 env,ironment. This information also will be
useful when selecting the remedial alternative that will be most cost effective at eliminating or I abating these threats should natural attenuation alone not:prove to be sufficient.
15
-- ------ -- --- - --
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 or Container, Sample Fixed-Base
Matrix Analvsis Method/Reference Comments Data Use Analvsis Preservation Laboratorv
Soil Aromatic and SW8260A Data are used to Each soil sampling Sample volume Fixed-base
Chlorinated determine the extent of round approximately 100 ml;
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.
xylene [B1EX]; removal.
Chlorinated
Compounds
Soil Biologically Under development HCI extraction Optional method that One round of Minimum 1 inch Laboratory
Available Iron followed by should be used when sampling in five diameter core samples
(Iii) quantification of fuel hydrocarbons or borings, five cores collected into plastic
released iron (Ill) 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 ----------. --
-----------· . ---·vinYi"ctilOridC Via"ifOn
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 TOC ground water is 4oe,
dependent upon the
amount ofTOC in the
aquifer 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
voes comnnunds 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.
--
---- ---- --- - -------
Table 2.1 (Continued)
Recommended Sample Volume, Field or
Frequency of Sample Container, Fixed-Dase
J\tatrl't Analvsis Method/Reference Comments Data Use Analvsis Samnle Preservation Laboraton
Water Alkalinity Hach Alkalinity test kit Phenolphthalein General water quality Each sampling Collect 100 mL of Field
model AL AP MG-L method parameter used (I) a,;; a round water in glass container.
marker to verify that all site
samples are obtained from
I.he same ground-water
system and (2) to measure
the buffering capacity of
e.round water.
Water Aromatic and SW8260A Analysi:i. may be Method of analy,c;is 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 solvents/byproduct,;, which cool to 4"C: add
(BlEX. alkyl benzenes arc lhe primary target hydrochloric acid to
trimethylbenzene analytes for monitoring pH 2.
isomers, natural 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 Menic EPA 200.7 or EPA To determine if anaerobic One round of Collect 100 ml in a Laboratory
200.9 biological activity is sampling glass or plastic
solubilizing arsenic from container that is rinsed
lhe aquifer matrix material. in lhe field with the
ground water 10 be
sampled. Unfiltered
Sa!Jlples obtained using
low flow sampling
methods are preferred
for analysis of dissolved
metals. Adjust pH to 2
with nitric acid. Do not
imert pH paper or an
electrode into lhe
samole.
Water Chloride Hach Chloride test k.it Silver nitrate As above, and to guide Each sampling Collect 100 mL of Field
(optional, see model 8-P tit.ration selection of additional data round water in a glass
data use) point,:; in real time while in container.
the field.
--
-00
-- -
Table 2.1 (Continued)
Matrix Anah·sis
Water Chloride
Water Chloride
(optional, see
dat.a use)
Water Conductivity
Water Iron (II) (fe•2)
.Water -· Hydrogen (Hi)
Water Manganese
--
I\ I eth od/Rcf ere nee
Mercuric nitrate
titration A4500•CI~ C
Hach Chloride te.<;t kit
model g.p
El 20. l/SW9050, direct
reading meter
Colorimetric
Hach Method # 8146
Equilibr:ition wii.h gas
in I.he field.
Determined with a
reducinl! 11:as deteclOr.
EPA 200.7 or EPA
200.9
- ----
Comments Data Use
Ion chromalOgraphy General water quality
(IC) method E300 parame1cr used as a marker
or method SW9050 to verify that site samples
may also be used are obtained from lhe same
ground-water system. Final
product of chlorinated
solvent reduction.
Silver nitrate Ar. above, and to guide
tilnltion selection of additional <lat.a
poinls in real time while in
the field.
General water quality
parameter used as a marker
10 verify that site samples
are obtained from the same
e.round-water svstem.
Filter if mrbid. May indicate an anaerobic
degradation process due lo
deple(ion of oxygen,
nitrate, and manganese.---
Optional Detennined terminal
specialized analysis electron accepting process.
Predicts 1he possiblity for
reductive dechlorination.
To determine if anaerobic
biological activity is
solubilizing manganese
from the aquifer matrix
material.
---- ---
Recommended Sample Volume, Field or
Frequency of Sample Container, Fixed.Base
Analvsis Sanmle Preser.·alion Laboratorv
Each sampling Collect 250 mL of Fixed•base
round water in a glass
container.
Each sampling Collect 100 mL of Field
round water in a glass
cont.ainer.
Each sampling Collect 100 to 250 mL Field
round of water in a glass or
p_l_astic conlainer.
Each sampling Collect from a flow-Field
round through or over-flow
cell / analyze at the well ----head.------·----------------.
One round of Sampled at well head Field
sampling on requires the production
selected wells. of 300 mL per minute
of water for 30 minutes.
One round of Collect 100 ml in a Laboratory
sampling glass or plastic
cont.ainer that is rinsed
in the field with I.he
ground water to be
sampled. Unfiltered
samples ob(ained using
low flow sampling
methods are preferred
for analysis of dissolved
metals. Adjust pH to 2
wilh nitric acid. Do not
insert pH paper or an
electrode into the
samnle.
-----

-
---·----

-

-
--.. -
Table 2.1 (Continued)
Recommended Sample Volume, Field or
Frequency of Sample Container, Fixed-Ba~
Matrix Annh·sls l\lelhod/Reference Comments Data Use Analysis Sample Preservation Laboratorv
Water Methane, ethane. Kampbell et al. 1989 Method published The pre5ence of CH.i Each sampling Co1Iect water !iamp1es Fixed-ba5e
and elh.ene and 1998 or SW38IO by researcher.; at the suggesl,; DTEX degradation round in 50 mL gla~,; serum
Modified U.S. Environmental via methanogenesii.. bottles with gray butyl
Protection Agency. Ethane and ethcne data are {Tenon-faced septa and
-• Limited to few used where chlorinated crimp caps: add HiSO4
commercial labs. solvent,,; are suspected of to pH less than 2, cool
undergoing biological to 4°C.
transformation.,
Water Nitrate IC method EJ00 Substrate for microbial Each sampling Collect up to 40 mL of Fixed-base
respiration if oxygen is round water in a glass or
depleted. pla,;tic container; add
H:i$04 to pH Jess 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 electrodes; influences and is influenced round through cell or an over-
-"'
potential results are displayed by the nature of the flowing container filled
on a meter; protect biologically mediated from the bottom to
samples from degradation of prevent exposure of the
exposure to oxygen. contaminants; the ORP ground water to the --
~eport rec.ult,; ___ (expre.c.sed a,; Eh)-of -------atmosphere. --
--------------. --against a ground water may range
silver/silver chloride from more than 800 mV to
reference electrode. le.,;s than -400 mV.
(Eh) is calculated by
adding a correction
factor specific to the
electrode used.
Water Oxygen Dissolved oxygen meter Refer to The oxygen concentration 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 pathway.
Water pH Field probe with direct Field Aerobic and anaerobic Each sampling Measure dissolved Field
reading meter calibrated biological processe,,; are round oxygen on site using a
in the field according to pH-sensitive. flow-through cell or
the supplier's over-flow cell.
specifications.
--
N 0
--------- -----
Table 2.1 (Continued)
Recommcr:.dcd Sample Volume,
Frequency of Sample Container,
Matrix Anal,·sis I\ lethod/Ref ere nee Comments Data Use Analvsis Samnle Preservation
Water Sulfate (S0i1) IC method EJ00 If this method is Substrate for anaerobic Each ~mpling Collect up to 40 mL of
used for sulfate microbial respiration. round water in a glass or
analysis. do not use pla.,;lic container; cool
the field method. to4<>C.
Water Sulfate (S0,·2) Hach method # 8051 Colorimetric, if this Same as above. Each sampling Collect up to 40 mL of
method is u:i;cd for round water in a glass or
sulfate analysis, do pJa,;tic container; cool
not use the fixed-to 4°C.
base laboratory
method.
Water Temperature Field probe with direct Field only To determine if a well i5 Each 5ampling Read from oxygen
reading meter. adequately purged for round meter.
sanmlin2.
Water Total Organic SW9060 Laboratory U!i.ed to classify plume and Each 5ampling Mea5ure using a flow-
Carbon also to detennine if reducti \'e round through cell or over-
called DOC dechlorination is possible now cell.
in the absence of --.. ---·------
----. -· ----------.. · :illthforioienic cai"°ho~." ----- -
NOTF.S:
I. "Hach'" re[ers to the Hach Company catalog, 1990.
2. "A" refers to Standard Methods for the Examination of Water and Wmtewater, 18th edition, 1992.
3. "E" rc[ers to Metltodrfor C/temical Analysis of Water and Wastes. U.S. EPA, 1983.
4. "SW" refers to the Test Methods for E1·aluati11g Solid H'aste, Physical, and Chemical Methods, SW-846, U.S. EPA, 3rd edition, 1986.
-- -
Field or
Fixed-Base
Laboratorv
Fixed-base
Field
Field
Laboratory
-----
---- ------- - ---- --
Table 2.2 Objectives for Sensitivity and Precision lo Implement the Natural Allenualion Protocol. Analyses other than those listed in lhis table may be
required for regulatory compliance.
Matrix Analysis l\-tcthod/Reference l\linimum Limit of Precision Avallahlllty l'otcntlal Data Quality
Ouantification Prnblerm
Soil Aromatic and SW8260A l mg/Kg Coefficient of Variation of Common laboratory Volatile.<; lost during shipment
chlorinated 20 percent. analysis. to laboratory; prefer extraction
hydrocarhons in the field.
(benzene.
toluene.
ethylhenzene. and
xylene (BTEXJ:
chlorinated
conwounds)
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.
om
Soil Total organic SW9060 modified for 0.1 percent Coefficient of Variation of Common laboratory Samples must be collected
carbon (lUC) soil samples 20 percent. analysis. from contaminant-
transporting (i.e.,
transmissive) intervals.
Soil Gas Fuel and EPA Method TO-I 4 I ppm Coefficient of Variation of Common laboratory Potential for atmospheric
Chlorinated (volume/volume) 20 percent. analysis. dilution during sampling.
voe,
-Soil Gas-Methane,-Oi; C::02--Field Soil Ga,;-Analyzer · -I-percent· ---~ --Coefficient of Variation of· . ·Readily available field ·Jnstrument musrbe properly -.
(volume/volume) 20 oercent. instrument. calibrated.
Water Alkalinity Hach alkalinity te,,;t kit 50 mg/L Standanl deviation of 20 Common field analysis. Analyze sample within I hour
model AL AP MG-L m!!IL. 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
comnoundsl
Water Chloride IC method E3lXl I mg/L Coefficient of Variation of Common laboratory ----
20 percent. analvsis.
Water Chloride Hach Chloride test kit I mg/L Coefficient of Variation of Common field analysis. Possible interference from
(optional, see model 8-P 20 percent. turbidity.
data use)
Water Conductivity E120.l/SW9050, direct 50 µSiem' Standard deviation of 50 Common field probe. Improperly calibrated
reading meter µS/cm2• instrument.
-
-- -- -
Table 2.2 (Continued)
N N
Matri-,:
Water
Water
Water
Wa1er
Water
Water
Water
-. ---
Water
Water
Water
Water
Water
Notes:
Analysis
Hydrogen (Hi).,
Iron (II} (Fe2•J
xx
Major Cations
Methane, ethane,
and elhene
Nitrate
Oxidation-
reduction
potential (ORP)
Oxygen
-·----. -
Sulfate (SO/·)
Sulfate (SO/·)
xx
pH
Temperature
Total Organic
Carbon
-
----
~lethod/Rcrerence I\1ininmm Limit of
Quantification
See Appendix A 0.L nM
Colorimetric 0.5 mg/L
Hach Method# 8146
SW60IO I mg/L
Kampbell et al., 1989 or I µg/L
SW38 I0 Modified
IC method EJ(X) 0.1 mg/L
A2580B plus or minus
300mV
Dissolved oxygen meter 0.2 mg/L
. ·-~------------
IC method EJOO 5 mg/L
Hach method ff 805 l 5 mg/L
Field probe with direct 0.1 standard units
readinl?. meter.
Field probe with direct 0 degrees Celsius
reading meter.
SW9060 0.1 mg/L
------- --
Precision Availability Potentiel Data Quality
Problems
Standard deviation of Specialized field Numerous, see Appendix A.
0.lnM. analysis.
Coefficient of Variation of Common field analysis. Possible inlerference from
20 percent. turbidity (must filter if turbid).
Keep out of sunlight and
analyze within minules of
collection.
Coeflicient of Variation of Common laboratory Possible colloidal
20 percent. analysis. interferences.
Coefficient of Variation of Specialized laboratory Sample must be preserved
20 percent. analysis. against biodegradation and
collected without headspace
{to minimize volatilization).
Standard deviation of 0.1 Common laboratory Must be preserved.
mg/L analysis.
plus or minus 50 mV. Common field probe. Improperly calibrated
electrodes or inLroduction of
atmospheric oxygen during
samoline:.
Standard deviation of 0.2 Common field Improperly calibra1ed
mg/L. instrument. electrodes or bubbles behind
·-----------------· ---·the niembrane-ora-fOUICd --1··
membrane or introduction of
atmospheric oxygen during
samolinl?..
Coerficient of Variation of Common laboratory. Fixed-base.
20 oercent.
Coefficient of Variation of Common field analysis. Possible interference from
20 percent. turbidity (must filter if turbid).
Keep sample cool.
0.1 standard units. Common field meter. Improperly calibrated
instrument; time sensitive.
Standard deviation of 1 Common field probe. Improperly calibrated
de1?.rees Celsius. instrument; time sensitive.
Coefficient of Variation of Common laboratory
20 percent. analysis.
** Filter if turbidity gives a response from the photometer before addition of the reagents that is as large or larger than the specified minimum quamification 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 chl6rinated solvents (e.g., Bouwer et al., 1981;
' Miller and Guengerich, 1982; Wilson and Wilson, 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
I and McCarty, 1991a, 1991b; DeStefano et al., 1991; Henry, 1991; McCarty et al., 1992; Hartmans
and de Bont, 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
chlorinated 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. In 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 electr,on 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 eiectron 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
I 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 ou\ the reactions, but they will facilitate only
those oxidation-reduction reactions that have a net yield of energy. ' .
2.2.1. I 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 chlotine 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 occ'/rs 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 pfoducts. 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 rhore common intermediate than trans-1,2-
DCE, and that I, 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 @ Chlorine Atom
Cl Cl © Carbon Atom
u 0 Hydrogen Atom
Cl Single Chemical
Bond
TCE -Double Chemical
Bond
Cl Cl
r!?
1, 1-DCE cis -1,2, -DCE trans-1,2-DCE
Cl
Vinyl Chloride
Ethene
%
· Complete Mineralization V~@
Ethane
Figure 2.2 Reductive dehalogenation of chlorinated ethenesl
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the accumulation of daughter products and an increase ii/ 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 ~nd Richardson (1993) have postulated that
this rate decrease may explain the accumulation of VC: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, occur under sulfate-reducing a~d 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.
2.2.1.1.2 Electron Donor Reactions
I
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 chlortna_ted aliphatic hydrocarbons (e.g., VC) can
be used as the primary substrate in biologically media:ted 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) d~scribe 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 Jong 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 (1997) show mineralization of DCE to carbon dioxide
under aerobic, Fe(III) reducing, and methanogenic conditions, respectively.
2. 2. 1. 1. 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 of 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 dqcumented 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 of BTEX or other carbon sourcbs, 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 1 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 o,f 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 l 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~ plume exhibiting Type I behavior, the
following questions must be answered:
I) Is the electron donor supply adequate to allow microbial reduction of the chlorinated
I organic compounds? In other words, will the micrporganisms "strangle" before they
"starve" (i.e., will they run out of chlorinated aliph/ltic 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) Is VC oxidized, or is it reduced? ' I
' Appendices B and C discuss what these questions mean and how they are answered. Type l
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 u1tilization of this natural carbon source
I 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 answerdd. Type 2 behavior generally results in ' slower biodegradation of the highly chlorinated solvents than Type I 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. 1
2.2.1.2.3 Type 3 Behavior 1 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 cap~ble of bi ode gradation of chlorinated
solvents.
26
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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 l 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 l 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
I
mixed behavior.
PCE➔ TCE➔DCE➔ VC-i: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 tha( no ethene is produced during this reaction.
Vinyl chloride is removed from the system much faster 11nder these conditions than it is under VC-
reducing conditions. :
A less desirable scenario, but one in which all co~taminants may be entirely biodegraded,
I involves a plume in which all chlorinated aliphatic hydrocarbons are reductively dechlorinated via
Type l or Type 2 behavior. Vinyl chloride is reduced tJ 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 .ind Gossett ( 1989). In this type of plume,
I VC degrades more slowly than TCE, and thus tends to accumulate.
2.2.2 Bioattenuation Screening Process i
An accurate assessment of the potential for natural bi ode gradation of chlorinated compounds
should be made before investing in a detailed study of n1tural 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 mc\ney are expended. If the site is regulated
under CERCLA, much of the data required to make the preliminary a·ssessment 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 procesk is designed to recognize geochemical
environments where reductive dechlorination is plau:sible. It is recognized, however, that
bioodegradation of certain halogenated compounds caj, also proceed via oxidative pathways.
Examples include DCE, VC, the dichloroethane,s, chloroethane, dichlorobenzenes,
monochlorobenzene, methylene chloride, and ethylene di.bromide.
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. Fig\lre 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 directio~ of ground-water flow.
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Analyze Available Site Data
Along Core of Plume
' k'=---1 Collect More Screening Data
to Determine if Biodegradation
is Occurring
No or
Insufficient
Data
Locate source(s)and
potential points of
exposure. Estimate
extent of NPAL,
residual and free-phase
Determine Groundwater Flow and
Solute Transport Parameters Along
Core of Plume using
Site-Specific Data; Porosity and
Oispersivity 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 Natur'al 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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Table2.3 Analytical Parameters and Weighting for Preliminary Screening for Anaerobic
Biodegradation Processes" '
Concentration In
Most Contaminated
Analysis Zone ' Interpretation Value
KJxygen· <0.5 mg/L Tolerated, suppress~s the reductive pathway at higher 3
r.oncentrations ' n..,.,r.en· >5mQ/L Not tolerated; however, VC mav be oxidized aerobicallv -3
Nitrate· <1 mall At hinher concentrations mav comoete with reductive nathwa• 2
Iron 11" >1 mg/l Reductive pathway possible; VC may be oxidized under Fe(III 3
L.educina conditions .
~ulfate· <20 mn/l At hi□her concentrations mav comnete with reductive oathwa, 2
"-ulfide* >1 mall Reductive nathwav oossible 3
Methane· <0.5 mg/l VC oxidizes 0
>0.5 mail· ,ultimate reductive dauahter oroduct, VC Accumulates 3
Oxidation Reduction 1<50 millivolts (mV) Reductive pathway possible 1
Potential" (OAP) <-100mV Reductive pathway likely 2
against Ag/AgCI : .... lectrode
DH° ~<pH < 9 Optimal range for reductive pathway 0
, > oH >9 hutside ontimal ranae for reductive nathwav -2
JOC o-20 mg/l Carbon and energy source; drives dechlorination; can be 2
,atural or anthroooa0nic .
emoerature• > 2d'C At T >20'-'r. biochemical orocess is accelerated 1
'"'arbon Dioxide >2x backaround Ultimate oxidative da"uahter oroduct 1
A1kalinitv >2x. backaround Results from interaction between CQ and anuifer minerals 1
-hloride .. >2x backaround Daunhter nroduct of oraanic chlorine 2
-rudroaen >1 nM Reductive nathwav oOssible. VC mav accumulate 3
Hvdrooen <1 nM VC oxidized 0
Jolatile Fatty Acids > 0.1 mg/l Intermediates resulting from biodegradation of more complex 2
comoounds; carbon .ind enern" source
BJEX" > 0.1 mail Carbon and enernv s·ource; drives dechlorination 2
etrachloroethene Material released 0
tfrichlor□ethene· Material released ' 0
Dauohter nroduct of PCE 2'1
DCE" !Material released ' 0
Daughter product of TCE 2"
f cis is > 80% of total DCE it is likely a daughter product
1 1-DCE can be chemical reaction oroduct of JCA
vc· Material released I 0
Dauohter nroduct of DCE 2"
1.1 1-Trichloroethane· Material released
'
0
DCA Daunhter nroduct of TCA under reducinn conditions 2
Carbon Tetrachloride Material released I 0
:::hloroethane• Dau□hter oroduct of DCA or VC under reducina conditions 2
Ethane/Ethane >0.01mg/l Daughter product of ~C/ethene 2
>0.1 moil . 3
:;hJoroform Material released 0 Dau□hter oroduct of Carbon Tetrachloride 2
Dichloromethane Material released
Dauohter oroduct of Chloroform
0
2
• Requm:d analysis. n/ Points nwarded only 1f 11 can be shown 1hat the compound 1s a daughter product (1.e .. not n conSIICIICOf 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: ! .
1) Determine if biodegradation is occurring using geochemical data. If biodegradation 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 altema-
' 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 giyen 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 migra~on is accepable or unacceptable with
respect to site-specific remediation objectives. 1 •
3) Locate source(s) and potential points of exposure. 1Ifsubsurface NAPLs are sources,
estimate extent of residual and free-phase NAPL. ·
' 4) Estimate the biodegradation rate constant. Biodegradatio.n 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 m9dified 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 biodegradation 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 1
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 (I) the most contaminated portion ~f 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 sam'pling should be conducted in accordance
with Appendix A.
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Dissolved Contaminant Plume
Source Area
0
0
0
Direction of Plume Migration
O Representative Sampling Location
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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. IJ conjunction with the sample collected in
the NAPL source zone, samples collected in the dissol ✓ed plume downgradient from the NAPL
source zone allow the investigator (I) to determine if th6 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 remediJted. The upgradient and lateral samples
allow delineation of the plume and determination of ba'ckground concentrations of the electron
acceptors and donors. · !
After these samples have been analyzed for the paraineters 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 biodegrJdation 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
I for each score. If the score totals 15 or more points, it is likely that bi ode gradation 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 chlorin ted organics
6 to 14 Limited evidence for anaerobic biodegradation* of chlorinate organics
15to20 Adequate evidence for anaerobic biodegradation* of chlorina ed organics
>20 Strong evidence for anaerobic biodegradation* of chlorinated organics
*reductive dechlorination
The following two examples illustrate how Step I of the screening procei s is implemented.
The site used in the first example is a former fire training area contaminate, 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 favo able. The second
example contains data from a dry cleaning site contaminated only with chlorina ed solvents. This
site was contaminated with spent cleaning solvents that were dumped into a shallo w dry well situated
just above a well-oxygenated, unconfined aquifer with low organic carbon concentt ations of dissolved
organic carbon.
Example 1: Strong Evidence for Anaerobic Biodegradation (Reductive Dec, lorination) of
Chlorinated Organics .
Analyte Concentration in Most Contaminated Zone Points A warded
Dissolved Oxygen 0.1 mg/L 3
Nitrate U,j mg, L 1,
lronr11, IU mg/I j
:Sultate 1. mg11 L.
1v1etnane .) ffi0'/1 j
ORP -190inV 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 ti P/L 2
Total Points Awarded 23 Points
In this example, the investigator can infer that biodegradation is likely occu ring at the time of
sampling and may proceed to Step 2.
Example 2: Anaerobic Biodegradation (Reductive Dechlorination) Unlikely
Analyte Concentration in Most Contaminated Zone Point< Awarded
Dissolved OxvPen 3 mP/L .3
Nitrate 0.3 mg/L 2
Iron (II) Not Detected (ND) 0
Sulfate IO mg/L 2
Methane ND 0
ORP + 100 mV 0
Chloride background 0
TCE (released) 1,200 Ug/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 o curring or is
occurring too slowly to contribute to natural attenu.ation at the time of the sampling. In this case,
the investigator should evaluate whether other natural attenuation processes can me t the cleanup
objectives for the site (e.g., abiotic degradation or transformation, volatilization o sorption) or
select a remedial option other than MNA.
Step 2: Determine Ground-water Flow and Solute Transport Parameters
After it has been shown that biodegradation is occurring, it is important to qua tify ground-
water flow and solute transport parameters. This will make it possible to use a so ute transport
model to quantitatively estimate the concentration of the plume and its direction and ate of travel.
To use an analytical model, it is necessary to know the hydraulic gradient and hydrauli conductivity
for the site and to have estimates of porosity and dispersivity. It also is helpful to know e 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 hav site-specific
hydraulic gradient and hydraulic conductivity data. To determine the ground-water fl wand solute
transport direction, it is necessary to have at least three accurately surveyed ells in each
hydrogeologic unit of interest at the site. The porosity and dispersivity are gener ly 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 oefficient of
retardation; otherwise, it is conservative to assume that the solute transport and round-water
velocities are the same. Techniques to collect these data are discussed in the append ces.
Step 3: Locate Sources and Receptor Exposure Points
To determine the length of flow for the predictive modeling to be conducted i Step 5, it is
important to know the distance between the source of contamination, the leading e ge along the
core of the dissolved plume, and any potential downgradient or cross-gradient rece tor exposure
points.
Step 4: Estimate the Biodegradation Rate
Biodegradation is the most important process that degrades contaminants in th 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 biodegradation rates estimated from field data c llected along
the core of the plume. Calculation of site-specific bi ode gradation rates is discussed in Appendix C.
If it is not possible to determine site-specific biodegradation rates, then literature v lues may be
used in a sensitivity analysis (Table C.3.5). A useful approach is to start with avera e values, and
then to vary the model input to predict "best-case" and "worst-case" scen,ario . Estimated
biodegradation rates can be used only after it has been shown that biodegradation is ccurring (see
Step 1). Although literature values may be used to estimate biodegradation rates in the ioattenuation
screening process described in Section 2.2, additional site information should b collected to
dete1mine biodegradation rates for the site when refining the site conceptual model, a 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 ate of solute
transport to the rate of attenuation is best accomplished using an analytical model. S veral 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 att nuation is to
determine if natural attenuation processes will be capable of attaining site-specifi remediation
objectives in a time period that is reasonable compared to other alternatives (i.e., to uantitatively
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estimate if site contaminants are attenuating at a rate fast enough to prevent further lume migration
and restore the plume to appropriate cleanup levels). The analytical model BIOS REEN can be
used to determine whether natural attenuation processes will be capable of meet· g site-specific
remediation objectives at some distance downgradiant of a source. The numerical mo el BIOPLUME
III can be used to estimate whether site contaminants are attenuating at a rate fast e ough to restore
the plume to appropriate cleanup levels It is important to perform a sensitivity nalysis to help
evaluate the confidence in the preliminary screening modeling effort. For the u oses of the
screening effort, if modeling shows that the screening criteria are met, the investiga or can proceed
with the natural attenuation evaluation.
Step 6: Determine if Screening Criteria are Met
Before proceeding with the full-scale natural attenuation enluation, the inv tigator 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 e known (or
estimated) time since the contaminant release and the contaminant velocity in ground
water, as calculated from site-specific measurements of hydraulic conducti ity and
hydraulic gradient, and estimates of effective porosity and contaminant ret dation?
• ls it likely that site contaminants are attenuating at rates sufficient to meet r mediation
objectives for the site in a time period that is reasonable compared to other ltematives?
If the answers to these questions are "yes," then the investigator is encouraged t proceed with
the full-scale natural attenuation demonstration.
2.3 COLLECT ADDITIONAL SITE CHARACTERIZATION DATA TO EV LUATE
NATURAL ATTENUATION AS REQUIRED
It is the responsibility of the proponent to "make the case" for natural attenu tion. Thus, a
credible and thorough site assessment is necessary to document the potential for natu al attenuation
to meet cleanup objectives. As discussed in Section 2.1, review of existing site c aracterization
data is particularly useful before initiating site characterization activities. Such eview should
allow identification of data gaps and guide the most effective placement of additional ata collection
points.
There are two goals during the site characterization phase of a natural attenuatio investigation.
The first is to collect the data needed to determine if natural mechanisms of contamin t attenuation
are occurring at rates sufficient to attain site-specific remediation objectives in a tim period that is
reasonable compared to other alternatives: The second is to provide sufficient site-s ecific data to
allow prediction of the future extent and concentrations of a contaminant plume thro gh solute fate
and transport modeling. Thus, detailed site characterization is required to achieve ese goals and
to support this remedial option. Adequate site characterization in support of natur 1 attenuation
requires that the following site-specific parameters be determined:
• Location, nature, and extent of contaminant source area(s) (i.e., areas contai ing mobile
or residual NAPL or highly contaminated ground water).
• Chemical properties (e.g., composition, solubility, volatility, etc.) of contami ant source
materials.
• The potential for a continuing source due to sewers, leaking tanks, or pipelin s, or other
site activity.
• Extent and types of soil and ground-water contamination.
o 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 nontransmi sive units;
-Potential pathways for NAPL migration (e.g., surface topography and di 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, a d exposure
points.
Many chlorinated solvent plumes have enough three-dimensional expressi n to make it
impossible for a single well to adequately describe the plume at a particular locatio on a map of
the site.
Figure 2.5 depicts a cross section of a hypothetical site with three-dimensional xpression of
the plume. A documented source exists in the capillary fringe just above the wate table. Such
sources are usually found by recovering, extracting, and analyzing core material. Thi material can
be (I) a release of LNAPL containing chlorinated solvents; (2) a release of pure chlori ated. solvents
that has been entrapped by capillary interactions in the capillary fringe; or (3) ma rial' 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 throu h this source
produces a plume that appears to dive into the aquifer as it moves away from the sourc . This effect
can be caused by recharge of clean ground water above the plume as it moves downg adient of the
source, by collection of the plume into more hydraulically conductive material at e bottom of
aquifer, or by density differences between the plume and the unimpacted ground wat r.
Below the first hydrologic unit there is a second unit that has fine-textured mate ial at the top
and coarse-textured material at the bottom of the unit. In the hypothetical site, the me-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 form a second source of ground-water c ntamination
in the first unit. Because DNAPL below the water table tends to exist as diffuse and wi ely extended
ganglia rather than of pools filling all the pore space, it is statistically improbable tha the material
sampled by conventional core sampling will contain DNAPL. Because these sources a e 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 ma rial and has
formed a second cryptic source area at the bottom of the second hydrologic unit. Comp e 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 e plume in
the first unit. Biological processes occurring in one hydrological unit may not occur n another; a
plume may show Type 2 behavior in one unit and Type 3 behavior in another.
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Figure 2.5 A cross section through a hypothetical release, illustrating the three-dimensio al character
of the plumes that may develop from a release of chlorinated solvents.
· Documented NAPL
.. / .
>' --=----~-=:>
Figure 2.6 A stacked plan representation of the plumes that may develop from the hypoth tical release
depicted in Figure' 2.5. Each plan representation depicts a separate plume th t can
originate from discrete source areas produced from the same release of chlori ated solvents.
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As a consequence, it is critical to sample and evaluate the three-dimensional haracter of the
site with respect to (1) interaction of contaminant releases with the aquifer ma ·x material, (2)
local hydological features that control development. and migration of plumes, and (3) e geochemical
interactions that favor bioattenuation of chlorinated solvents.
The following sections describe the methodologies that should be imple
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 hydro geologic system and to det ine the three-
dimensional distribution of mobile and residual NAPL that can act as a continuing s urce 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 contaminati n in the vadose
zone. Depending on the status of the site, this work may have been completed uring previous
remedial investigation work. The results of soils characterization will be used as i ut into a solute
fate and transport model to help define a contaminant source term and to sup ort the natural
attenuation investigation.
The purpose of sampling soil and aquifer matrix material is to determin the subsurface
distribution of hydrostratigraphic units and the distribution of mobile and residua NAPL, as well
as pore water that contains high concentrations of the contaminants in the dissolv d phase. These
objectives can be achieved through the use of conventional soil borings or dire t-push methods
(e.g.; Geoprobe® or cone penetrometer testing), and through collection of soil g s samples. All
samples should be collected, described, analyzed, and disposed of in accordance ith local, State,
and Federal guidance. Appendix A contains suggested procedures for sample c llection. These
procedures may require modification to comply with local, State, and Federal gulations 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 cument natural
attenuation, including the effects of sorption, volatilization, and _biodegradation. Each analyte is
discussed separately below.
• Volatile Organic Compounds: Knowledge of the location, distribution, oncentration,
and total mass of contaminants sorbed to soils or present as mobile or i obile NAPL is
required to calculate contaminant partitioning from NAPL into ground w ter. 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 ater to the
atmosphere or other identified receptor for vapors is required to estimate xposure of the
human population or ecological receptors to contaminant vapors. If the x of vapors
can be compared to the discharge of the contaminants in ground water, th 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 atnx 1s
important for sorption and solute-retardation calculations. TOC samples hould be
collected from a background location in the stratigraphic horizon(s) wher most
contaminant transport is expected to occur.
• Oxygen and Carbon Dioxide: Oxygen and carbon dioxide soil gas mea rements can be
used to identify areas in the unsaturated zone where biodegradation is oc urring. This
can be a useful and relatively inexpensive way to identify NAPL source eas, particularly
when solvents are codisposed with fuels or greases (AFCEE, l 994).
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• Fuel and Chlorinated Volatile Organic Compounds: Knowledge of the istribution of
contaminants in soil gas can be used as a cost-effective way to estimate the xtent of soil
contamination,
2.3.2 Ground-water Characterization
To adequately determine the amount and three-dimensional distributio of dissolved
contamination and to document the occurrence of natural attenuation, ground-wate samples must
be collected and analyzed. Biodegradation of organic compounds, whether natural or nthropogenic,
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 import ce of natural
attenuation at a site.
Ground-water sampling is conducted to determine the concentrations and istribution 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 collecte , handled, and
disposed of in accordance with local, State, and Federal guidelines. Appendix A con ins suggested
procedures for ground-water sample collection. These procedures may need to e modified to
comply with local, State, and Federal regulations or to accommodate site-specific c nditions.
The analytical protocol for ground-water sample analysis is presented in T ble 2.1. This
analytical protocol includes all of the parameters necessary to delineate dissolved ontamination
and to document natural attenuation, including the effects of sorption and biodegr dation. Data
obtained from the analysis of ground water for these analytes is used to scientific lly document
natural attenuation and can be used as input into a solute fate and transport model. e following
paragraphs describe each ground-water analytical parameter and the use of each nalyte in the
natural attenuation demonstration.
2.3.2.1 Volatile and Semivolatile Organic Compounds
These analytes are used to determine the type, concentration, and distribution o contaminants
and daughter products in the aquifer. In many cases, chlorinated solvents are foun commingled
with fuels or other hydrocarbons. At a minimum, the volatile organic compound ( OC) analysis
(Method SW8260A) should be used, with the addition of the trimethylbenzene i omers if fuel
hydrocarbons are present or suspected. The combined dissolved concentrations f BTEX and
trimethylbenzenes should not be greater than about 30 mg/L for a JP-4 spill (Smith al., 1981) or
about 135 mg/L for a gasoline spill (Cline et al., 1991; American Petroleum Instil te, 1985). If
these compounds are found in higher concentrations, sampling errors such as em lsification of
LNAPL in the ground-water sample likely have occurred and should be investigate
Maximum concentrations of chlorinated solvents dissolved in ground water fro 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 sol bilities, then
sampling errors such as emulsification of NAPL in the ground-water sample have Ii ely occurred
and should be investigated.
2.3.2.2 Dissolved Oxygen
Dissolved oxygen is the most thermodynamically favored electron acceptor use by microbes
for the biodegradation of organic carbon, whether natural or anthropogenic. Anae obic bacteria
generally cannot function at dissolved oxygen concentrations greater than about .5 mg/L and,
hence, reductive dechlorination will not occur. This is why it is important to hav a source of
carbon in the aquifer that can be used by aerobic microorganisms as a primary subs ate. During
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aerobic respiration, dissolved oxygen concentrations decrease. After depletion o 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 RP of the ground
· water downward into the range within which reductive dechlorination can ccur. Reductive
dechlorination is most effective in the ORP range corresponding to sulf te reduction and
methanogenesis, but dechlorination of PCE and TCE also may occur in the O range associated
with denitrification or iron (III) reduction. Dehalogenation ofDCE and VC gen ally are restricted
to sulfate reducing and methanogenic conditions.
Dissolved oxygen measurements should be taken during well purging and i
and after sample acquisition using a direct-reading meter. Because most well p
can allow aeration of collected ground-water samples, it is important to minimi
aeration as described in Appendix A.
2.3.2.3 Nitrate
mediately before
rging techniques
the potential for
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 ia denitrification.
In order for reductive dechlorination to occur, nitrate concentrations in the conta ·nated portion of
the aquifer must be less than 1.0 mg/L.
2.3.2.4 Iron (ID
In some cases, iron (III) is used as an electron acceptor during anaerobic iodegradation of
organic carbon. During this process, iron (III) is reduced to iron (II), which may b soluble in water.
Iron (II) concentrations can thus be used as an indicator of anaerobic degradation o fuel compounds,
and vinyl chloride (see Section '.l.2.1.1.2). Native organic matter may also suppo reduction of iron
(II). Care·mu,t be taken when interpreting iron (11) concentrations because they ay be biased low
by reprecipitation as sulfides or carbonates.
2.3.2.5 Sulfate
After dissolved oxygen and nitrate have been depleted in the microbiologic l treatment zone,
sulfate may be used as an electron acceptor for anaerobic biodegradation. This rocess is termed
"sulfate reduction" and results in the production of sulfide. Concentrations of s !fate greater than
20 mg/L may cause competitive exclusion of dechlorination. However, in many lumes 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, r carbon dioxide
is used as an electron acceptor, and is reduced to methane. Methanogenesis gene ally occurs after
oxygen, nitrate, and sulfate have been depleted in the treatment zone. The prese ce of methane in
ground water is indicative of strongly reducing conditions. Because methane is n t present in fuel,
the presence of methane above background concentrations in ground water in co tact with fuels is
indicative of microbial degradation of hydrocarbons. Methane also is associated ith spills of pure
chlorinated solvents (Weaver et al., 1996). It is not known if the methane comes rom 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 inc eased alkalinity.
Increases in alkalinity result from the dissolution of rock driven by the production f carbon dioxide
produced by the metabolism of microorganisms. Alkalinity is important in th maintenance of
ground-water pH because it buffers the ground water system against acids gener ted during both
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aerobic and anaerobic biodegradation. 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/ AgCI) as the reference electrode. This protocol
uses the potential against the Ag/ AgCI electrode as the screening potential, not Eh as would be
measured against the standard hydrogen electrode.
2.3.2.9 Dissolved Hydrogen
ln 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 arc 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 a\ld Goodwin, 1988;
Lovley et al., 1994; Chapelle et al., I 995). 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(Ill),
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(Ill) reducers are slightly less efficient and thus maintain somewhat higher H2 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, H2 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
Methane 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 en 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, H2 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 Jess 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 denitrification (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 1 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 CJ· to CI•', the chloride form (CJ·) 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 coliect 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 th'e 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. I .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 (1993) and Molz et al. (1994) 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 abl.e 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
groundswater 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 inany 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
contaminantmass 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 v.ertical 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 can ]Jrovide 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.
Biodegradation 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 refineml'nt 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 permeability, 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 strat;graphic 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 Jogs 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 section~ should be prepared from boring logs and/or CPT data. A minimum of
two hydro geologic 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 ofNAPL 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 hydrogeologic 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, contarninantcontour 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, thes_e
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 confident that 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 con~_entrations 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 CaCO,). 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 C~lculations "
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 NAPUWater 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. ( 1988), 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 equilihrium
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 fosi-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 det,ermining first-order rate constants are de~cribed 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 Bu3check
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 piumes 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 ~ite-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 adequ_ate 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,
sorptiun; 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 i'i a_viable remedy, the proponent of natural 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 exposure .pathways analysis. If conservative
model input parameters are used, the solute fate and transport model should give conservative
estimates ofcontaminant plume migration. From this information, the potential for irripacts on
human health. and the environmerit from contamination present at the site can be assessed.
2.7 EVALUATESUPPLEMENTAL 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 contaminant
source.term; this will allow for a reevaluation of the exposure pathways analysis.
In so'me 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, orto
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 a·:tain 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. Longcterm 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: I) 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 cour.lation. 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. ;
Dissolved Contaminant Plume
S ourca Area Plume of Geochemlco.l lndlcators
Direction of Plume Migration
0 Long Term Monitoring Wells
e Performance Evaluation Wells
0
•
•
•
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 nu,ipb~~t:n~,placement,of long-term monitoring wells and performance
evaluation wells should be dctcrinined through regulatory negotiation, the locations of long-term
monitoring welh 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 erisure that
all data required for regulatory decision making are collected. Water level and NAPL thi~kness
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 behaviorof the plume. After the first year, an appropriate sampling frequency
should be c,;tablished 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 arl!
low, the time required for ground water to move from up gradient monitoring wells to down gradient
monilc;ring 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 FIN.DINGS
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, µre-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 artain site-specific remediation
objectives in a time period that is reasonable compared to other alternatives, and that human he,:lth
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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--...,._ ......
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Banerjee, P., Piwoni, M.D., and Ebeid, K., 1985, Sorption of organic contaminants to a low
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1-and 2-carbon organic compounds: Environ. Sci. Technol., 15(5):596-599.
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Bouwer;,EJ. and Wright, J.P., 1988, Transformations of trace halogenated aliphatics in anoxic
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in groundwater beneath a manufacturing facility: Ground Water, 32(2):215-226.
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