HomeMy WebLinkAbout201706201013-1TECHNICAL MEMORANDUM
Update of Cape Fear River Water Quality Model for
Potential Outfall Structure Relocation for DuPont
PREPARED FOR: E.I. du Pont de Nemours and Company (DuPont)
PREPARED BY: CH2M HILL
DATE: January 19, 2007
Executive Summary
This Technical Memorandum (TM) presents a discussion of the development and
application of a QUAL2E and QUAL2K model in the Cape Fear River from Buckhorn Dam
to Lock and Darn 1 near Kelly. The purpose of the modeling was to evaluate whether
DuPont can relocate its outfall structure from downstream of Lock and Dam 3 to upstream
of Lock and Dam 3 on the Cape Fear River near Fayetteville. The work was conducted for
DuPont for submittal to the Division of Water Quality (DWQ).
The modeli
ng effort indicates that predicted dissolved oxygen is not sensitive to the location
of DuPont's outfall location. The modeling supports the conclusion that DuPont's NPDES
permit limits for oxygen consuming wastes would not need to be reduced to protect water
quality in the Cape Fear River if the discharge point is moved upstream of the Lock and
Dam 3 structure.
DuPont's effluent data also indicate that there are low nutrient concentrations in its effluent,
and its nutrient loads are well below those outlined in DWQ's interim nutrient loading
strategy. Moving the outfall upstream of Lock and Darn 3 should not negatively impact
water quality.
Introduction
The purpose of this TM is to evaluate whether DuPont can relocate its outfall structure from
downstream of Lock and Dam 3 to upstream of Lock and Dam 3 on the Cape Fear River
near Fayetteville. DuPont's effluent channel is eroding, which results in sediment inputs to
the Cape Fear River: Communication with local citizens indicates that the effluent charnel
was a small stormwater conveyance ditch prior to the DuPont facility's startup in 1970.
Because of the easily erodible soils in this region, the effluent channel is now approximately
forty to sixty feet wide and fifteen to twenty feet deep. In order to remove this sediment
load from the river and enhance water quality, DuPont would like to relocate its outfall to
upstream of Lock and Darn 3. Under this scenario, only stormwater from the forested area
outside DuPont's fence would enter the current effluent channel during rain events.
DuPont has examined other alternatives including sediment basins at the bottom of the
channel, but has concerns about the ongoing maintenance that would be needed. This TM
evaluates whether DuPont's current NPDES permit limits would need to be reduced to
UPDATE OF CAPE FEAR RIVER WATER QUALITY MODEL FOR POTENTIAL OUTFALL STRUCTURE RELOCATION FOR DUPONT 11
protect water quality in the Cape Fear River if the discharge point is moved upstream of the
Lock and Dam 3 structure.
The objectives of the modeling effort were to:
• Evaluate whether DuPont can relocate its outfall structure from downstream of Lock
and Dam 3 to upstream of Lock and Dam 3 on the Cape Fear River near Fayetteville
based on predicted water quality.
• Determine the predicted dissolved oxygen (DO) at DuPont's existing NPDES permit
limits at its current outfall location and at the proposed outfall location.
• Evaluate DuPont's nutrient data and determine whether current loads comply with the
DWQ's interim nutrient strategy for the Cape Fear River.
Dissolved Oxygen
Historical Modeling Information
The Cape Fear River was modeled by CH2M HILL in March 2002 and October 2001 using
QUAL2E from Buckhorn Dam to Lock and Dam 1 near Kelly. This model was an update of
two existing 1995 models developed by DWQ. These two models were combined and
recalibrated using in situ data collected by the Middle Cape Fear River Basin Association
(MCFRBA) during July 1999. Appendix A contains Technical Memorandum Number 3
"Cape Fear QUAL2E Model Development and Application" dated October 2, 2001 revised
March 5, 2002, which provides the details regarding the previous modeling efforts.
QUAL2E Model Update
CH2M HILL updated the existing QUAL2E model to reflect a change in the DuPont outfall �
structure location. Lock and Dam 3 is represented in the model at reach 15, element 2. The
outfall structure was moved from reach 15, element 13 to reach 14, element 17 above Lock /
and Dam 3 based on information provided by DuPont on the proposed outfall relocation
(Figure 1). Appendix B contains the output files for the relocation of the DuPont outfall
structure. I
HAI OM
Model Results
Table 1 summarizes the predicted water quality with the DuPont outfall structure in its
current location at the current and proposed outfall locations as well as at each of the lock
and dam structures, the predicted dissolved oxygen sag points in the model. Table 2
represents the predicted conditions with the DuPont outfall structure relocated above Lock I
and Dam 3. All model runs were performed using the Base Case scenario 1B outlined in the
prior modeling report (CH2M HILL, 2001 and Appendix A). Table 3 represents the change
in predicted water quality values at each river location due to the relocation of the DuPont
outfall structure. The predicted changes in water quality due to the outfall relocation are
not significant.
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