NC0005088_7. CSS CAP Part 2_Appx D_FINAL_20160212

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Surface Water Mixing Model Approach
Cliffside Steam Station Ash Basin
Overview of Modeling
The relatively simple morphology of the receiving waters adjacent to Cliffside Steam Station
(CSS) makes this 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
(NCDEQ) surface water quality standards, including determination of applicable low river flow
statistics.
The river design flows were used along 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 4.2.1 of Corrective Action Plan (CAP) Part 1 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 (fag/L);
Cgs is the groundwater model concentration entering the river (fag/L);
Crier is the upstream (background) river concentration (fag/L); and
DF is the groundwater dilution factor.
Alternately, the resulting surface water concentration can be calculated using the following mass
balance equation:
_ QgwXCgw+QriverXCriver
CSW
Qgw+Qriver
where: Qg, is discharge rate from the groundwater model (cfs);
Cg,,is the groundwater model concentration entering the river (fag/L);
Qri,eris the upstream river design flow (cfs); and
C,1Ve, is the upstream (background) river concentration (fag/L).

Surface Water Mixing Model Approach
Cliffside Steam Station Ash Basin
For each groundwater COI that discharges to surface waters at a concentration exceeding its
applicable groundwater quality standard or criteria (as outlined in Section 1.9.2 of CAP Part 1),
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 standard or criteria to
determine surface water quality standard compliance
Historical river flow data were available for the Broad River near Boiling Springs, North Carolina
(USGS #02151500, from 1925 to 2015), which is located approximately 4 miles downstream of
the CSS site. Daily river flow data from this gage were analyzed to calculate the 1 Q10, 7Q10,
and mean annual design flows for the Broad River near Boiling Springs, North Carolina.
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.
These river design flows were scaled down using a drainage area ratio to correct for additional
drainage from minor tributaries between the CSS site and the downstream USGS gage location
Drainage area ratios where developed using information from the USGS StreamStats web
application (http://water.usgs.gov/osw/streamstats/).
Suck Creek is ungaged, so to estimate the creek flow, a similar discharge scaling ratio was
developed using the drainage area of Suck Creek versus the total drainage area of the USGS
Broad River gage (#02151500).
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 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.
• When surface water data were not available, or when surface water data were reported
at the method detection limit (MDL), half of the MDL was used in the mixing model
calculations.
• The analysis in Suck Creek is limited by a lack of gaged discharge to develop creek
low -flow design statistics (i.e., 1Q10, 7Q10, and annual mean).
Mixing Model Development
The mixing model approach requires the assignment of upstream low river design flows for the
fraction of the river as specified in Section 4.2.1 of CAP Part 1 for the acute, chronic, water
supply and human health mixing zone limitations. The calculated 1Q10, 7Q10, and mean

Surface Water Mixing Model Approach
Cliffside Steam Station Ash Basin
annual river design flows used for the Broad River near the CSS site and Suck Creek are
provided in Table 1.
Table 1 Broad River and Suck Creek Design Flows
Design Condition
Broad River Flow (cfs)
Suck Creek Flow (cfs)
1Q10
146
3.1
7Q 10
204
4.4
Mean Annual
1,031
22.1
Limited surface water quality data are available in the Broad River just upstream of the CSS
site. Some historical water quality data were collected at the USGS gauge near Boiling Springs
(#02151500); however, that gauge is located downstream of the CSS site and the data were
collected more than 35 years ago and are not representative of present conditions. To perform
mixing zone calculations for the Broad River, it was assumed that upstream surface water
concentrations were equivalent to half of the MDL for each COI.
Surface water samples were collected in Suck Creek at surface water sample location SW-2,
which is located near the upstream extent of the Duke Energy property boundary (see Figure 1).
This sampling location is upgradient of the CSS ash basins and is a suitable surface water
background station for the Suck Creek mixing zone calculations. If a COI at SW-2 was reported
at the MDL, then half of the MDL was used in the mixing zone calculations. Note that the
upstream surface water concentrations for Suck Creek are very similar to the half MDL
concentrations used for the Broad River mixing zone calculations.
The CSS groundwater modeling discussed in Section 4.1 of CAP Part 1 was used to provide the
groundwater flow and COI concentrations into the adjacent receiving waters (Suck Creek and
Broad River). Figure 1 presents the location of the groundwater model calculated flow inputs
into these adjacent receiving waters, and Table 2 presents the total groundwater flow along the
two flow boundaries 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 or criteria at the mixing zone boundaries in Suck Creek and the Broad River.
Table 2 Model -Calculated Groundwater Flows
Waterbody
Groundwater Flow
3
(ft /day)
(cfs)
Suck Creek
12,375
0.14
Broad River Total
69,261
0.80
Notes:
1. ft3/day = cubic feet per day
2. Broad River total includes groundwater inflow to Suck Creek
Total loading of groundwater COls to the Broad River at the CSS site includes direct loading to
the Broad River and local loading to Suck Creek, which discharges to the Broad River. Table 3
provides flux -weighted average COI concentrations in groundwater discharging to the Broad
River adjacent to the CSS site, as well as assigned upstream surface water concentrations.
These values were used in the mixing zone dilution calculations presented in Section 4.2.2 of

Surface Water Mixing Model Approach
Cliffside Steam Station Ash Basin
CAP Part 1. Table 3 also lists for comparison the surface water quality standards or criteria
applicable to each COI.
Table 3 Broad River Dissolved COI Concentrations and Water Quality Standards
COI
Groundwater
Concentration
(pg/L)
Surface Water
Concentration
(pg/L)*
Acute
WQS
(pg/L)
Chronic
WQS
(pg/L)
HH / WS
WQS
(pg/L)
Antimony
4.24
0.25
ns
ns
640 / 5.6
Arsenic
8.50
0.25
340
150
10 / 10
Boron
81.45
25
ns
ns
ns / ns
Total chromium
8.59
0.25
ns
ns
ns / ns
Hexavalent chromium
1.22
0.25
16
11
ns / ns
Cobalt
9.72
0.25
ns
ns
4/3
Lead
8.47
0.05
14
0.54
ns / ns
Nickel
8.74
0.25
140
16
ns / 25
Sulfate
32,119
500
ns
ns
ns / 250,000
Thallium
8.457
0.05
ns
ns
0.47 / 0.24
Vanadium
9.02
0.50
ns
ns
ns / ns
Notes:
1. All COls are shown as dissolved except for total chromium
2. * — Values set to'/z MDL, except hexavalent chromium set to'/z MDL for total chromium
3. WQS — water quality standard
4. HH / WS — Human Health / Water Supply (15A NCAC 02B .0211, 15A NCAC 02B .0216, effective
January 1, 2015
5. ns — no water quality standard
Mixing zone calculations were also performed separately for Suck Creek and include only COI
loads from local groundwater sources. Table 4 provides model flux -weighted average COI
concentrations in groundwater discharging to Suck Creek, as well as assigned upstream
surface water concentrations. These values were used in the mixing zone dilution calculations
presented in Section 4.2 of CAP Part 1. Table 4 also lists for comparison the surface water
quality standards or criteria applicable to each COI.

Surface Water Mixing Model Approach
Cliffside Steam Station Ash Basin
Table 4 Suck Creek Dissolved COI Concentrations and Water Quality Standards
COI
Groundwater
Concentration
(pg/L)
Surface Water
Concentration
(pg/L)*
Acute
WQS
(pg/L)
Chronic
WQS
(pg/L)
HH / WS
WQS
(pg/L)
Antimony
4.73
0.25
ns
ns
640 / 5.6
Arsenic
9.82
0.21
340
150
10 / 10
Boron
44.14
25
ns
ns
ns / ns
Total chromium
9.79
0.52
ns
ns
ns / ns
Hexavalent chromium
1.41
0.52
16
11
ns / ns
Cobalt
10.42
0.16
ns
ns
4/3
Lead
9.46
0.05
14
0.54
ns / ns
Nickel
9.46
0.42
140
16
ns / 25
Sulfate
7,371
1,100
ns
ns
ns / 250,000
Thallium
9.672
0.05
ns
ns
0.47 / 0.24
Vanadium
4.88
0.56
ns
ns
ns / ns
Notes:
1. All COls are shown as dissolved except for total chromium
2. * — Values from sampling station SW-2 or'/2 MDL, except hexavalent chromium set to SW-2 value for total
chromium
3. WQS — water quality standard
4. HH / WS — Human Health / Water Supply (15A NCAC 02B .0211, 15A NCAC 02B .0216, effective
January 1, 2015
5. ns — no water quality standard
For both Tables 3 and 4, the aquatic life water quality standards or criteria for arsenic,
hexavalent chromium, lead, and nickel 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.

Surface Water Mixing Model Approach
Cliffside Steam Station Ash Basin
1 Y
r
1 ,
,'� ` � � 1 ! 1, — :,--- .-.. • ...,
�SW-2
Lege
Suck Creek GW Model Discharge
Broad River GW Model Discharge
0 0.125 0.25 0.5 0.75 1
Miles
Figure 1 CSS Groundwater Model Flow Locations and Surface Water Stations
6