NC0004979_Allen CAP Part 1_Appx E_Final_20151120

This page intentionally left blank

Surface Water Mixing Model Approach
Allen Steam Station
Overview of Modeling
The relatively simple morphology of the receiving waters adjacent to Allen Steam Station (Allen)
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 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;
Qgw 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 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+Cgw
CSW DF
where: Csw is the surface water concentration at the edge of the mixing zone (fag/L);
Cgw is the groundwater model concentration entering the river (fag/L);
Craver 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: Qgw is discharge rate from the groundwater model (cfs);
Cgw is the groundwater model concentration entering the river (Ng/L);
Qriver is the upstream river design flow (cfs); and
Criver is the upstream (background) river concentration (fag/L).

For each groundwater Constituent of Interest (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 (WQS) compliance.
Historical river flow data are available for the Catawba River at Catawba, North Carolina (USGS
#02142500; 1896 to 1962 with gaps from 1900 to 1935), which is located approximately 52
miles upstream of the Allen site. Daily river flow data from this gage (from 1935 to 1962)' were
analyzed to calculate the 1Q10, 7Q10 and mean annual river design flows for the Catawba
River at Catawba, North Carolina.
The 1 Q1 0 flow is the annual minimum 1-day average flow that occurs once in ten years; the
7Q10 flow is the annual minimum 7-day average flow that occurs once in ten 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 up using a drainage area ratio to account for additional
drainage from minor tributaries between the USGS gage location and the Allen site. 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:
• 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; and
• The analysis is limited by the availability of contemporary USGS gage data to develop
river design flows of interest (i.e., 1Q10, 7Q10, and annual mean).
Flows prior to 1935 appear to have been unregulated and are unrepresentative of present conditions.
2

Mixing Model Development
The mixing model approach requires the assignment of upstream river design flows for the
fraction of the river as specified in Section 4.2.1 for the acute, chronic, water supply, and human
health mixing zone limitations. The calculated 1Q10, 7Q10, and mean annual river design flows
used for the Catawba River (Upper Lake Wylie) are provided in Table E-1.
Table E-1. Catawba River Design Flows
Design Condition
Catawba River Flow (cfs)
1Q10
139
7Q10
263
Mean Annual
3,011
No site -specific surface water quality data were available in the Catawba River just upstream of
the Allen site. Therefore, to perform mixing zone calculations, it was assumed that upstream
surface water concentrations were equivalent to half of the MDL for each COI.
The Allen groundwater modeling discussed in Section 4.1 was used to provide the groundwater
flow and COI concentrations into the adjacent receiving waters (i.e., Catawba River at Upper
Lake Wylie). Figure E-1 provides the location of the groundwater model calculated flow inputs
into the adjacent receiving waters, and Table E-2 provides the total groundwater flow along the
flow boundary noted on Figure E-1. These groundwater flows were used to assess the impact
on surface water concentrations and compliance with the applicable water quality standards2 or
criteria at the mixing zone boundaries in the Catawba River (Upper Lake Wylie).
Table E-2. Model -Calculated Groundwater Flows
Waterbody
Groundwater Flow
3
(ft /day)
(cfs)
Catawba River
41,029
0.48
Notes:
1. ft3/day = cubic feet per day
2. cfs = cubic feet per second
Table E-3 provides flux -weighted average COI concentrations in groundwater discharging to the
Catawba River adjacent to the Allen 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. Table E-3 also lists for comparison the surface water quality standards or criteria
applicable to each COI.
2 Water quality standards were published by NCDEQ in North Carolina Administrative Code 15A NCAC
2B, amended effective January 1, 2015.
3 U.S. Environmental Protection Agency (USEPA) National Recommended Water Quality Criteria.

Table E-3. Catawba River Dissolved COI Concentrations & WQS
COI
Groundwater
Concentration
(pg/L)
Surface Water
Concentration
(pg/L)*
Acute
WQS
(pg/L)
Chronic
WQS
(pg/L)
HH / WS
WQS
(pg/L)
Antimony
5.19
0.25
ns
ns
640 / 5.6
Arsenic
11.64
0.25
340
150
10 / 10
Barium
30.63
4.5
ns
ns
200,000 / 1,000
Boron
823
25
ns
ns
ns / ns
Chromium (VI)
2.62
0.25
16
11
ns / ns
Cobalt
12.23
0.25
ns
ns
4/3
Selenium
7.18
0.25
ns
5
ns / ns
Sulfate
96,964
500
ns
ns
ns / 250,000
Vanadium
7.73
0.50
ns
ns
ns / ns
Notes:
1. All COIs are shown as dissolved fraction except for total chromium, which is total recoverable metal
2. * — Values set to'/z MDL, except chromium (VI) set to'/2 MDL for total chromium
3. HH / WS —human health / water supply
4. ns — no water quality standard
In Table E-3, the water quality standards for arsenic and chromium (VI) assume a water effects
ratio (WER) of 1, which expresses the difference between toxicity measured in a laboratory and
toxicity in site water. Site -measured WER 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 WER of 1 provides a
conservative assumption in the surface water quality assessment for these COls.

r•_
e�
E.
Legend
GW Model Output
0 0.125 0.25 0.5 0.75'