2021.03.01_CCOA.p1_ChemoursRepsonsesToNCDEQCommentsOnMassLoadingProtocol

Geosyntec Consultants NC P.C.
RESPONSES TO NCDEQ COMMENTS
Chemours Fayetteville Works

TR0795 1 March 2021
On February 1, 2021, Chemours received a second round of comments from the North Carolina Department of Environmental Quality (NCDEQ) on the protocol document “Cape Fear River PFAS Mass Loading Calculation
Protocol (Version 2)” submitted by Chemours on November 18, 2020 as required under Paragraph 1 (a) and (b) of the Addendum to Consent Order Paragraph 12. The table below summarizes the comments received from
NCDEQ, Chemours’ responses to those comments, and the subsequent changes that will be made in this third version of the protocol document.

Comment # Protocol
Document Comment Description Response Section Edit
1
Cape Fear
River Mass
Loading
Hydraulic Conductivity values in the groundwater model(s) should be replaced with values from
aquifer pumping tests for their respective hydrologic units.
Hydraulic conductivity values for the groundwater term of the mass loading model are
routinely evaluated and updated as appropriate. Aquifer pump tests performed during the pre-
design investigation provide updated hydraulic conductivity values for the Black Creek
Aquifer, and slug tests will continue to be used to determine hydraulic conductivity values for
the Floodplain Deposits. The protocol document will be updated to reflect these hydraulic
conductivity updates.
Section
4.4.4.2
Table D1
Appendix D
2
Cape Fear
River Mass
Loading
Prior to conducting aquifer pumping tests, please provide a brief aquifer test workplan to DEQ
for review. Please submit electronic copies of these workplans to Rick Bolich of DWR and Joe
Ghiold of DWM.
Aquifer tests were completed in 2020 and the findings will be reported as attachment to 60%
Design Report due on August 15, 2021 per Consent Order Addendum, Paragraph 3.
Section
4.4.4.2
Table D1
Appendix D
3
Cape Fear
River Mass
Loading
In consideration of Comment #7 in the “Responses to DEQ Comments” table, DEQ believes
that the sample location we specified for OLDOF-1 in the Cape Fear River Mass Loading
Calculation Protocol is critically important for accurate monitoring of the flux of PFAS
compounds into the Cape Fear River. It may be necessary to reach this sampling location by
boat due to the access concerns for this location that were cited by Chemours.
An inspection of the area was performed on November 6, 2020 by Chemours’ contractor. The
inspection confirmed that there are no additional surface water additions (e.g., seeps) between
the current sampling location and the mouth of the Old Outfall 002 to the Cape Fear River. At
the time of inspection, the water at the mouth of Old Outfall 002 was 8 inches which
precludes boat access. The area close to the mouth is open and equipment placed here could
be easily vandalized. Therefore, the present location where composite samples can be
successfully collected is recommended to be retained as the sampling location.
--
4
Cape Fear
River Mass
Loading
Bullet for Transport Pathway #4. Are there any sanitary sewer inflows to the wastewater
treatment plant?
Yes, there are sanitary sewer inflows to the wastewater treatment plant. Sanitary sewage is
conveyed to the WWTP from various buildings and is treated and discharged to Outfall 001
(Fayetteville Works NPDES NC0003573 Permit Application Update. Transmittal from Brian
D. Long to Julie Grzyb, NCDEQ. December 15, 2020).
--
5
Cape Fear
River Mass
Loading
Section 4.4.4.1, Page 15. The text indicates that four seeps were identified that discharge
directly to the Cape Fear River (River); however, five seeps are then listed in the text. Please
clarify.
The Protocol text will be modified to clarify five seeps in total, where four are onsite and one
is offsite:
“Four onsite seeps at the Site have been identified that discharge directly to the Cape Fear
River: Seep A, Seep B, Seep C, Seep D. Additionally, the Lock and Dam Seep is an offsite seep
that also discharges directly to the Cape Fear River (Figure 4).”
Section
4.4.4.1
6
Cape Fear
River Mass
Loading
Section 4.4.4.2, Page 16. The last sentence of the first paragraph below the bulleted items does
not read clearly, specifically “the Floodplain Deposits are downgradient of the Black Creek
Aquifer and hydraulically connected to the Black Creek Aquifer.” Stratigraphically, the
Floodplain Deposits are located above the Black Creek Aquifer (BCA). The Floodplain
Deposits are connected to the BCA without apparent confining or semi-confining units (i.e.
clayey strata) only in the southeastern portion of the site, as reflected in Segment #6 and
Segment #7 of Figure D2. Please clarify where the Floodplain Deposits are hydraulically
connected to the Black Creek Aquifer.
The lithology observed at boreholes PIW-7S/D and PIW-8D in the area southeast of the site in
segments 6 and 7 respectively, indicated that the Floodplain Deposits are hydraulically
connected to the BCA. Here the Floodplain Deposits lie uncomfortably on top of the BCA
with no clay layers in between. In this area, the Floodplain Deposits are stratigraphically
adjacent to and on top of the BCA and hydraulically downgradient of the BCA. The hydraulic
connection between Floodplain Deposits and the BCA can also be inferred from the similar
Darcy velocities observed from passive flux meters deployed in Floodplain Deposit wells and
BCA wells. This is further discussed in the Response to Comment 9 below.
Section
4.4.4.2,
Appendix D
7
Cape Fear
River Mass
Loading
In item #4 of the “Approach” Section, the text indicates that the slug test results from well LTW-
03 will be used to estimate the hydraulic conductivity for groundwater upwelling from the BCA
into the River at Segment #4. Well LTW-03 screens the Floodplain Deposits and not the Black
Creek Aquifer according to Table 3. According to the drilling log for LTW-03 from January
2006, the well is screened from 15 to 30 feet below ground and Clay was documented from 25 to
32 feet below ground. Segment #4 on Figure D2, generated from CTech’s Earth Volumetric
We agree that the boring logs from LTW-3 and PIW-6S do not indicate the presence of the
BCA from 10-24 ft AMSL. Therefore, the EVS model has been updated to prioritize data
from PIW-6S resulting in a localized "pinch-out" of the BCA in Segment 4. Both the cross-
section and the thickness of the BCA in Segment 4 have been updated accordingly. This
model update has no significant impact on the mass discharge as the cross-sectional area of
Table 3
Appendix D
Figure D2
Geosyntec Consultants NC P.C.
RESPONSES TO NCDEQ COMMENTS
Chemours Fayetteville Works

TR0795 2 March 2021
Comment # Protocol
Document Comment Description Response Section Edit
Studio (EVS) software, shows the well screen extending past a confining unit into the BCA
which does not appear accurate. DEQ is unclear as to how the EVS software modeled the BCA
for Segment #4, since LTW-03 screens the Floodplain Deposits and the drilling log shows Clay
from 25 to 32 feet, the maximum depth of the boring. The drilling log for nearby shallow well
PiW-6S showed that Clay continued to a 40-foot depth and was continuous except for lignite
lenses from 33 to 34 feet below ground. Therefore, more permeable strata in the BCA does not
appear to exist at the LTW-3/PiW-6S area (Segment #4) between 26 and 40 feet below ground
which corresponds with an elevation of 10 to 24 feet above mean seal level.
the BCA in Segment 4 is estimated to be 2% of the total cross-sectional area (previously it
was estimated to be 6%).
8
Cape Fear
River Mass
Loading
Appendix E, Figure E1. The buffer polygons do extend 1 mile from the river; however, the scale
bar in Figure E1 is not correct and does not match the width of the areas on either side of the
river. DEQ suggests that a ratio scale (i.e. 1:63360) be placed next to the bar scale (or
elsewhere) to serve as a double check. If the working scale of the figure is intended to be 1 inch
= 1 mile, and the ratio scale is not 1:63360, then the author or user of the map will know there
is a problem with the scale bar.
The one-mile buffer and scale bar have been corrected. Appendix E
9
Cape Fear
River Mass
Loading
Total Table 3+ concentration data from February 2020 sampling is used in calculations for
PiW-1S (Segment #1), LTW-03 (Segment #4) and PiW-7S (Segment #6). These three wells are
assigned to the Floodplain Deposits as shown in Table 3 of the Protocol. The PFAS
concentration data for wells installed into the BCA should be used, including PiW-1D (Segment
1) and PiW-7D (Segment 6). A representative concentration for Segment 4 is more difficult
since there is not a currently identified well screening the BCA within this segment. DEQ
suggests installing an additional groundwater monitoring well to obtain a representative PFAS
concentration for Segment 4.
For Segments 1 and 6, the onsite groundwater term will be updated to use concentration data
from BCA wells only (i.e., PIW-1D and PIW-7D, respectively), as opposed to taking an
average concentration from wells located in the BCA wells and the Floodplain Deposit wells.
For Segment 4, it is appropriate to use a Floodplain Deposits well (LTW-03) since the BCA is
not observed in borehole logs along this segment (LTW-03 and PIW-6S) and therefore no
additional well is required ( the EVS model has been updated to reflect this, as discussed in
the Response to Comment #7). Further pre-design investigation results showed that the two
floodplain wells instrumented with passive flux meters had groundwater Darcy velocity
ranges consistent with Black Creek Aquifer deposits suggesting these two units are in
hydraulic connection and Floodplain Deposit concentrations can serve as surrogate in this
situation. Attached to these responses to comments, see Table 1 which provides the Darcy
velocity data and Figure 1 which shows the Darcy velocity reported for locations where
passive flux meters were deployed. These data will also be reported as an attachment to the
60% Design Report due on August 15, 2021 per Consent Order Addendum, Paragraph 3.
Appendix D
10
Cape Fear
River Mass
Loading
DEQ back-calculated the aquifer thickness by dividing each segment cross-sectional area by the
segment length. Then DEQ reviewed the drilling logs and Hydraulic Profiling Tool (HPT) logs
to find generally sandy strata representative of the BCA. In our opinion, the thickness of
Segment #3 should be changed from 6.3 to 8 feet (drilling log shows sand from a 30 to 37-foot
depth and the screen interval goes to 38 feet). This 8-foot thickness also corresponds to the BCA
thickness shown on Cross Section B-B’ of the On-site/Off-site report. Drilling borehole logs and
the HPT log do not show sandy horizons present in the BCA near Segment #4 as noted above.
Therefore, the aquifer thickness should be decreased from 11.4 feet to a smaller amount to
account for thin sandy lenses that might have been missed by the soil classification/HPT work.
A 5-foot thickness is recommended. Segment #5 should be changed from 23.2 to 26.5 feet
(drilling log shows sand extending from 21 to 47.5 feet). The thickness of Segment #7 should be
changed from 19.4 to 11.5 feet (the drilling log shows sand from a 33.5 to 44-foot depth and the
HPT log (HP-11) showed good estimated K values to a 45-foot depth).
The thickness of the BCA for each segment is estimated based on the EVS model output.
Therefore, this modelled thickness will not match exactly to the thickness of the BCA
observed in each nearby borehole. For Segment 3, the BCA thickness estimated by EVS was
6.3 feet and the thickness observed at LTW-02 (located near Segment 3) was approximately 8
feet. For Segment 5, the thickness estimated by EVS was 23.2 feet and the thickness observed
at PZ-22 (within Segment 5) was approximately 19 feet. For Segment 7, the thickness
estimated by EVS was 19.4 feet and the thickness observed at PIW-8D (Segment 7) was
approximately 20 feet. These differences are considered to be reasonable, given the variation
in the thickness of the BCA near the Cape Fear River at the site. As noted above in Response
to Comment #7 for Segment 4, the groundwater term is being updated in the protocol
document to use the thickness of the Floodplain Deposits, since the BCA is not observed in
borehole logs along this segment. If PFAS mass discharge was recalculated using the
thicknesses suggested for Segments 3, 5, and 7, the total onsite groundwater mass load for
Total Attachment C from all segments to the Cape Fear River would decrease by 3% (if
considering lower bound mass discharge) or increase by 5% (if considering upper bound mass
discharge).This is not an unreasonable amount of variation, considering the heterogeneity
observed in the subsurface.

Appendix D
Geosyntec Consultants NC P.C.
RESPONSES TO NCDEQ COMMENTS
Chemours Fayetteville Works

TR0795 3 March 2021
Comment # Protocol
Document Comment Description Response Section Edit
11
Cape Fear
River Mass
Loading
A note at the bottom of the table indicates that Figure D3 was used to obtain the hydraulic
gradients; however, Figure D3 of Appendix D in the protocol is a hydrograph. The horizontal
distances between most of the wells are large, between 710 and 830 feet. Can wells that are
more closely spaced be evaluated for hydraulic gradient?
The horizontal distances used to estimate hydraulic gradient are based on groundwater
elevation contours developed from sitewide groundwater elevation measurements, as opposed
to distances between individual well pairs. This method was suitable to represent groundwater
fluxes discharging from the BCA to the Cape Fear River. As stated in Appendix D of the
Mass Loading Protocol, hydrographs from wells along the river suggest that hydraulic
gradients in the BCA are relatively constant over time and respond to flux in the Cape Fear
River in a subdued manner, with the exception of those time periods when there are large
changes in river level (i.e., significant flooding events such as hurricanes or tropical storms).

The PFAS mass loading from onsite groundwater to the Cape Fear River that was estimated
from the Mass Loading Model in the third quarter of 2020 shows good agreement with data
collected during the pre-design investigation, suggesting that the methods described in the
Mass Loading Protocol for the onsite groundwater term are reasonable. More specifically,
onsite groundwater PFAS mass discharge was calculated using passive flux meters deployed
during the pre-design investigation (as described in Response to Comment #9). These mass
discharge values were within the range (i.e., at the median) of those estimated from the Mass
Loading Model. Attached to these responses to comments, see Table 1 which provides the
Darcy velocity data and Figure 1 which shows the Darcy velocity reported for locations where
passive flux meters were deployed. These data will also be reported as an attachment to the
60% Design Report due on August 15, 2021 per Consent Order Addendum, Paragraph 3.
Appendix D

TABLE 1
PASSIVE FLUX METER DARCY VELOCITY RESULTS
Chemours Fayetteville Works, North Carolina
Geosyntec Consultants of NC P.C.
Depth below
top of well casing Darcy Velocity
(ft)(cm/day)
PIW-10DR-48-58-G PIW-10DR 53 3.0
PIW-11-47-57-G PIW-11 52 3.6
PIW-12-64-74-G PIW-12 69 3.0
PIW-13-54-64-G PIW-13 59 3.2
PIW-14-56-66-G PIW-14 61 3.0
PIW-15-34-44-G PIW-15 39 3.3
PIW-1D-25-30-G PIW-1D 28 5.3
PIW-2D-40-50-G PIW-2D 45 3.1
PIW-3D-20-25-G PIW-3D 23 7.2
PIW-4D-32-37-G PIW-4D 35 4.5
PIW-6S-18-28-G PIW-6S 23 3.3
PIW-7D-29-34-G PIW-7D 32 3.2
PIW-7S-7-17-G PIW-7S 12 8.4
PIW-8D-35-40-G PIW-8D 38 3.1
PIW-9D-40-45-G PIW-9D 43 2.8
PW-10R-57-67-G PW-10R 62 6.6
PW-11-54-64-G PW-11 59 3.6
DUP-1G PIW-15 39 3.3
PIW-15-34-44-GMS PIW-15 39 3.3
PIW-15-34-44-GMSD PIW-15 39 3.2
Notes:
Darcy velocity data reported by EnviroFlux
All values rounded to 2 significant digits
ft - feet
cm/day - centimeters per day
Sample ID Location ID
TR0795 1 of 1 March 2021
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'ACape Fea
r
R
i
ve
r
Willis Creek
Ol
d

O
u
t
f
a
l
l

0
0
2
PIW-7D
PIW-10DR
PIW-11
PIW-12
PIW-13
PIW-14
PIW-15
PIW-2D
PIW-3D
PIW-4D
PIW-6S
PIW-7S
PIW-8D
PIW-9D
PW-10R
PW-11
PIW-10DR
PIW-11
PIW-12
PIW-13
PIW-14
PIW-15
PIW-2D
PIW-3D
PIW-4D
PIW-6S
PIW-7S
PIW-8D
PIW-9D
PW-10R
PW-11
PIW-7D
PIW-1DPIW-1D
Monitoring Wells Where PassiveFlux Meters Were Installed
Chemours Fayetteville Works, North Carolina
Figure
5Raleigh
³Path: P:\PRJ\Projects\TR0795\Database and GIS\GIS\Pre-design Investigation\TR0795_PassiveFluxMeterLocations.mxd; JKasunic; 2/22/2021February 2021
Projection: NAD 1983 StatePlane North Carolina FIPS 3200 Feet; Units in Foot US
Notes:1. Due to the scale of the map, pairs of wells that are in close proximity have been offset for visibility. Therefore, the placement of these wells on this map do not reflect their true geographic coordinates.2. The outline of Cape Fear River is approximate and is based on open data from ArcGIS Online and North Carolina Department of Environmental Quality Online GIS (MajorHydro shapefile).3. PIW and PW-well locations were surveyed by a licensed North Carolina Surveyor.4. Basemap source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.
Legend
!'A Floodplain Deposits
!'A Black Creek Aquifer
Nearby Tributary
Observed Seep
Site Boundary
1,000 0 1,000500 Feet
Notes:
Site map with PFM locations is shown for reference. Darcy velocity bars are spaced to approximate their relative locations on the site map.
The Darcy Velocity is the rate at which a volume of water moves across a surface of specified area, giving units of volume per area per time which reduces
to distance per time. This quantity is not the travel speed of groundwater.
PFM - Passive flux meter
cm/day - centimeters per day
!'A
!'A !'A!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'A
!'ACape Fea
r
R
ive
r
Willis Creek
Ol
d

O
u
t
f
a
l
l

0
0
2
PIW-7D
PIW-10DR
PIW-11 PIW-12PIW-13
PIW-14
PIW-15
PIW-2D
PIW-3D
PIW-4D
PIW-6S
PIW-7S
PIW-8D
PIW-9D
PW-10R
PW-11
PIW-10DR
PIW-11 PIW-12PIW-13
PIW-14
PIW-15
PIW-2D
PIW-3D
PIW-4D
PIW-6S
PIW-7S
PIW-8D
PIW-9D
PW-10R
PW-11
PIW-7D
PIW-1DPIW-1D
Monitoring Wells Where PassiveFlux Meters Were Installed
Chemours Fayetteville Works, North Carolina
Figure
5Raleigh
³Path: P:\PRJ\Projects\TR0795\Database and GIS\GIS\Pre-design Investigation\TR0795_PassiveFluxMeterLocations.mxd; JKasunic; 2/22/2021February 2021
Projection: NAD 1983 StatePlane North Carolina FIPS 3200 Feet; Units in Foot US
Notes:1. Due to the scale of the map, pairs of wells that are in close proximity have been offset for visibility. Therefore, the placement of these wells on this map do not reflect their true geographic coordinates.2. The outline of Cape Fear River is approximate and is based on open data from ArcGIS Online and North Carolina Department of Environmental Quality Online GIS (MajorHydro shapefile).3. PIW and PW-well locations were surveyed by a licensed North Carolina Surveyor.4. Basemap source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.
Legend
!'A Floodplain Deposits
!'A Black Creek Aquifer
Nearby Tributary
Observed Seep
Site Boundary
1,000 0 1,000500 Feet
Inset Map Legend
!'A Floodplain Deposits
!'A Black Creek Aquifer
Nearby Tributary
Observed Seep
Site Boundary"P:\PRJ\Projects\TR0795\Database and GIS\Illustrator\Mass Loading\Figure 1 - PFM Results Mass Loading TR0795.ai"0 1 2 3 4 5 6 7 8 9
PIW-11
PIW-12
PIW-13
PIW-14
PIW-15PIW-1D
PIW-2DPIW-3D
PIW-4D
PW-10R
PIW-6S
PIW-7SPIW-7D
PIW-8D
PIW-9D
PIW-10DR
PW-11
Darcy Velocity (cm/day)
Passive Flux Meter Results - Darcy Velocities
Chemours Fayetteville Works, North Carolina
Figure
1Raleigh
Path: P:\PRJ\Projects\TR0795\Database and GIS\Illustrator/Pre-design Investigation\Passive Flux Meter Results TR0795.aiMarch 2021