2021.09.14_CCOA.p3b_DEQ Comments on Pre-Design Investigation Summary

DWM Comments, Review of Pre-Design Investigation Summary, Version 2
Chemours Fayetteville Works,
Geosyntec Consultants of NC, P.C., 6-29-2021

Version 1- March 24, 2021 and Version 2 – June 29, 2021

• Version 2 of the Pre-Design Investigation Summary included additional information to fill
data gaps along the proposed barrier wall alignment. Page 9 of the digital PDF has a table
showing the revisions.

Overall Comments

Limited Reporting of PFAS Results: The cover letter and the Pre-design Investigation Summary
(PDI) address only the Table 3+ PFAS compounds, and do not include the PFAS analyzed and
reported by Modified Method 537 in the “Other PFAS” table. Geosyntec stated in the report
that they would focus on the Table 3+ PFAS because they are predominantly related to
manufacturing operations at the site. However, the June 14, 2021 Geosyntec Response to
NCDEQ Comments on Consent Order Paragraph 18 Memo states that PFCAs are byproducts
of processes at the Chemours facility. Therefore, these compounds are related to
manufacturing operations. Data from the Characterization of PFAS in Process and Non-
Process Wastewater and Stormwater Quarterly Reports #1 and #2 (dated July 31, 2019 and
October 31, 2019) show the presence of both PFCAs and PFSAs in Chemours Process
Wastewater. PFCAs and PFSAs are Method 537 compounds and should be monitored using
Method 537 and reported in all monitoring reports. They did not include perfluorooctanoic
acid (PFOA), which is chemically related to Ammonium perfluorooctanoic acid (APFO), which
was manufactured at the Fayetteville Works facility between approximately 2002 and 2013.
The PDI references the Corrective Action Plan (CAP) where Table 3+ PFAS were the focus. The
CAP, submitted to the DEQ in December 2019, has still not been approved.

Paragraph 16.c of the 2019 Consent Order references the CAP and states “…include the
collection and reporting of accurate baseline concentrations for all PFAS for which test methods and lab standards have been developed within each groundwater monitoring well installed pursuant to this paragraph, in addition to existing long-term wells (LTWs) along the Cape Fear River. As test methods and lab standards are developed for additional PFAS, the
CAP shall be amended to address those PFAS.

Modified Method 537 was in existence at the time of the CAP and the PDI. Chemours is not
providing a complete picture of groundwater contamination at the site. Several compounds
on the Method 537 list have been detected widely in the groundwater samples and include
Perfluoro-butanoic acid (PFBA), Perfluoroheptanoic acid (PFHpA), Perfluorohexanoic acid
(PFHxA), Perfluoro-pentanoic acid (PFPeA), Perflurononanoic acid (PFNA) and PFOA. Before
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2017, DuPont sampled several monitoring wells at the site for APFO, and later PFOA and
PFOS.

Action Item: Laboratory testing must continue for EPA Method 537 PFAS compounds and the
results for these analytes must be reported. In addition, Chemours must also analyze for
HFPO-TA, HFPO-TeA, trifluoroacetic acid (TFA), PFMOPrA, and PFMOBA. Analysis of all
compounds should be carried out with a method capable of achieving accurate results for all
analytes. J-flagged data should be kept to a minimum for sporadic analytical issues. Data
where the majority of values are J-flagged are indicative of poor method performance. In
these cases, samples should be reanalyzed to achieve acceptable method performance.

Section 2.3: Passive Flux Meter (PFM) Installation and Analysis and Table 3 (referenced in text):
• Table 3: The footnote for Apparent DTBfinal (copied below) is confusing. It appears the
values in this column represent the depth to the top of the PFM, whether one five-foot
section was used or if two sections were stacked in the well screen interval.
Apparent DTB final - apparent depth to bottom after PFM placement and
confirmation of depth to top of PFM
• The word “measured” is spelled wrong in the legend for DTWinitial.
Action Item: Please restate the footnote to clarify, and correct the typographical error.

Section 2.5 and Appendix D: Geotechnical Exploration
Section 4.7 of the geotechnical engineering report (Appendix D, Page 25) provides an
overview of four different alternatives to design of the barrier wall. The advantages and
disadvantages are presented for the four alternatives.
• In Section 4.7.6 (Page 30), the report indicates that the depth of embedment will be
based on the results of seepage modeling during the design process.
Action Item: Respond to question - Chemours/Geosyntec must considering potential
methods (and technology) in the design to measure potential seepage through, below
or around the wall after it is completed to show the effectiveness of the barrier wall and
identify any installation or other problems that cause a decrease in efficacy. The NCDEQ
will require periodic performance testing after it is constructed.

Section 3.1: PFAS (Table 3+) Distribution and Flux

Testing was not done by EPA Method 537 Mod as was mentioned on the first page of these
comments.

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Action Item: The DEQ requests expanding of testing to include more analytes as noted on
the first page.

Section 3.2.1.: Step Test Results (including Appendix H and Table 11)

The PDI report indicated that the northern extraction wells (EW-1 and EW-4) exhibited lower
yield conditions indicative of a leaky confined system. Extraction wells EW-3 and EW-5 had
comparatively higher yields but also are indicative of leaky confined conditions, which suggest
that some groundwater may migrate through the overlying confining unit. Geosyntec
indicated that the results from EW-2 were indicative of a high yield unconfined system which
was consistent with boring logs for the area. In the June 14, 2021 Chemours response to the
On and Off-site Assessment, Figure 7.1 indicated negative (upward) vertical gradients for well
pairs near EW-3 and EW-5. 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.

Action Item: May the coarser sediments in the Black Creek Confining Unit potentially play a
role with the negative vertical gradients? Will the extraction wells planned in this area for
the final remedy span the majority of the Black Creek Aquifer?

Section 4.2: Regional Hydrogeology
Action Item: Provide a description of the Upper Cape Fear Aquifer further in the section,
below the descriptions for the other aquifers.

Section 5.1.1: PFAS distribution in Groundwater and Surface Water
Groundwater samples were collected from new monitoring wells PIW-11 through PIW-15,
PIW-16S and PIW-16D, and tested for PFAS by Table 3+ laboratory SOP. Four of the seven
new wells are located close to existing monitoring wells SMW-12, PIW-1S and PIW-1D that
were tested for PFAS by Method 537M (“Other PFAS”) in July 2020. New monitoring wells
PIW-11, PIW-16S and PIW-16D are not located near existing wells that were sampled for
PFAS by Method 537M.

Table 6 – Field Parameters of Groundwater Samples. The turbidity for PIW-11 is unusually
high compared to the other samples. The text did not explain the reason(s) for the turbidity
and efforts undertaken to reduce the turbidity level prior to sampling.

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Action Items:

• Comment why the turbidity was so high in PIW-11 and the steps taken to lower the
turbidity. Re-development or replacement of this well with a well screen and filter
pack appropriate for the formation may be necessary.
• Chemours should test samples from wells PIW-11, PIW-16S and PIW-16D by Method
537M.
• During future site-wide sampling events, the analysis program should include PFAS
by Method 537M (“Other PFAS”) in addition to Table 3+.

Section 5.2 (Aquifer Properties) and Appendix A (Field Methods, aquifer test well design
section):
Note (7/27/2021) - The following comment and two bulleted action items were addressed
during a conference call with Geosyntec and Chemours. Originally, design plans were
written for some of the extraction wells and they were included in the Field Methods
appendix. Geosyntec indicated that subsurface conditions required the longer well screens
and adjustment of the screen interval placement from the original design.
The screen intervals for extraction wells EW-1 to EW-3 are much greater than originally designed
and stated in the aquifer test well design information, included as a section (appendix) of the field
methods (Appendix A). The largest screen interval for these three wells is 30 feet for well EW-3
(positioned at 37 to 67 feet below ground, or at an elevation of 10 to 40 ft. MSL). The design plan for
the EW-3 screen interval was 15 feet. The aquifer test well design information does not include the
proposed details for EW-4 or EW-5. The report indicates that 20 and 30 feet of screen were used for
construction of EW-4 and EW-5, respectively.
Action Items:
• Why were longer screen intervals chosen than specified in the design?
• What decision-making led to the EW-3 screen not being installed from 53 to 67 feet
per the initial plan? The stated target depth interval for the screen was between
approximately 50 and 70 feet BLS.

Based on the log in Appendix C, the EW-3 well screen interval spans two sand sections above
and below an 11-foot section of fat clay, USCS type CH, from 42 to 53 feet below ground. The
unit is described on the boring log as FAT CLAY (CH); dark gray; medium stiff; moist; medium
plasticity; greasy. On the detailed cross section (Figure 10B), this clay unit is shown as the Black
Creek Confining Unit. Neither the location of the extraction wells nor their screened intervals
were shown on Figure 10A or 10B. When the Hazardous Waste Section drew the screen interval
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for EW-3 on the detailed cross-section, the well screen does not extend to near the base of the
Black Creek Aquifer.
Action Items:

• For well EW-3, was a packer used to seal off the borehole either above or below the 11-
foot thick clay unit for the purposes of completing aquifer testing?
• Will the extraction wells planned for construction in the Black Creek Aquifer near EW-3
screen more of the Black Creek Aquifer?

Section 7: Updated Geological Interpretation
Note (9/09/2021): The 60% Design submittal provided an updated geological interpretation.
The following action items were addressed but are included here for clarity.
Action Items:
• 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?
• This figure would be more useful if the extraction wells located near the barrier wall alignment
could be overlain. 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.