2024.10.15_CCOA.p2.c.v_Chemours Response to DEQ Comments PMP5

Table 1: Comments on CFR Long -Term Remedy Performance Monitoring Report #5, January — March 2024, Submitted June 27, 2024
No
Report Section
Comments by NCDEQ
Chemours Response
Background for Reader. This comment set covers the January 1 to
March 31, 2024, period documenting operation and effectiveness
From NCDEQ:
of the long-term remediation (LTR) system installed on the east
No Comment is required for this row.
side of the Chemours property near the Cape Fear River (CFR).
The LTR system is a hydraulic control remedy in which
contaminated groundwater and surface water are withdrawn,
conveyed, and then treated in a treatment plant before the cleaned
water is discharged under permit to the CFR. To further restrict
contaminant migration and increase the performance of the LTR,
an engineered barrier wall was constructed below ground using
"One Pass" trenching methodology. During trenching, the native
soil was pulverized by a 3-foot-wide array of drilling teeth which
effectively acted like a saw blade that extended vertically
downward into the top of the confining unit for the Upper Cape Fear
Aquifer. Portland cement, clean water and occasionally bentonite
clay were added to create a slurry with the native soils, that after
hardening, severely restricts water in the Surficial and Black Creek
aquifers from passing through the wall. The underground barrier
all is approximately 5,834 feet long and averages about 80 feet in
B-1
Not Applicable
depth.
This report by Geosyntec Consultants of NC, P.C. (Geosyntec)
describes the treatment operations and effectiveness by the
various elements of the LTR system. Additionally, the report
documents the continued operation and effectiveness of "Flow -
through Cells" (FTCs) installed between December 2020 and June
2021 as an interim remedy to treat groundwater seep and surface
water crossing the property in four small tributaries that discharge
into the CFR. Each FTC contains two chambers of granular
activated carbon (GAC) filtering media. Water typically moves by
gravity -flow through the FTC and is cleaned by the GAC.
Groundwater recharge to the tributaries downgradient of the
hydraulic control remedy has been severely restricted which was
predicted by operation of the LTR. Recharge to the tributaries is
now generally caused by precipitation and flooding. Each FTC is
now operated in batch mode where water in the impoundment
before the influent gate is allowed to rise before passing through
the carbon beds. During dry periods, each FTC may not operate for
lengthy periods.
Page 1 of 7

Table 1: Comments on CFR Long -Term Remedy Performance Monitoring Report #5, January — March 2024, Submitted June 27, 2024
11 No I Report Section I Comments by NCDEQ I Chemours Response 11
3
Data Validation. Matrix interference studies have shown that
quantitation of R-PSDA, Hydrolyzed PSDA, and R-EVE is
inaccurate due to interferences by the sample matrix (Geosyntec,
1.1 2020). What steps are underway to modify methods to ensure
reliable and accurate reporting?
Section 6.2.2
and Figures 6-8
and 6-9
6.2.2
Table 6-2
Table 6-3
>urface Water. In February 2024, total PFAS at surface water
;amples WC-1, WC-2, and WC-3, were 1420, 875, and 230 ng/L,
espectively. This is a consistent pattern in sampling events over
ime. PFAS in WC-1 (downstream end of Willis Creek) is higher
han WC-2 (mid -reach), and PFAS in WC-2 is higher than WC-3
upstream). While WC-1 would be expected to have higher
:oncentrations than WC-2 based on predominant groundwater flow
firections from the facility to the northeast, the report does not
liscuss why WC-2 is higher than WC-3. The results from WC-2
ind WC-3 suggest a source of PFAS is entering Willis Creek
)etween WC-2 and WC-3. Nothing is stated as to the remedy's
ibility to control or minimize it or the Performance Monitoring (PM)
plan to assess it. Potentiometric maps in the report from the 2023
annual sampling event also indicate that groundwater in the central
and western portion of the manufacturing area flows to the north
owards Willis Creek.
Section name as above including Tables 6-2 and 6-3: There are
elevated detection limits for multiple analytes. This comment is
similar to #6 in the NCDEQ technical review comments for the Q4
2023 report. Chemours responded to #6 in a July 19, 2024
submittal that "samples with a smaller sample volume (called the
"high level method") have elevated reporting limits for both Table
3+ and EPA 537 Mod analytes." The samples from wells with
higher concentrations of Table 3+ PFAS are designated for "high
method" testing. Response #6 continued "use of a smaller sample
size during sample preparation is standard procedure for diluting
high concentration samples in analytical laboratories. It is critical to
analyze these samples using the high-level method instead of the
low-level method since using the low-level method may severely
contaminate the laboratory's analytical instruments."
:hemours is presently conducting analytical experiments
valuating the use of direct injection and extended analytical run
mes to reduce the interferences by the sample matrix on the
uantitation of R-PSDA, Hydrolyzed PSDA, and R-EVE.
he long-term groundwater remedy was constructed according
> the extent specified in August 2020 Addendum to Consent
irder Paragraph 12 (CO Addendum) paragraph 3(b) as
epicted in Attachment 5 to the CO Addendum and per the
onditions described in the NCDEQ's approval on October 26,
022. The remedy's northern end coincides with the location of
✓C-2 in Willis Creek. Location WC-3 is further upstream in Willis
-reek. Therefore, the section of Willis Creek between WC-3 and
✓C-2 is not evaluated as part of the remedy. Additionally, the
resence of Table 3+ PFAS compounds in Site groundwater
djacent to Willis Creek between WC-3 and WC-2, and that Site
roundwater flowed towards Willis Creek have been previously
;ported to NCDEQ. For instance, these data were presented in
ie September 2019 On and Offsite Assessment and the
lecember 2019 Corrective Action Plan.
he results from the February 2024 samples for WC-3 produced
sable data that provided quantitation for the compounds found
t highest concentration in the sample, including HFPO-DA at 50
g/L, PFMOAA at 22 ng/L and PMPA at 83 ng/L. Reporting
nits were increased in the specific case of the WC-3 February
024 sample due to low isotope dilution analyte recoveries. The
iboratory addressed this issue by extracting a lower volume of
ie field samples (by approximately 10x) which raised the
sporting limits to a maximum value of 14 ng/L for one
ompound R-PSDCA which has never been detected in the
mg -term remedy performance monitoring data set at WC-3.
his is in stark contrast to the much higher reporting limits
ncountered when the high-level method is used which often
)nge between 140 ng/L for R-PSDCA to 24 ng/L for Hydro -EVE
,cid with most reporting limits between 40 and 100 ng/L.
Page 2 of 7

Table 1: Comments on CFR Long -Term Remedy Performance Monitoring Report #5, January — March 2024, Submitted June 27, 2024
11 No I Report Section I Comments by NCDEQ I Chemours Response 11
0
The DEQ acknowledges the desire by the contract laboratory to
prevent instrument down time; however, inferring the
contamination level within individual water samples often leads to
conservatively high reporting limits that limit the data's usefulness.
An example of a sample that has not exhibited significant
contamination in the past is Willis Creek sample WC-3 (Table 6-3).
These surface water and well locations have been sampled many
times and historical concentrations from past sampling events
should be considered. Results with exceptionally high reporting
limits resulting in non -detects should be reanalyzed at a lower
dilution rate.
Total PFAS and PFOA concentrations in OW-57 continue
o be elevated compared to surrounding wells. The concentrations at
he westernmost extraction wells also had high PFAS (EW-1, EW-2)
and high PFOA (EW-2) concentrations when they were last sampled
n April 2023 (NCDEQ has not received the results from the most
-event sampling event from Spring 2024). Observation well OW-11 is
nstalled about 50 feet west of EW-1 and a new monitoring well was
)bserved on July 31, 2024 to the west of OW-11. Provide NCDEQ
vith laboratory test results from when both wells were last sampled
Ilona with well construction information for the new well.
Overall, we believe that the western -most portion of the northern
7 alignment is not sufficiently assessed, and the existing hydraulic
Figure 6-7C control remedy may not be sufficient to capture migration of PFAS in
roundwater around the western end of hydraulic control remedy.
lonitoring data for Willis Creek surface water samples WC-2 and
VC-3 consistently show higher PFAS concentrations at mid -reach
ample WC-2 vs. upstream sample WC-3 as indicated in comment
2. Particle track output from groundwater flow modeling by NCDEQ
nd our contractor, in addition to potentiometric maps presented in
ast site -wide groundwater monitoring reports, suggests groundwater
elow the site flows from the PPA and the northwest part of the
ufacturing area towards Willis Creek west of EW-1.
small relative percentage of the Willis Creek contribution of PFAS
mours continues to keep a record of expected
:entrations at sample locations and provides laboratories
an indication of the concentration range so the appropriate
ple dilution can be used. For example, the April 2024
ples results from WC-3, reported in the 2Q 2024 Long -Term
iedy Report, were analyzed using the low-level method and
rting limits for non -detect compounds ranged from 2 to 3
Fhe monitoring well that was recently installed west of OW-11 is
NSITU-02R, which replaced INSITU-02, which was consistently
iry. NCDEQ requested a deeper well in this location. INSITU-
)2R was installed as a 2-inch diameter PVC well screened from
i to 25 feet below ground surface. It will be included in monthly
)auging events in the PMP program. Laboratory analytical data
or INSITU-02R are included as an attachment to this response
etter. OW-11 is used for hydraulic head monitoring only and is
iot sampled in the PMP program. Results for EW-1 were
)rovided in the recent submittal (September 30, 2024)
iocumenting the long-term remedy's performance in Q2 2024.
he remedy in Willis Creek is preventing groundwater from
oing around the remedy. At OW-11 to the west of EW-1 at the
lost distal end of the extraction wells, the groundwater
levation is approximately 3.70 feet less than pre -startup
onditions. EW-1 extracts approximately 12.9 gallons per
iinute, one of the highest flow rates in the Willis Creek
lignment. There is no evidence that the long-term remedy is
Ilowing groundwater to migrate around the western end of the
;medy. Regarding PFAS loading to Willis Creek to the west of
W-1, from locations WC-3 to WC-2: Please see response to
omment 2 that the remedy was constructed to the extent
efined Attachment 5 in the COA and per the conditions
escribed in the NCDEQ's approval on October 26, 2022.
hemours rejects any suggestion of non-compliance with the
will commence with PFM deployment in OW-57 in
Page 3 of 7

Table 1: Comments on CFR Long -Term Remedy Performance Monitoring Report #5, January — March 2024, Submitted June 27, 2024
No
Report Section
Comments by NCDEQ
Chemours Response
to the Cape Fear River is acknowledged. The migration of PFAS
October 2024.
around the western edge of the hydraulic control remedy does not
comply with Paragraph 3.b. of the Consent Order Addendum (COA)
which indicates that Chemours shall construct and operate an
extraction system sufficient to prevent groundwater migration around,
above or under the barrier wall [sic]. When the COA was authorized
on August 13, 2020, the underground barrier wall was planned for the
entire alignment (see Attachment #5 of the COA). The Groundwater
and Seeps Remedy 90% Design Report, dated March 25, 2022,
indicated that results from groundwater modeling indicated that
groundwater in the northern alignment portion could be intercepted by
extraction wells alone (Geosyntec and GEOServices, 2022). After
follow-up comments, meetings and submittal of supplemental data,
the NCDEQ approved the 90% design report on October 26, 2022.
NCDEQ agrees with the installation of the Passive Flux Meter (PFM)
in well OW-57 for darcy velocity analysis. Based on further work
including the PFM, the NCDEQ may request additional monitoring
ells in the northern alignment area.
It is recognized that the current remedy was not designed to lower
Charts showing the long-term PFAS and groundwater elevation
PFAS levels downgradient of the remedy but it is reducing the
trends in monitoring wells downgradient of the long-term remedy
head gradient which in turn is decreasing the mass flux to the CFR
will be prepared and included in the PMP report for Q3 2024,
and Willis Creek. Nevertheless, it is important to understand and
which will be submitted in December 2024.
monitor any long-term changes in groundwater PFAS
concentrations in the area between the remedy and the
5
Figures 6-7 A-C
waterbodies receiving contaminated groundwater discharge. To
be able to easily evaluate changes in PFAS over time
downgradient of the remedy, it would be useful to have total PFAS-
time plots at selected high -concentration downgradient well
locations (e.g. OW-57, PZ-22/LTW-04, LTW-05, PIW-8D, etc.)
similar to the plot in Figure 6-8.
Hydraulic Containment of Willis Creek Black Creek Aquifer. This
Chemours agrees to install a new BCA monitoring well in the
comment is similar to Comment #5 from our Q4 2023 response
vicinity of SMW-09.
6.2.1,Subpart
and reflects Chemours response received on July 19, 2024. In that
6
3b
Figure 6-6C
response, Chemours stated that "it may be unlikely that an
additional Black Creek Aquifer (BCA) monitoring well would inform
the hydraulic control evaluation" in response to NCDEQ suggesting
Page 4 of 7

Table 1: Comments on CFR Long -Term Remedy Performance Monitoring Report #5, January — March 2024, Submitted June 27, 2024
No
Report Section
Comments by NCDEQ
Chemours Response
a new BCA well near SMW-09. In contrast, we have an optimistic
view that a new BCA well near SMW-09 will assist in tightening up
the spacing of the potentiometric surface contours and lead to a
better hydraulic control evaluation. The new well near SMW-09
would be approximately 630 feet from the remedy alignment vs.
SMW-03B which is about 2,100 feet from the alignment. In the
future reports, also include gauging data from BCA-01 in the
drawdown and potentiometric surface figures.
Section name as above. The last paragraph on Subpart 3 in
Chemours assumes that the references to SW-03 (highlighted in
Section 6.2.1 indicates that a lower water level at SW-05 is
yellow at left) are referring to SW-05 (which is initially referenced
discussed with respect to well OW-57. Based on past site visits, we
in the comment and is downgradient of OW-57). The Willis
observed a considerable change in ground surface elevation
Creek hydraulic containment remedy was not designed to lower
between OW-57 and SW-03. Could the change in ground surface
he groundwater elevation at the interface of groundwater and
elevation help explain why the SW-03 elevation is lower?
surface water, but rather to intercept groundwater. In hydraulic
containment remedies, there are stagnation points beyond the
extraction wells that represent the limit of the downgradient
Staff gauge (water elevation) readings associated with surface
capture zone (see at bottom of this table Figure C-1 from the
water sampling events should be indicated in the report, preferably
USGS circular 1139, Groundwater and Surface Water: A
within a separate table because Table 6-1 is already lengthy.
Common Resource). Stagnation points can be conceptualized
as a groundwater divide. On one side groundwater is contained
Figure 6-6A displays only the surface water elevations for SW-04
by the remedy and flows downgradient to the extraction wells,
and SW-05. In Figure 6-613 and 6-6C, the elevations for SW-02 are
and on the other side water flows away from the extraction wells
6.2.1, Subpart
missing.
towards Willis Creek. In the Willis Creek remedy alignment, the
position of stagnation points from cumulative pumping varies
7
Table 6-1
across the alignment, and is closer to Willis Creek in some
Figure Set 6
locations (SW-03) than others (SW-05) due to hydrogeologic
heterogeneity and due to the variable distance to the creek
along the alignment.
Surface water elevation data can be provided in a separate table
or the PMP report for Q3 2024, which will be submitted in
December 2024.
In Figure 6-6A for the January 2024 gauging event, flooding of
he river and Willis Creek prevented access to SW-01, SW-02,
and SW-03. For location SW-02, accessibility is challenging, and
he current location at the western edge of the creek has
occasionally been dry, potentially in part due to pumping effects
at this location. Geosyntec is evaluating potential alternative
locations further into the creek and is anticipating a reinstallation
attem t in October 2024. It is noted that Willis Creek surface
Page 5 of 7

Table 1: Comments on CFR Long -Term Remedy Performance Monitoring Report #5, January — March 2024, Submitted June 27, 2024
No
Report Section
Comments by NCDEQ
Chemours Response
water elevation is still being robustly monitored at four other
locations along the remedy alignment.
Add note at the bottom that describe to the reader what "Series" and
Series treatment indicates the influent flow is directed first to the
"Parallel" mode refer to. Batch mode is described already.
lead filter bed and then the lag filter bed next, whereas Parallel
8
Table 2-1A to D
treatment indicates the influent flow is divided roughly equally
into both filter beds, for a temporary increase in flow capacity.
This clarification will be added to the PMP report for Q3 2024,
which will be submitted in December 2024.
Designate by color or bolding which wells are screened in the Surficial
This clarification will be added to the PMP report for Q3 2024,
9
Table 4-3
Aquifer vs. the Black Creek Aquifer.
which will be submitted in December 2024.
References Cited:
References Cited:
Geosyntec, 2020. Matrix Interference During Analysis of Table
USEPA, 1994. Methods for Monitoring Pump -and -Treat
3+ Compounds. Chemours Fayetteville Works. June 30, 2020.
Performance. EPA/600/R-64/123. June 1994.
N/A
Geosyntec and GEOServices, 2022, Groundwater and Seeps
USEPA, 1997. Design Guidelines for Conventional Pump-and-
Remediation 90% Design Submittal Chemours Fayetteville Works,
Treat Systems. EPA/540/S-97/504. September 1997.
Geosyntec and GEOServices, LLC, March 25, 2022.
USGS, Groundwater and Surface Water: A Single Resource.
1998.
Page 6 of 7

Table 1: Comments on CFR Long -Term Remedy Performance Monitoring Report #5, January - March 2024, Submitted June 27, 2024
A
Recharoe a,Ca
` f r------------
- — Land surface t _
Water table ry
Uncanfined aquifer
Confining bed
rr
-- Water table
Unconfired aquifer 1� �� �r Lj
�~
F -- Confining bed
Ci
_ Land au face
Water table — y`
Uaconfired aquifer
Confining bed
Figure C-1. In a schematic hydrologic
setting where ground water discharges
to a stream under natural conditions (A),
placement of a well pumping at a rate
(01) near the stream will intercept part
of the ground water that would have
discharged to the stream (B). if the well
is pumped at an even greater rate (02),
it car, intercept additional water that
would have discharged to the stream
in the vicinity of the well and can draw
water from the scream to the well (C).
Figure C-1 excerpted from USGS circular 1139 "Groundwater and Surface Water: A Single Resource" showing in panel (8) capture of groundwater and a stagnation point /
groundwater divide between extraction well and waterbody.
Page 7 of 7

Sampling Program
CAP GW Sampling 3Q24
Location ID
INSITU-02R
Field Sample ID
CAP3Q24-INSITU-02R-081624
Sample Date
8/16/2024
QA/QC
Sample Matrix
LIQUID
Sample Delivery Group (SDG)
320-114697-1
Lab Sample ID
320-114697-5
537 Modified (ng/L)
Hfpo Dimer Acid
890
PFMOAA
270
PFO2HxA
1,200
PFO3OA
150
PFO4DA
200
PFO5DA
<130
PMPA
690
PEPA
180
PS Acid
<50
Hydro -PS Acid
99
R-PSDA
<35
Hydrolyzed PSDA
<34
R-PSDCA
<180
NVHOS, Acid Form
<160
EVE Acid
<50
Hydro -EVE Acid
40
R-EVE
170 J
Perfluoro(2-ethoxyethane)sulfonic Acid
<36
PFECA B
<78
PFECA-G
<36
PFPrA
1,500
Total Table 3+ (17(18 compounds)'''
3,700
Total Tablc 3+ compounds)"'
5,200
Total Table 3+ (21 compounds)'
5,400
Analytical Results for INSITU-02R
Chemours Fayetteville Works
Fayetteville, North Carolina
Page 1 of 2
October2024

537 Modified (ng/L)
10:2 Fluorotelomer sulfonate
<84
11CI-PF3OUdS
<40
1H,1H,2H,2H-perfluorodecanesulfomte (8:2 FTS)
<58
1H,1H,2H,2H-perfluorohexanesulfonate (4:2 FTS)
<30
2-(N-ethyl perfluoro-l-octanesulfonamido)-ethanol
<110
2-(N-methyl perfluoro-l-octanesulfonamido)-ethanol
<180
6:2 Fluorotelomer Sulfonate
<310
9CI-PF3ONS
<30
DONA
<50
N-Ethyl Perfluorooctane Sulfommidoacetic Acid
<160
N-ethylperfluoro-l-octanesulfonamide
<110
N-methyl perfluoro-l-octanesulfonamide
<54
N-Methyl Perfluorooctane Sulfonamidoacetic Acid
<150
Perfluorobutane Sulfonic Acid
<25
Perfluorobutanoic Acid
<300
Perfluorodecane Sulfonic Acid
<40
Perfluorodecanoic Acid
<39
Perfluorododecane Sulfonic Acid (PFDoS)
<120
Perfluorododecanoic Acid
<69
Perfluoroheptane Sulfonic Acid (PFHpS)
<24
Perfluoroheptanoic Acid
<31
Perfluorohexadecanoic Acid (PFHxDA)
<110
Perfluorohexane Sulfonic Acid
<71
Perfluorohexanoic Acid
<73
Perfluorononanesulfonic Acid
<46
Perfluorononanoic Acid
<34
Perfluorooctadecanoic Acid
<120
Perfluorooctane Sulfonamide
<120
Perflooropentane Sulfonic Acid (PFPeS)
<38
Perfluoropentanoic Acid
<61
Perfluormetradecanoic Acid
<91
Perfloorotridecanoic Acid
<160
Perfluoroundecanoic Acid
<140
PFOA
<110
PFOS
<68
Notes:
1 - Total Table 3+ was calculated including J qualified data but not non -detect data. The total Table 3+
sum is rounded to two significant figures.
2 - Total Table 3+ (17 Compounds) does not include R-PSDA, Hydrolyzed PSDA and R-EVE.
3 - Total Table 3+ (18 compounds) does not include PFHpA, R-PSDA, Hydrolyzed PSDA, and R-EVE.
Bold - Analyte detected above associated reporting limit.
B - Not detected substantially above the level reported in the laboratory or field blanks.
J - Analyte detected. Reported value may not be accurate or precise.
ng/L - nanograms per liter
QA/QC - Quality assurance/ quality control
SOP - standard operating procedure
UJ — Analyte not detected. Reporting limit may not be accurate or precise.
-- - No data reported
< - Analyte not detected above associated reporting limit.
ND - No Table 3+ compounds were detected above their associated reporting limits.
Analytical Results for INSITU-02R
Chemours Fayetteville Works
Fayetteville, North Carolina
Page 2 of 2
October2024