NC0003417_App A_20160916

TECHNICAL MEMORANDUM
Date: August 31, 2016 File: 1026.104.09
To: Project File
Cc:
From: Judd Mahan, Regina Graziano
Subject: H. F. Lee Groundwater Flow and Transport Modeling
WATER SUPPLY WELL CAPTURE ZONE ANALYSIS
A well capture zone analysis was performed to delineate well capture zones for the water
supply wells near the Duke Energy property boundary at H.F. Lee. A well capture zone is the
area of an aquifer in which all the water will be removed by pumping wells within a specified
time period (Grubb, 1993). Consequently, mobile constituents within the capture zone would
also be removed by pumping within this timeframe. Groundwater pumping produces a low
pressure area in the groundwater flow field that induces groundwater flow towards the well.
The pressure front does not propagate evenly through the aquifer as groundwater upgradient
from the well has higher potential energy and is already flowing towards the well (positive
kinetic energy). The pressure front extends outward into the aquifer until equilibrium is
reached. At this point, the aquifer volume contributing water stabilizes and the flow rate of
water into the well equals the pumping rate. Consequently, the miscible and mobile
constituents in groundwater, if present, within the capture zone are removed by pumping.
In the capture zone analysis, a model is used to simulate the normal pumping of the water
supply wells and the capture zones generated by the model are compared to the footprints of
the ash basin waste boundary. This comparison can be used to identify potential impacts to the
recharge areas used by the water supply wells. The capture zone analysis accounts for the
potential effect of adjacent water supply wells pumping at the same time, and considers the
effects of local groundwater flow and other relevant conditions, such as the location and water
elevations of the ash basin system.
In an unconfined aquifer system, such as at H.F. Lee, groundwater flow normally follows the
surface topography flowing from areas of higher elevation (higher water levels) to areas of
lower elevation (lower water levels) due to gravity (Freeze and Cherry, 1979). Typically,
aquifer recharge occurs at higher elevations where vertical downward flow is predominant.
Aquifer discharge areas (e.g., a groundwater seep line near a stream) are found at lower
elevations. The well capture zone will extend further upgradient than downgradient, as that is
the predominant source of water recharge to the well.
P: \ Duke Energy Progress.1026 \ 104. Lee Ash Basin GW Assessment \ 1.9 Groundwater Modeling \ Tech Memo Aug 31
2016.docx

Groundwater Flow and Transport Modeling August 31, 2016
H.F. Lee Energy Complex, Goldsboro, NC Page 2 of 3
METHODOLOGY
The steady-state groundwater flow and contaminant transport model developed for the H.F.
Lee CAP-2 report (SynTerra, 2016) was utilized for this study. The MODFLOW NWT
simulation from the H.F. Lee CAP 2 report (SynTerra, 2016) was converted to MODFLOW 2000
in order for the MODPATH simulation to run. Using the MODFLOW NWT simulation caused
an error when the particle tracking is in contact with a dry grid cell when using MODPATH.
Converting the MODFLOW NWT file to MODFLOW 2000 required some changes in the model
in order for the simulation to process. First the grid cells within and adjacent to the Neuse River
decreased in size, the Neuse River hydraulic heads were slightly higher, and the cooling pond
hydraulic head also increased. These changes did not affect the results of the capture zone
simulation since the Neuse River and cooling pond are downgradient of the water supply wells.
The capture zone was simulated using a similar approach as the contaminant transport model
to predict concentrations; however, MODPATH is used in the calibrated flow model instead of
MT3DMS. The model MODPATH, widely used by the United States Geological Survey (USGS),
is a "particle tracking" model that traces the groundwater flow lines from any desired starting
position. MODPATH can be used with the reverse tracking feature to trace the groundwater
flow lines around each well to see where the water that is pumped from the well originates.
This well-known procedure is called a well capture zone analysis, because it identifies the zone
from which all of the water entering the well is captured. The groundwater flow model was
calibrated to match water levels measured in June 2015 and specifically considered the potential
effects of local water table mounding from the ash basins due to enhanced infiltration from the
basins. A primary concern at each of the ash basins was possible impacts to water supply wells
from the ash basins. The calibrated groundwater flow model was used to assess these possible
impacts by considering pumping from all of the water supply wells within the model domain of
the site.
The model solution for groundwater flow for the H.F. Lee CAP-2 was performed under existing
conditions. Although pumping rates for the individual household wells at the site were not
available, an assumption equal to the average US household water use rate of 400 gallons per
day (USEPA, 2008) was used. No information was available for public water supply well L19
and it was assumed to have a pumping rate of about 39,600 gallons per day. It was assumed
that all of the wells in the model domain were pumping continuously unless they were known
to be inactive. Additional information on the groundwater model is included in CAP-1 Section
2 and CAP-2 Section 3 (SynTerra, 2015b, 2016).
RESULTS
A numerical capture zone analysis for the H.F. Lee site was conducted to evaluate potential
impact to upgradient or side -gradient water supply pumping wells. The analysis for water
supply wells near the H.F. Lee site indicates that well capture zones projected well into the
indefinite future are limited to the flow upgradient and do not extend toward the ash basins as
illustrated in the figure below. Further, none of the particle tracks originating in the ash basins
moved into the well capture zones.
P: \ Duke Energy Progress.1026 \ 104. Lee Ash Basin GW Assessment \ 1.9 Groundwater Modeling \ Tech Memo Aug 31
2016.docx

Groundwater Flow and Transport Modeling August 31, 2016
H.F. Lee Energy Complex, Goldsboro, NC Page 3 of 3
Well Capture Zone
Ash Basin Boundary
Figure 1. Supply well capture zones for the H.F. Lee Site. The yellow particle tracks are the
well capture zones and the ash basins are outlined in orange.
P: \ Duke Energy Progress.1026 \ 104. Lee Ash Basin GW Assessment \ 1.9 Groundwater Modeling \ Tech Memo Aug 31
2016.docx