NC0024406_07. BCSS CAP Part 2_Appx D_FINAL_20160307

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Surface Water Mixing Model Approach
Belews Creek Steam Station Ash Basin
Overview of Modeling
Groundwater modeling showed that groundwater at the Belews Creek Steam Station (BCSS)
flows generally northward toward the Dan River and away from Belews Lake. The relatively
simple morphology of the Dan River receiving waters adjacent to the BCSS makes the site
amenable to the Mixing Model Approach. For this approach, river flow data from the U.S.
Geological Survey (USGS) were analyzed to determine upstream river design flows and assess
compliance with North Carolina Department of Environmental Quality surface water quality
standards, including determination of 1Q10, 7Q10, and mean annual river flows. The 1Q10 flow
is the annual minimum 1-day average flow that occurs once in 10 years; the 7Q10 flow is the
annual minimum 7-day average flow that occurs once in 10 years; and the mean annual flow is
the long-term average annual flow based on complete annual flow records.
The river design flows are combined with groundwater model discharge results to calculate
effluent dilution factors using the following equation:
DF = Qgw+Qriver
Qgw
where: DF is the groundwater dilution factor;
Qg, is discharge rate from the groundwater model (cubic feet per second [cfs]);
and
Qriver is the upstream river design flow (cfs).
The mixing zone sizes presented in Section 4.2 of the Corrective Action Plan (CAP) Part 1
Report' for the different water quality standards were used in this equation to determine the
appropriate dilution factor to assess compliance with the applicable water quality standards. The
applicable dilution factor was then used with the groundwater model concentration and
upstream concentration for the constituent of interest (COI) to determine the resulting surface
water concentration at the edge of the mixing zone, using the following equation:
_ (DF-1)XCriver+C
Csw DF
where: CSC, is the surface water concentration at the edge of the mixing zone
(micrograms per liter [pg/L]);
Cgs is the groundwater model concentration entering the river (pg/L);
C,i,e, is the upstream (background) river concentration (pg/L); and
DF is the groundwater dilution factor.
' HDR. 2016. Corrective Action Plan Part 1. Belews Creek Steam Station Ash Basin, December 8, 2015.
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Surface Water Mixing Model Approach
Belews Creek Steam Station Ash Basin
Alternately, the resulting surface water concentration can be calculated using the following mass
balance equation:
Csw
Qgw XC9w+QriverXCriver
-
Qgw +Qriver
where: Qg,, is discharge rate from the groundwater model (cfs);
Cg, is the groundwater model concentration entering the river (fag/L);
Q,i,e,is the upstream river design flow (cfs); and
C,i,er is the upstream (background) river concentration (tag/L).
For each groundwater COI that discharges to surface waters at a concentration exceeding the
North Carolina Groundwater Quality Standards, as specified in T15A NCAC .0202L (2L
Standards), the appropriate dilution factor and upstream (background) concentration were
applied to determine the surface water concentration at the edge of the applicable mixing zone.
This concentration was then compared to the applicable water quality standards to determine
surface water quality standard compliance.
Historical river flow data are available for the Dan River near Wentworth, North Carolina (USGS
#02071000, 1941 to 2015), which is located approximately 21 miles downstream of the
BCSS. Daily river flow data were analyzed to calculate the 1Q10, 7Q10, and mean annual
design flows. These design flows were scaled down using a drainage area ratio to estimate river
design flows near the BCSS. Drainage area ratios where developed using information from the
USGS StreamStats web application (http://water.usgs.gov/osw/streamstats/).
Key Assumptions and Limitations for Each Model
The key model assumptions and limitations include, but are not limited to, the following
• River design flows can be derived by drainage -area scaling of flows recorded at
upstream or downstream USGS gauges.
• Groundwater flow mixing in the receiving water occurs over the entire cross-section of
the mixing zone area (e.g., over 10% of the river width for the acute water quality
assessment).
• COI transformations are not represented in the analysis (i.e., all COls are treated as
conservative substances without any decay).
• The analysis is limited by the availability of surface water data used to assign upstream
river COI concentrations.
Mixing Model Development
The mixing model approach requires the assignment of upstream critical river design flows for
the fraction of the river as specified in CAP Part 1 Report Section 4.2 for the acute, chronic,
water supply, and human health (carcinogen or non -carcinogen) mixing zone limitations. The
1 Q10, 7Q10, and mean annual river design flows for Dan River at the BCSS are presented in
Table 1.
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Surface Water Mixing Model Approach
Belews Creek Steam Station Ash Basin
Table 1 Dan River Design Flows
Design Condition
Dan River Flow (cfs)
1 Q10
70
7Q10
81
Mean Annual
544
Site -specific surface water quality data were available for two sampling events in the Dan River
adjacent to the BCSS at a location just upstream of the confluence with the unnamed stream
which flows from the ash basin dam, locally known as Little Belews Creek (Station SW-DR-U/D,
Figure 1). Upstream COI concentrations used in the mixing model calculations were derived
from these data. Concentrations for beryllium, boron, cobalt, and thallium were below the project
method detection limits (MDL). Therefore, to perform mixing zone calculations for the Dan River,
it was assumed that upstream surface water concentrations for these COls were equivalent to
half of the project MDL. Hexavalent chromium was not measured, so its upstream surface water
concentration was set to the measured concentration for total chromium.
The BCSS groundwater modeling discussed in CAP Part 2 Report Section 4.1 was used to
provide the groundwater flows and COI concentrations into the adjacent receiving waters.
Figure 1 depicts the location of the groundwater model calculated flow inputs into the adjacent
receiving waters (Dan River), and Table 2 provides the total groundwater flow for the regions
noted on Figure 1. In developing these values, it was assumed conservatively that any
groundwater discharging to Little Belews Creek would eventually reach the Dan River. These
groundwater flows were used to assess the impact on surface water concentrations and
compliance with the applicable water quality standards at the mixing zone boundaries in the
Dan River. The groundwater flows are approximately six times those estimated during the initial
modeling in CAP Part 1. The increased flow resulted from increased recharge based on newly
calculated values and a slight increase in hydraulic conductivity based on site -specific aquifer
testing results.
Table 2 Model -Calculated Groundwater Flows
Waterbody
Groundwater Flow
3
(ft /day)
(cfs)
Dan River
99,439
1.151
Notes:
1. ft3/day = cubic feet per day
Table 3 provides flux -weighted average COI concentrations in groundwater discharging to the
Dan River adjacent to the BCSS, as well as upstream surface water concentrations in the
Round 1 sample (SW-DR-U). These values were used in the mixing zone dilution calculations
presented in the CAP Part 1 Report Section 4.2. The table also lists for comparison the surface
water quality standards applicable to each COI.
The COI concentrations in groundwater are generally higher than those estimated during the
initial modeling in CAP Part 1 and are over three orders of magnitude higher for hexavalent
chromium and thallium. The increased concentrations from the groundwater model are due to
the use of proposed provisional background concentrations, which elevates the concentration of
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Surface Water Mixing Model Approach
Belews Creek Steam Station Ash Basin
most constituents over the entire model domain. The model background concentrations are 3.2
pg/L for hexavalent chromium and 10 pg/L for thallium; the highest groundwater concentrations
in both Round 1 and 2 samples were 14 pg/L for hexavalent chromium and 3.9 pg/L for thallium,
and the majority of sample results were below the reporting limit.
Table 3 Dan River Dissolved COI Concentrations and Water Quality Standards
COI
Groundwater
Concentration
N /L
Surface Water
Concentration
N /L
Acute
WQS
N /L
Chronic
WQS
N /L
HH / WS
WQS
N /L
Arsenic
8.50
0.21
340
150
10 / 10 (c)
Beryllium
0.421
0.10*
65
6.5
ns / ns
Boron
1,763
25*
ns
ns
ns / ns
Chloride
119,898
3,300
ns
ns
ns / 250,000 (nc)
Total Chromium
7.954
0.87
ns
ns
ns / ns
Hexavalent
chromium
3.159
0.87*
16
11
ns / ns
Cobalt
18.42
0.25*
ns
ns
4 / 3 (nc)
Thallium
9.926
0.05*
ns
ns
0.47 / 0.24 (nc)
Notes:
1. All COls are shown as dissolved fraction except for total chromium, which is total recoverable metal
2. * — Values set to '/2 MDL, except hexavalent chromium set to measured value for total chromium
3. HH / WS= human health /water supply (15A NCAC 02B .0211, 15A NCAC 02B .0216, effective January 1, 2015)
4. WQS = water quality standard
5. c = carcinogen
6. nc = non -carcinogen
7. ns = no water quality standard
In Table 3, the water quality standards for arsenic, beryllium, and hexavalent chromium assume
a water effects ratio of 1, which expresses the difference between toxicity measured in a
laboratory and toxicity in site water. Site -measured water effects ratios are typically less than 1
due to complexing parameters in the site water (e.g., dissolved organic carbon) that reduces
site toxicity as compared to laboratory measured toxicity for metals. Thus, using a water effects
ratio of 1 provides a conservative assumption in the surface water quality assessment for these
COIs.
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Surface Water Mixing Model Approach
Belews Creek Steam Station Ash Basin
Figure 1 Belews Creek Steam Station Groundwater Model Flow Locations