7 MSS CAP Part 2_Appx D_FINAL

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Surface Water Mixing Model Approach
Marshall Steam Station
Overview
The Marshall Steam Station (MSS) is located on a semi -enclosed arm of Lake Norman. The
MSS withdraws condenser non -contact cooling water from the head -end of the lake arm and
discharges via a canal to a different arm of Lake Norman, which will induce a relatively
unidirectional flow past the MSS. The MSS study area and intake and discharge flow locations
from and to Lake Norman are presented on Figure 1. This induced flow in the lake arm receiving
waters adjacent to the MSS makes the site amenable to the Mixing Model Approach. For this
approach, upstream design flows were derived based on the permitted intake flow of condenser
non -contact cooling water that is pulled into the lake arm and past the MSS groundwater flow
discharge.
The upstream design flows were combined with groundwater model discharge results to
calculate effluent dilution factors using the following equation:
DF = Qgw+Qupstream
Qgw
where: DF is the groundwater dilution factor;
Qgw is discharge rate from the groundwater model (cubic feet per second [cfs]);
and
Qupstream is the upstream design flow (cfs).
The mixing zone sizes presented in Section 4.2 of the MSS 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) X Cupstream+Cgw
CSw DF
where: C, , 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);
Cupstream is the upstream (background) concentration (pg/L); and
DF is the groundwater dilution factor.
Alternately, the resulting surface water concentration can be calculated using the following mass
balance equation:
CS _ QgwXCgw+QupstreamXCupstream
Qgw+Qupstream

Surface Water Mixing Model Approach
Marshall Steam Station Ash Basin
where: Qg,, is discharge rate from the groundwater model (cfs);
Cgs, is the groundwater model concentration entering the river (Ng/L);
Qupstream is the upstream design flow (cfs); and
Cupstream is the upstream (background) concentration (fag/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.
Key Assumptions and Limitations for Each Model
The key model assumptions and limitations include, but are not limited to, the following:
• Groundwater flow mixing in the receiving water occurs over the entire cross-section of
the mixing zone area (e.g., over 10% of the lake arm width for the acute water quality
assessment).
• Surface water flow past the MSS that is available for groundwater dilution is comprised
of the MSS non -contact cooling water intake flow.
• 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 unavailability of surface water data used to assign
upstream river COI concentrations, so half of the project specific method detection limit
(MDL) was used in the mixing model calculations.
Mixing Model Development
The mixing model approach requires the assignment of upstream design flows 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. For the MSS, the upstream design
flows used here are based on the permitted intake non -contact cooling water flow of 1,093
million gallon per day (1,691 cfs). The upstream design flows for lake induced flow past the
MSS are presented in Table 1.
Table 1 Lake Norman Design Flows
Design Condition
Design Flow (cfs)
10% of Induced Flow
169
50% of Induced Flow
845
100% of Induced Flow
1,691
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Surface Water Mixing Model Approach
Marshall Steam Station Ash Basin
Historical water quality data have been collected in Lake Norman at a number of North Carolina
Department of Environment and Natural Resources stations; however, most of the data was
collected prior to 2002, and none of the stations are located directly upstream of the MSS.
Limited sulfate data were collected in Lake Norman in 2007, with concentrations at eight
stations all averaging around 3.6 milligrams per liter. Given the limited water quality data, it was
assumed that upstream surface water concentrations were equivalent to half of the project MDL
for each COI except barium, chloride, and hexavalent chromium. Measured groundwater
concentrations for barium and chloride always exceeded their MDLs, so upstream surface water
concentrations for those COls were set to half of the minimum reported groundwater
concentration. Upstream surface water concentration for hexavalent chromium was set to half of
the project MDL for total chromium.
The MSS groundwater modeling discussed in the 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, and the Table 2 provides the total groundwater flow along the flow boundary
noted on Figure 1. 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 Lake Norman.
Table 2 Model -Calculated Groundwater Flows
Waterbody
Groundwater Flow
(ft3/day)
(cfs)
Lake Norman
31,314
0.362
Notes:
ft3/day = cubic feet per day
Table 3 provides flux -weighted average COI concentrations in groundwater discharging to Lake
Norman adjacent to the MSS, as well as assigned upstream surface water concentrations.
These values were used in the mixing zone dilution calculations presented in CAP Part 2 Report
Section 4.2. The table also lists for comparison the surface water quality standards applicable to
each COI.
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Surface Water Mixing Model Approach
Marshall Steam Station Ash Basin
Table 3 Lake Norman Dissolved COI Concentrations and Water Quality Standards
COI
Groundwater
Concentration
(jig/L)/L
Surface Water
Concentration
*
Acute
WQS
/L
Chronic
WQS
/L
HH / WS
WQS
/L
Arsenic
4.97
0.25
340
150
10 / 10 (c)
Barium
156
4.25
ns
ns
200,000 / 1,000 (nc)
Beryllium
1.34
0.10
65
6.5
ns / ns
Boron
915
25
ns
ns
ns / ns
Chloride
75,148
400
ns
ns
ns / 250,000 (nc)
Total Chromium
9.02
0.25
ns
ns
ns / ns
Hexavalent
chromium
1.76
0.25
16
11
ns / ns
Cobalt
2.40
0.25
ns
ns
4 / 3 (nc)
Selenium
7.14
0.25
ns
5
ns / ns
Sulfate
75,313
500
ns
ns
ns / 250,000 (nc)
Thallium
0.50
0.05
ns
ns
0.47 / 0.24 (nc)
Notes:
1. All COls are shown as dissolved fraction except for total chromium and selenium, which are total recoverable metal
2. * — Values set to'/Z MDL, except hexavalent chromium, set to'/z MDL for total chromium; and barium and chloride
set to of minimum reported groundwater concentration
3. HH / WS = human health / water supply
4. WQS = water quality standard
5. c = carcinogen
6. nc = non -carcinogen
7. ns = no water quality standard
In the above table, 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
cols.
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Surface Water Mixing Model Approach
Marshall Steam Station Ash Basin
Figure 1 Marshall Steam Station Groundwater Model Flow Locations

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