Geosyntec Consultants of NC P.C.

RESPONSES TO COMMENTS
Chemours Fayetteville Works
Cape Fear River PFAS Mass Loading Model Scope of Work
Comments
FromCommentID

CFRW

1

CFRW

2

CFRW

3

CFRW

4

CFRW

5

CommentResponses

With regards to Section 4.4.2, page 14, more information is needed about
the long-term wells, or LTWs, being used to analyze groundwater
discharge from the Black Creek Aquifer to the Cape Fear River. In
particular, Chemours should provide all the borehole logs and monitoring
well construction details to verify Geosyntec’s findings.
Accurately analyzing PFAS mass loading into the Cape Fear Riverrequires at least one wet weather sampling event taken during wet months,or during a period with higher precipitation.
In Section 4.3 page 13, Chemours should be evaluating aerial deposition
from the facility to the Cape Fear River, as well as to its
tributaries—Georgia Branch and Willis Creek. It is not currently clear
from the plan that the company is doing so.

Chemours has provided Geosyntec with the LTW borehole logs and pastmonitoring data. Geosyntec will integrate this information into its
assessment of PFAS mass loading from the Site to the Cape Fear River.

As discussed in section 4.7 page 16 of the Modeling Scope of Work
document, wet weather samples will be collected at the mouths oftributaries, seeps and from locations in the Cape Fear River.
Aerial deposition potentially occurring from the facility to Cape Fear Rivertributaries will be incorporated into the model by directly measuring PFAS
concentrations in surface water samples collected at the mouths of thetributaries. These water samples include potential PFAS in the tributaries
from air deposition. Geosyntec will also estimate the direct aerial
deposition component to the creeks using the same methods used for theCape Fear River (deposition rate iso-contours, surface areas and tributary
flow velocities).
With regard to Sections 4.3 (page 13) and 4.6 (page 15), it is not clear that To capture potential upstream offsite groundwater contributions to Sitethe extent of aerial deposition from the facility has been fully evaluated,associated PFAS loadings, samples will be collected from the Cape Fear
which is important to understanding these pathways. Geosyntec appears to River directly upstream of the Site. These results in combination withbe evaluating off-site groundwater using only residential well sampling.
private well data and samples collected adjacent to the drinking water
However, it is possible that the residential well sampling program assesses intakes at Bladen Bluffs and Kings Bluff Canal will be used to assess
an area smaller than the aerial deposition from the facility, in which case upstream and downstream offsite groundwater contributions. Note that thethe PFAS mass loading of aerial deposition off-site and to the Cape Fear sensitivity analysis discussed in section 5.2 will further evaluate theRiver and its tributaries will not be fully evaluated.
potential contribution from offsite groundwater.

It is not clear that this plan assesses surface water contamination fromOnsite stormwater runoff that is captured through the network of storm
runoff that comes in contact with on- and off-site contaminated soil. This is drains is accounted for in the model as transport pathway 4 (section 4.5).
a source of PFAS loading into the Cape Fear River and its tributaries, and Further, onsite stormwater runoff that is not captured through the network
should be assessed.
of storm drains is anticipated to flow through the catchment areas of WillisCreek, Georgia Branch Creek, the seeps and Old Outfall 002, which arebeing assessed in the model (transport pathways 2, 5, 7 and 9).
Contributions from offsite runoff will be assessed through a mass balance
approach (section 4.6) and measured river concentrations adjacent toBladen Bluffs and Kings Bluff Intake Canal. Initial modeling efforts
demonstrate that onsite stormwater is a minor component of PFAS loading
to the Cape Fear River. Additionally modeling efforts and HFPO-DAresults reported by CFPUA both suggest storm events are correlated withreductions in HFPO-DA concentrations due to additional dilution fromincreased river flow.
Page 1 of 3May 2019

Geosyntec Consultants of NC P.C.

RESPONSES TO COMMENTS
Chemours Fayetteville Works
Cape Fear River PFAS Mass Loading Model Scope of Work
Comments
From

Comment
ID

NCDEQ

6

NCDEQ

7

NCDEQ

Comment

Responses

1. The document does not describe the spatial extent of the model or how
the boundaries will be or were decided. Please clarify.

The upstream spatial extent of the modeling effort will be the upstream
river location where measured concentrations of Site-associated PFAS arenon-detect. Over May 22 and 23, 2019 samples were collected from theriver 8 and 20 miles upstream of the Site in support of this modeling work.
Upon receipt of laboratory results, the results will be reviewed to assess if
Site-associated compounds are non-detect. If there are detections, then anadditional set of samples will be collected at further upstream to determine
spatial extent. The downstream spatial extent of the modeling effort will beat the Kings Bluff Intake. Over May 22 and 23, 2019 samples werecollected from these locations.

2. When the model is completed for baseflow conditions and during astorm event, DWR would like a complete copy of the model including
supporting data for all calculations and assumptions.
3. It is unclear why the proposed model appears to require so few
independent samples. Please explain. For example,Geosyntec will provide DWR with the excel file used to prepare themodeling analysis.

8NCDEQ

9NCDEQ

10NCDEQ

11a. How many samples will be used to estimate Cape Fear River
concentration and flow data? Taken under what conditions? (p.12)
Please also specify the number of samples that will be used tocharacterize Willis Creek, Georgia Branch Creek, and Old Outfall 002.
(p. 13)b. Why are only two seep samples proposed? (p. 14)
c. Samples will be taken from Outfall 002 every two weeks. How many
samples in total will be used for model development? (p. 15)

Page 2 of 3

The number of sampling events outlined in the modeling scope of work
document were constrained by the time available to collect, analyze andthen interpret the data before the consent order deliverable date of August26, 2019 for the Paragraph 12 Reductions Plan. The Reductions Plansubmission will outline a quarterly sampling and flow gauging schedule for
the Seeps, Old Outfall 002, Outfall 002, Willis Creek, Georgia Branch
Creek and the Cape Fear River upstream and downstream of the Site. Thissampling will be implemented for a period of two years upon which time
the scope and frequency of sampling will be re-evaluated in consultation
with NCDEQ. The continued additional data collection will help assess
potential variability over time, seasons, etc., The modeling analysis will beupdated at minimum twice a year to account for newly collected data.
Specific responses to NCDEQ comment number 3 (comment ID #8) subbullets are listed below.
a) Two sampling and flow gauging events of Georgia Branch Creek, Willis
Creek, Old Outfall 002 and the Cape Fear River will be performed. Cape
Fear River flow data will come from USGS river flow gauging locations.
Samples will be collected during dry and wet weather conditions. Weekly
Cape Fear River data published by CFPUA will also be considered in theParagraph 12 submission.
b) Please see response to NCDEQ comment 3, comment ID #8.
c) Two samples are collected per week from present Outfall 002 using acomposite sampler. These data will be considered in the model
development and Paragraph 12 submission.

May 2019

Geosyntec Consultants of NC P.C.

RESPONSES TO COMMENTS
Chemours Fayetteville Works
Cape Fear River PFAS Mass Loading Model Scope of Work
Comments
FromCommentID

NCDEQ

12NCDEQ

CommentResponses

d. Page 17 seems to indicate that a total of two samples will be used foreach location, at least for calibration purposes. Please explain.

b) Please see response to NCDEQ comment 3, comment ID #8.

4. Does Outfall 002 have any groundwater contribution? (p. 9)

On April 24th 2019 the Chemours contractor team conducted dry weather
sampling at locations in the Site drainage network to assess the potential forgroundwater contributions to PFAS detected in water sampled from Outfall
002. An additional set of samples was collected May 30 2019 from theOutfall channel between where DuPont area stormwater enter the channel
and the Outfall 002 sump where the water enters the pipe to the river. The
results of these sample analyses will be described in the Paragraph 12Reductions Plan submission.
On April 24th the Chemours contractor team conducted dry weather
sampling of the influent to the Wastewater Treatment Plant which includes
contributions from Kuraray that flow through the Terracotta Pipe. The
treated effluent of the Wastewater Treatment Plant discharges throughOutfall 001 towards Outfall 002. The results of these sample analyses will
be described and interpreted in the Paragraph 12 Reductions Plansubmission.

135. Onsite groundwater paths seem to miss the potential for the terracotta
pipe to add to PFAS loading to Outfall 002. Has this possibility been
analyzed? (p. 13)
NCDEQ

14Notes
CFPUA - Cape Fear Public Utility Authority
CFRW - Cape Fear River Watch
DWR - Division of Water Resources
HFPO-DA - hexafluoropropylene oxide dimer acid
LTW - long-term wells
NCDEQ - North Carolina Department of Environmental Quality
ORD - Office of Research and Development
PFAS - Per- and polyfluoroalkyl substances
USGS - United States Geological Survey

Page 3 of 3

May 2019