NC0001422_App B GW Modeling Report_20160201

CAP PART 2 SIMULATIONS FOR L.V. SUTTON ENERGY
COMPLEX, WILMINGTON, NC

December 16, 2015

Prepared for
SynTerra
148 River Street
Greenville, SC 29601

Investigators
Ronald W. Falta, Ph.D.
Scott E. Brame, M.S.
Regina Graziano, M.S.
Lawrence C. Murdoch, Ph.D.

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The simulated June, 2015 concentration distributions described in the Corrective Action Plan
Part 1 were used as initial conditions in a predictive simulation of future flow and COI transport
at the Sutton Site. The simulation runs from 2017 through 2045, with results presented at 5 years
(2020), 15 years (2030), and 30 years (2045). It should be noted that these simulation does not
consider the high boron concentrations that were found deep in the PeeDee formation. The
simulations only include constituent concentrations that emanated from the coal ash sources on
the site.
This simulation uses the Geosyntec design for ash removal from the FADA and ash basins, with
construction of a lined and capped ash landfill east of the current basins. The ash removal plan
is based on a figure provided by Geosyntec (2015a) that shows the “Wet Option”. With this
plan, all ash is removed from the FADA, and Lake Sutton is allowed to fill that excavation. Ash
is removed from the 1984 basin, and that area is graded so that it gently slopes towards Lake
Sutton. Ash is removed from the 1971 basin, and the excavation extends nearly to the PeeDee
formation in the zone where deep ash is located. The majority of the 1971 basin excavation is
then connected to Lake Sutton by breaching the dike on the southwest side of the basin.
The plan for the new lined and capped landfill calls for it to be constructed east of the current ash
basins, close to the eastern property line (Figure 1). The landfill will occupy approximately 100
acres, and the plan calls for two stormwater retention ponds, one at the north end of the landfill,
and one at the south end of the landfill. The northern pond will have an area of about 12 acres,
while the southern pond will have an area of about 10.5 acres (Geosyntec, 2015b).
These stormwater basins will have a large effect on the groundwater flow in the area. The ponds
are unlined, and are designed to capture all runoff from the 100 acre landfill cap. The ponds
have been sized to hold large storm event without overflowing. Considering the very high
hydraulic conductivity of the surficial aquifer, it is expected that most of the water entering the
stormwater basins will recharge the groundwater. The recharge rate in these basins was
estimated by assuming that about half of the annual rainfall (30 inches per year) runs off the
landfill cap, and is captured by the stormwater basins. This would be a total of 250 acre-feet of
water per year. Assuming an infiltration area of about 20 acres for the two ponds, this
corresponds to a recharge rate of 150 inches per year. This value was used in the CAP2
simulation.
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The model assumes that site geometry changes rapidly so that the new design is largely in effect
by June, 2017, when the simulation begins. The Lake Sutton constant head zone is enlarged to
include the FADA and most of the 1971 ash basin. The concentrations in this the constant head
zone are maintained at zero. The deep excavation in the 1971 basin is given a very high
conductivity, and is also maintained at zero concentrations. The remaining 1971 and 1984 ash
basin areas are given the background recharge rate of 12 inches per year. The new landfill area
is given a recharge rate of zero. All water supply pumping rates are assumed to remain constant
at the rates used in last step of the transport flow model.
The specified concentration zones that were used to represent the COI sources in the ash basins
and FADA (Figure 13) are removed, but COI concentrations in all layers outside of the ash are
initialized to their simulated June, 2015 values.
A groundwater extraction system consisting of 18 pumping wells is assumed to begin operation
in June 2017 at the start of the simulation. The extraction system consists of a line of 12
groundwater extraction wells located along the eastern property line, adjacent to the new landfill
(Figure 1), and a second line of 6 wells located just outside the current eastern boundary of the
ash basins. In the model, these wells are screened in the lower part of the Surficial Aquifer, from
an elevation of -15 ft MSL to -35 ft MSL.
A variable pumping system is used where the 12 outer wells only operate until 2020, by which
the boron plume has been pulled back to the property line. Wells 13 through 18 operate
continuously through the end of the simulation, and serve to pull the plume back inside the
eastern compliance boundary. During the 3 years of operation, wells 1-3 pump at a rate of 25
gpm, wells 4-8 pump at 50 gpm, and wells 9-12 pump at 25 gpm. Wells 13-18 operate at a rate
of 50 gpm. This pumping scheme produces a cone of depression along the property line and
inside of the compliance boundary during the period of 2017-2020 (Figure 1). After the outer
wells are turned off, the pumping system results in a cone of depression inside of the compliance
boundary (Figure 2).
The simulated boron concentrations over the simulation period are shown in Figures 3 to 8. The
simulation predicts that the boron plume will be pulled back to the property line by 2020. By
2030, only a small part of the plume extends beyond the compliance boundary to the east, and by
2045, none of the plume extends beyond the eastern compliance boundary.
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REFERENCES

Geosyntec, 2015a, Option 8 – Wet Option IV, L.V. Sutton Steam Station Closure Grading
Options.
Geosyntec, 2015b, Site Development Plan, Construction Permit Application Drawings, Onsite
CCR Disposal Facility, L.V. Sutton Energy Complex, Wilmington, NC.

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Figure 1. Simulated hydraulic heads in model layer 7 in 2017-2020 before the outer wells are
turned off.

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Figure 2. Simulated hydraulic heads in model layer 7 in 2020-2045 with only the inner wells
operating.

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Figure 3. Simulated 2020 boron concentration (ug/L) in the second model layer of the surficial
aquifer (layer 4).

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Figure 4. Simulated 2020 boron concentration (ug/L) in the lowest model layer of the surficial
aquifer (layer 7).

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Figure 5. Simulated 2030 boron concentration (ug/L) in the second model layer of the surficial
aquifer (layer 4).

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Figure 6. Simulated 2030 boron concentration (ug/L) in the lowest model layer of the surficial
aquifer (layer 7).

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Figure 7. Simulated 2045 boron concentration (ug/L) in the second model layer of the surficial
aquifer (layer 4).

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Figure 8. Simulated 2045 boron concentration (ug/L) in the lowest model layer of the surficial
aquifer (layer 7).