DEQ-CFW_00034486

North Caroli m-
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Department of Environment and Natural Resources
Division of Waste Management
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June 5, 2003
Corporate Remediation. Group
DuPont Engineering
6324 Fairview Road
Charlotte, NC 28210
Re: Site Conceptual Model/Groundwater Flow Directions
RCRA Phase I
DuPont Fluoroproducts - Fayetteville Works
EPA ID # NCD 047 368 642
V I 6� �111 �
Part of the May 29' telephone conversation between DuPont's Corporate Remediation
Group (CRAG) and the Hazardous Waste Section (HWS) focused on the groundwater flow aspect
of the DuPont -Fayetteville facility's site conceptual model. As pointed out by the CRG,
subsurface data indicates that the groundwater flow direction in the vicinity of the Former Fire
Training Area is toward the southwest. The groundwater flow direction at SWMUs 9A and 9B
(Former WWTP Lagoons) is in the same direction, However, groundwater flow in the area of
SWW 6-Process Sewer System is toward the northeast and in the direction of the Cape Fear
River.
The HWS believes the groundwater flow directions at the units listed above are accurate
interpretations of the subsurface data. The topography in this area of the facility is relatively flat,
and Section 5.1.2 of FQR JTha;s_e1_Report--DnRont Fayetteville Works seems to imply that
these flow directions are influenced by the dip of a clay layer which lies beneath the entire site.
During a site visit in May 2002, however, there was a brief discussion about the control a channel
located west of the former WWTP lagoons could have on the direction of groundwater flow at
these units (i.e., SVVMUs 9A, and 913).
In an unconfined aquifer, a groundwater divide can develop between two channels (or
canals) even in the absence of significant topography. The height of such a groundwater divide
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DEQ-CFW-00034486

Mr. Buskirk
Page 2
1u-te 5, 2003
and its location would be controlled by aquifer properties, distance between the channels,
hydrostatic heads at the channels, and recharge to the aquifer. Analytical solutions have been
derived which express this concept mathematically.
Since a groundwater divide could profoundly influence the distribution of contaminated
groundwater, the HWS recommends that the CRG expand its site conceptual model to include
discussions of the groundwater flow reversals documented in the Phase I RFI report. As an
explanation for the location and likely areal extent of the divide, the influence of a western
channel should be considered. However, this is just one of several hypotheses that could explain
the different groundwater flow directions documented at the site. DuPont -Fayetteville and the
CRG should select the hypothesis which, in their judgment, best fits all the data.
Concerning a separate topic, the "Nafion channel" was also discussed during the site visit
*f 2002. It was proposed that this channel could intercept contaminated groundwater originating
in the Nafion area before it reached the Cape Fear River or was discharged at downgradient
springs. The fact that this theory was developed should be mentioned in the Phase I report. In
kddition, data collected during Phase I investigations which may help define the role of the
channel in the local groundwater flow regime should be discussed.
Hopefully, this letter will prove to be a useful follow-up to our recent telephone
conversation. If your office has questions concerning this correspondence, please call me at
[phone] extension
Sincerely,
Larry Stanley
Hydrogeologist
cc: Narindar Kumar, US EPA, Region 4
Bobby Nelms
Michael Johnson, DuPont Fayetteville Works
Larry Stanley
Abbe
Vp Bud M C
cl Bob Glasp
Karim Pathan
Larry Stanley;nllps
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DEQ-CFW-00034487