2021.09.14_CCOA.p3b_DEQ 60% Barrier Wall Comment Response Table

DEQ Comment/Response Table to Groundwater and Seeps Remedy 60% Design Submittal (August 2021)
No.
VERSION 5-B - September 14, 2021
Section(s), Chemours Groundwater and Seeps Remedy
60% Design Presentation and Submittal
DEQ Comment Action Item for Chemours
GREEN - Action Not Needed
TAN - Approval Pending further Review of Limited Data
RED - Deficient Area, Focus Needed
Chemours Response
General Comments about the Submittal, dated 8-13-2021
GEN. 1 (Page vi). Professional Signatures and Seals
All Appendices
The report related to activities of a geological nature is signed and sealed by a Professional Geologist. The report indicates
that the engineering activities are being supervised by a licensed Engineer, however the document does not state who the
licensed Engineer is and is not co-signed or contains a P.E. Stamp. The Engineering Board Rules (21 NCAC 56) includes Section
.1103 ( c ) which provides an exception for required certification related for documents in progress. Item (2) states "Progress
Drawings - Do not use for Construction".
The Geosyntec text in the Professional Signature and Seals section
reads "in-progress and not for construction". However, additional
text is needed on this page to clarify to the public whom was
responsible for the engineering portions of the document and why
an engineer has not signed the document (e.g. citing the P.E. Board
rule and exception). Considerable technical knowledge is shown in
these documents and authors should be specified.
GEN. 2 All Appendices
There are seven appendices to the document and no authors from GEOServices and/or Geosyntec are specified. Considerable
technical knowledge is shown within these documents. Appendices A and G are documents previously submitted to the
NCDEQ. When the PDI report was originally submitted it was certified by a Professional Geologist but it is not certified here.
Certify Appendix A and Appendix G. See the engineering board rule
21 NCAC 56 .1103 ( c ) for attribution that is allowed for documents
previously certified by a P.E. but not certified in subsequent
submittals. The Geologists Licensing Act, N.C.G.S. 89E-13 indicates
that geologic papers or documents involving geologic work
prepared or approved by a Licensed Geologist shall be signed and
sealed if documents are submitted for the public record.
GEN. 3 Remediation items not addressed in the remedy as described, but what must be
considered
The NCDEQ desires that the groundwater recovery and treatment system be augmented by installation of extraction wells
closer to the source areas (see comments 5 and 6). We also request improvements in the recovery of highly contaminated
groundwater in the Monomers IXM and WWTP areas where Chemours is pumping small volumes from three monitoring wells.
We acknowledge that the existing monitoring well pumping has decreased some PFAS concentrations; however, the quarterly
reports demonstrate that groundwater contamination remains very high.
Refer to the action items for comments 5 and 6.
Specific Comments about the Submittal, dated 8-13-2021
1 (Page ES-1). Executive Summary The Site Hydrogeology does not list the Upper Cape Fear Aquifer which underlies the Chemours Site or the Floodplain
Deposits next to the River. Revise the Section to include the Upper Cape Fear Aquifer.
2 (Page 6) Section 2.1.1 - Site Geology and Hydrogeology
What is the nature of the soils in the perched zone, above the clay unit? The clay (perched clay) is discussed, but the not the
strata above it.
The Surficial Aquifer and the Upper Cape Fear Aquifer are not included in the list of water bearing units.
Revise the text to include the topics and answers to the questions
at left.
3 (Page 11) Section 3.1.1 - Summary of PDI Report and Appendix A
NCDEQ has prepared comments for the PDI Report, Version 2, separately from this document. Version 2 of the PDI report was
signed and sealed by a North Carolina Licensed Geologist (E. Beaumont Hodge) but the PDI report in Appendix A does not
contain the signature of Mr. Hodge or certification. See Comment Gen. 2.
Refer to comments from the PDI Report, version 2 and make
corrections to this report and Appendix A. Also address the action
item for comment Gen. 2.
Page 1 of 5
DEQ Comment/Response Table to Groundwater and Seeps Remedy 60% Design Submittal (August 2021)
No.
VERSION 5-B - September 14, 2021
Section(s), Chemours Groundwater and Seeps Remedy
60% Design Presentation and Submittal
DEQ Comment Action Item for Chemours
GREEN - Action Not Needed
TAN - Approval Pending further Review of Limited Data
RED - Deficient Area, Focus Needed
Chemours Response
4 (Page 12) Figure 3 High Resolution Cross Section with Interpreted Geology
[included within Section 3.1.1].
There is a darker green, solid shaded area in the UCF confining Unit that is not explained in the legend. What does this pattern
represent?
A pattern shown in the log for S-3 from an elevation of 9 to 13 feet is not explained in the legend. This similar pattern is shown
in about 9 other locations and has fewer black larger diameter dots than shown in the patch for Clayey Gravel. Does the S-3
patch represent sand with coarse grain to gravel sized particles?
In comparing the extraction well screened intervals with the high-resolution cross-section, we noted that dotted patterns in
the Black Creek Confining Unit are present. The legend indicates that these dotted patterns correspond with a zone of
interspersed coarser sediments. Near OW-7 and OW-8, the coarser sediments are present for almost the whole thickness of
the Black Creek Confining Unit]
The numbering sequence designating stations on the plans in Appendix D increase from north to south and are opposite the x-
axis scale shown (increases from south to north). Add the station numbers to this figure so the extraction well site plans may
be easily compared.
Please answer the three questions at left and update the document to
reflect this information, including adding the corresponding station
numbers (e.g. 30+00) onto the X axis of the Figure.
5 (Pages 11-17) Section 3.1.1 (Summary of PDI Report) and 3.1.3 (Remedy
Development)
The report states that results from modeling indicate that the groundwater in the northern portion of the alignment has lower
transmissivity based on the aquifer tests. The report also indicates that modeled contaminated groundwater in the northern
alignment can be captured by extraction wells alone.
1) The proposed barrier wall offers Willis Creek and the Cape Fear River protection from groundwater contamination
migrating to these receptors. Without the wall, the protection system is not as robust along the northern alignment. The
NCDEQ desires that this system be augmented by installation of extraction wells closer to the source areas, including the PPA
area where total Table 3+ PFAS groundwater concentrations exceeded 500,000 ng/L among three wells installed into the
surficial aquifer, SMW-05P, SMW-06B and SMW-08B. The PPA area is about 3,000 feet from the proposed extraction wells in
the northern alignment. We ask that extraction wells be set up with sufficient water extraction volumes per day to impact
existing contamination in this area and reduce migration of grounwater towards the barrier wall and surface water bodies.
2) A greater percentage of fine-grained soils (e.g. silt, clay) is reported in the saturated zone along the northern alignment
compared with the remainder of the barrier wall aligment. In fine-grained material the productivity of these wells may reduce
with time as screens become clogged. The radius of influence from the wells will decrease without maintenance. Please
describe the maintenance that will be performed to prevent clogging and to resolve clogging that occurs.
Reply to the comments at left. Recovery and remediation of
contaminated groundwater in the PPA area (surficial aquifer) has
not been addressed to date as required by the 15A NCAC 02L rules.
Please describe what will be done to address this issue. Perched
zone wells in the PPA area (SMW-05, SMW-06 and SMW-08) have
not been sampled in 2019 or 2020. Also refer to the next
comment.
6 (Page 17) Section 3.2.1 - Groundwater Extraction System Design
No attempt is made to specifically address removal or effective hydraulic control of the high concentrations of PFAS in the
perched zones, except for the recovery of contaminated groundwater from two monitoring wells in the Monomers IXM area
and one monitoring well near the Wastewater Treatment Plant. An additional groundwater extraction system should be
designed to address this contamination, or some type of hydraulic control, such as "Plumestop (R) Liquified Activated Carbon
(TM)", should be developed for this area.
The extraction well and barrier wall remedy do not address PFAS contaminated groundwater present in the floodplain
deposits spanned by wells PIW-1S, LTW-1, LTW-3, LTW-4 and PIW-7S. In 2020, PFAS contamination exceeded 100,000 ng/L
except for PIW-1S (~33,100 ng/L). Assuming that the barrier wall, groundwater extraction, and seep capture systems are
effective, contamination in the floodplain deposits should attenuate over the long-term. However, PFAS in these soils may
continue to act as a secondary source to the River.
The NCDEQ has discussed verbally and through email
correspondence our desire for Chemours to improve groundwater
yields and PFAS mass recovery below the Monomers IXM and
WWTP source areas as required in 15A NCAC 02L. The current
temporary pumping of the perched zone is not sufficient to reduce
concentrations to near NC Groundwater Standards. Address how
high HFPO Dimer Acid and other PFAS contaminants in
groundwater will be addressed in the perched zone throughout the
manufacturing area.
What plans are being considered to reduce the concentration of
PFAS in the floodplain deposits, over both the short term and long-
term?
Page 2 of 5
DEQ Comment/Response Table to Groundwater and Seeps Remedy 60% Design Submittal (August 2021)
No.
VERSION 5-B - September 14, 2021
Section(s), Chemours Groundwater and Seeps Remedy
60% Design Presentation and Submittal
DEQ Comment Action Item for Chemours
GREEN - Action Not Needed
TAN - Approval Pending further Review of Limited Data
RED - Deficient Area, Focus Needed
Chemours Response
7 (Page 18) Section 3.2.2 - Groundwater Conveyence System Design
The DEQ requests that design consideration be given for sudden, large volume discharges from the force main groundwater
conveyances. The design flow for the north main is about 855 gpm and about 370 gpm for the south main. The mains are
offset from the barrier wall about 20 feet. If the line is ruptured, the flow rate may cause water to daylight and this volume
should remain on the west, uphill side of the barrier wall. Are shutoff devices planned for the force main piping? What other
mechanisms or procedures are in place to limit the potential spill size?
Please respond to the questions listed in the column to the left.
8 (Page 19) Section 3.2.3 - Barrier Wall Design
A wall 0.5 meters (19.7 inches) in thickness does not seem sufficient to block migration of contaminants over the long term,
especially if bulges in the adjacent formation narrows the thickness. The verticality of the wall excavation will be extremely
important to facilitate generally smooth barrier wall surfaces over a 70 to 85 foot depth. How did the design engineers come
up with that thickness? How will they ensure verticality during construction? How will the wall be tested during and after
installation to ensure efficacy and no breakthrough of contaminated groundwater?
Silicified wood is common in the Black Creek Aquifer and the PDI report (Appendix A, Section 3.3.1) indicated wood chunks
were identified in some samples encountered in borings. How will the Barrier Wall mix account for the presence of wood
which may have greater porosity than the surrounding soils?
Additional comments are below related to Appendix D.
Please respond to the questions and comments listed in the column to
the left.
9 (Page 19) Section 3.2.3 - Barrier Wall Design
1) Please address the affects of possible chemical interactions between the known groundwater and soil contaminants and
the components of the barrier wall. Are the components of the wall compatible with PFAS?
2) Is the barrier wall design capable of stopping the transmission of known PFAS contaminants through the wall through
chemical diffusion? 3) Will pilot tests be done or how will you show the wall installation and materials were correct and
sufficient to ensure no gaps or breakthrough of the barrier wall in the various soil types and depths along the length of the
wall?
Please respond to the questions and comments listed in the column to
the left.
10 (Page 19) Section 3.2.4 - Constructability Considerations
Storms are becoming increasingly more severe, and it is probable that flooding will exceed the 100 year flood plain (elevation
of 68 ft. MSL) during the barrier's operational life. What measures will be taken to prevent erosion of the barrier wall near the
surface?
Will a portion of the barrier wall extend above ground to prevent water from overtopping the barrier and causing possible
damage to the extraction wells?
How will the barrier be marked on the surface with signage or markers to prevent damage from construction equipment or
future excavation?
Please respond to the questions listed in the column to the left.
11 (Page 20) Section 4 - Seeps A and B Ex-Situ Capture Remedy Design
Appendix F
The 60% Design Report indicates that several equalization basins and associated dams are planned to be constructed on the
site. The basins are indicated to retain the site runoff from 0.5" rainfall, and pass the runoff from storms greater than 1/2 inch
depth / 24 hours. Maximum spillway capacity is not indicated. Based on the intention to store contaminated fluids in these
basins, they will likely be classified as Class C (high-hazard) dams under 15A NCAC 02K (Dam Safety Regulations) due to
potential environmental damage resulting from a dam failure. The dam design drawings, specifications and associated design
reports and calculations must be submitted to the Division of Energy, Mineral and Land Resources Dam Safety Program for
review and approval prior to construction. Also, the dam and spillways will need to be designed to a much higher hydraulic
standard than that indicated in the 60% Report (1/3 of the Probable Maximum Precipitation or PMP). The 1/3 PMP rainfall for
the site area is approximately 14 inches in 24 hours.
Review the comments at left and submit the documents as requested by
the N.C. Dam Safety Program in the Division of Energy, Mineral and Land
Resources (DEMLR).
Page 3 of 5
DEQ Comment/Response Table to Groundwater and Seeps Remedy 60% Design Submittal (August 2021)
No.
VERSION 5-B - September 14, 2021
Section(s), Chemours Groundwater and Seeps Remedy
60% Design Presentation and Submittal
DEQ Comment Action Item for Chemours
GREEN - Action Not Needed
TAN - Approval Pending further Review of Limited Data
RED - Deficient Area, Focus Needed
Chemours Response
12 (Page 24) Section 5.2 - Groundwater Treatment Design
(Page 28) Section 8 - Operations and Maintenance
The 60% design report does not include discusson of a backup power supply to operate pumps in the extracton wells and the
seep basin pump stations (A and B). A strong storm could knock out power for several days. Particle tracking simulations
(Appendix B) noted that wrap around flow could occur over, around and through portions of the barrier after 7 days without
pumping of the wells. If there is no backup power supply, what will be done to repair and test the barrier wall after a period of
power outage?
Please respond to the questions listed in the column to the left.
13
Appendix B - 3-Dimensional Groundwater Flow Model, Geosyntec, August 2021
Sections 1 (Introduction and Objectives) and 3.2 (Flow Boundary Conditions)
Section 1: A groundwater flow model is being used to optimize the design of the groundwater remedy. The objective of the
remedy is to reduce the flow of PFAS contaminants into the Cape Fear River from groundwater baseflow. A groundwater flow
and contaminant transport model seems more appropriate for this task. Please explain if this was considered and if so, why it
was not utilized.
Section 3.2: "Bottom Boundary: Chosen as flat at an elevation of -20 ft above MSL which is located within the Upper Cape
Fear confining unit." Please provide the assumed elevation of the top of the Cape Fear confining unit and the elevation of the
bottom of the model domain with respect to MSL. It's not clear from the text or figures.
Please respond to the questions and comments listed in the column to
the left.
14
Appendix B - 3-Dimensional Groundwater Flow Model, Geosyntec, August 2021
Section 5: Remedial Design Simulations
1) "Geosyntec prepared a high resolution cross section along the groundwater remedy alignment." Can the flow model be
revised to simulate this more precise stratigraphy in the vicinity of the groundwater remedy?
2) Assigning the uppermost nodes of the northern model domain as constant heads and the underlying nodes as no flow
could account for the hydraulic barrier alone simulation causing almost all tracking particles to be captured by the extraction
wells since the wells are located so close to this boundary. The northern boundary should be extended further north of the
creek in order to provide a more realistic depiction of the groundwater flow in this scenario. Alternatively, since the location
of the extraction wells in this area appears to be based on the location of the previous barrier wall design, the optimal
placement of the extraction wells in this area should be re-evaluated.
Please respond to the questions and comments listed in the column to
the left.
15
Appendix B - 3-Dimensional Groundwater Flow Model, Geosyntec, August 2021
Section 5: Remedial Design Simulations
3) Please indicate which extraction wells are screened in the surficial aquifer and which are screened in the Black Creek
aquifer in the model simulation figures and resubmit.
4) A five year model run time was used to evaluate baseline flowpaths and the vertical barrier alone scenario. The model run
time was not indicated in the description of the evaluation of the hydraulic barrier alone and the hybrid (combination)
scenarios. Please provide the model run times for the hydraulic barrier alone and the hybrid scenarios.
5) The model was run assuming a hydraulic conductivity (K) of 1 x 10-6 cm/sec for the barrier. Was a sensitivity analysis
completed with regard to the barrier wall K value? How will you show that the installed wall meets this criteria?
6) The length of the barrier wall was reduced from 9,000 feet in the original design to 6,000 feet in the proposed optimized
scenario. What was used as the basis for determining the northern most extent of the barrier wall in the optimized scenario?
Please respond to the questions and comments listed in the column to
the left.
16
Appendix B - 3-Dimensional Groundwater Flow Model, Geosyntec, August 2021
Section 5.2: Vertical Barrier Alone
"Specifically, in the area near Seep A and B where there is high transmissivity, particles migrate over, around and through the
barrier and discharge to Cape Fear River at a relatively high rate." Please explain the mechanism(s) by which the particles
migrate through the barrier wall in this scenario.
Please respond to the comment listed in the column to the left.
Page 4 of 5
DEQ Comment/Response Table to Groundwater and Seeps Remedy 60% Design Submittal (August 2021)
No.
VERSION 5-B - September 14, 2021
Section(s), Chemours Groundwater and Seeps Remedy
60% Design Presentation and Submittal
DEQ Comment Action Item for Chemours
GREEN - Action Not Needed
TAN - Approval Pending further Review of Limited Data
RED - Deficient Area, Focus Needed
Chemours Response
17 Appendix C: Barrier Wall 60% Design Submittal, by GEOServices, LLC
The 60% Design report indicates that Chemours contractors' have consulted with barrier wall installation contractors but the
contractor has not been selected. NCDEQ requests that Chemours consider the following:
1) Chemours should carefully review the design and produce a strong performance specification. It will be incumbent on
the contractor to seek for the best method for creating a barrier wall with the stated permeability.
2) Barrier wall contractor selection to be based on past work experience.
3) Develop good specifications and testing protocols for review by NCDEQ. The chosen contractor must have a testing
laboratory onsite.
4) A robust QA/QC plan should be developed and NCDEQ requests review of the QA/QC plan.
5) Little information was provided in the report on the mix design. The design should be based on soil samples collected
from the site, bentonite and cement. Lab tests for permeability shall be conducted for varying proportions of the three
components to come up with the optimum design mix. QA/QC should be based on the testing protocol.
6) Post construction testing to test the integrity of the wall should be highlighted in the QA/QC Plan.
7) The QA/QC plan should include performance monitoring targeted towards assessment of the remediation system’s
impact on PFAS levels entering the Cape Fear River (CFR). The performance monitoring plan should consist of direct
monitoring of groundwater and surface water quality immediately prior to discharging into the CFR all along the site boundary
with surface water bodies, including downgradient of the barrier wall and groundwater extraction system and in areas where
there is no barrier wall. This performance monitoring could consist of monitoring wells, seepage meters or similar baseflow
monitoring measures, or some combination of these devices. Priority should be given to the installation and sampling of the
performance monitoring devices in order to provide an adequate establishment of “baseline” data before the
completion/operation of the barrier wall and groundwater extraction system.
8) Please explain how hydraulic control will be maintained during construction of the barrier wall. Will the groundwater
extraction system be capable of preventing significant hydraulic “bypasses” ahead of the barrier wall during construction, and
will there be inspections and monitoring of the barrier wall during construction to help ensure structural stability of the wall?
Please respond to the questions and comments listed in the column to
the left.
18 Appendix D: Groundwater Extraction & Conveyance System, 60% Engineering
Design Report, Geosyntec
1) Section 4.1.1: Proper well development can be an important factor for optimizing the efficiency of the extraction wells and
in minimizing the production of sediment and turbidity, which can have a detrimental impact on the groundwater extraction
and treatment system. Please provide an explanation of how the extraction wells will be developed, and how the
development water, which is likely to have very high levels of sediment and turbidity, will be handled. Please specify an
appropriate well development procedure and evaluation criteria (e.g., 10 NTU).
2) The 60% design report does not provide estimated screen depths for the wells. When comparing Figure 3 in Appendix B
and the proposed surficial aquifer well locations on the plans, it appears that several of the well screens will be shallow. Were
shallow interceptor trenches considered in place of shallow recovery wells in the lower elevaton areas?
3) Consider installing a long shallow trench to extract water upstream of the proposed extraction wells. The depth of the
trench will be less than 10 feet deep and it will capture the contaminants upstream rather than allowing the contaminants to
travel downstream and making it difficult to capture it through a series of extraction wells.
Please respond to the questions and comments listed in the column to
the left.
19 Appendix F: Seep Ex-Situ Capture Remedy Design Report, GEOServices, LLC,
August 2021
(Page 5), Section 2.2.1.2 - Why is a separate force main being used for the recovered seep water vs. the recovered
groundwater?
(Page 5), Section 2.2.2 - Are turbidity curtains being considered for the influents to the pump stations? They assisted in
lowering turbidity within inlet water going through the flow-through cells?
Drawings SC-1.0 and SC-2.0 - Wells EW-30, EW-31 and EW-42 are located in overflow ditches from Seeps A and B. These
ditches are designed to fill if rainfall events exceed 0.5 inches per 24 hours. What protection measures are being considered
to protect the well pads? How will the top of the barrier wall be protected where the overflow channels pass over the barrier
wall?
Please respond to the questions at left.
Page 5 of 5