NC0001422_Fact Sheet_20221213

NC0001422
DEQ/DWR
FACT SHEET FOR NPDES PERMIT DEVELOPMENT
Renewal
NPDES No. NC0001422
Facility Information
Applicant/Facility Name:
Duke Energy Progress, LLC/ L.V. Sutton Energy Complex
Applicant Address:
801 Sutton Steam Plant Road, Wilmington, NC 28401
Facility Address:
(same)
Permitted Flow
N/A
Type of Waste:
100 % Industrial
Facility/Permit Status:
Major Modification (WWTP Class I)
County:
New Hanover
Miscellaneous
Receiving Stream:
Cape Fear River
(001), Sutton Lake
(008)
Regional Office:
WiRO
Stream Classification:
C Sw (001)
C (008)
SI: 18-(63)
Quad
J27SW
Castle Hayne
303(d) Listed?:
Yes
Impaired for D.O.
(Cape Fear River)
Permit Writer:
Sergei Chernikov, Ph.D.
Subbasin:
030617 (CPF)
Date:
November 15, 2022
Drainage Area (mi2)-
Aildillir
Summer 7Q10 (cfs)
Tidally influenced
(Outfall 001);
Lake (Outfall 008)
30Q2 (cfs):
See above
Average Flow (cfs):
See above
IWC (%):
100 (Outfall 008)
22 (Outfall 001)
based on
CORMIX model
Primary SIC Code:
SUMMARY
This is a renewal of the NPDES wastewater permit for L.V. Sutton Energy Complex (Sutton). Duke
Energy Progress Sutton Plant is a natural gas -fired 620 MW combined cycle generation facility. The
power block consists of two combustion turbine generators (each with a HRSG — heat recovery steam
generator) and one steam turbine generator. Historically, the facility operated 3 coal-fired units. The
coal-fired units were shut -down in the fourth quarter of 2013. The facility is regulated by federal
effluent guidelines (40 CFR Part 423 — Steam Electric Power Generating Point Source Category) —
BPT/BAT.
On February 11, 2015 the Wilmington Regional Office delineated the Effluent Channel at the Sutton
Energy Complex in accordance with the requirements of 15A NCAC 02B .0228. The new Outfall 008
was established to accommodate discharge from this effluent channel.
Wastewater outfalls:
Outfall 001 — cooling pond discharge, recirculated cooling water, non -contact cooling water,
groundwater, landfill leachate, and treated wastewater from Outfall 004 (new ash pond). The new
Page 1 of 12

NC0001422
ash pond can discharge directly to Sutton Lake through Outfall 004 or to Cape Fear River through
Outfall 001. The Outfall 001 is discharging through the mixing box that was set-up to concurrently
discharge ash pond wastewater and water from Sutton Lake. The compliance point for Outfall 001 is
located within the mixing box. The location of Outfall 001 is: Latitude: 34°16'57.24"; Longitude:
77°59'20.32".
Outfall 008- Primarily consists of recirculating cooling water from the Combined Cycle generation
unit, contains flows from internal outfalls 005, 006, 007, 009, and stormwater outfalls. The location
of Outfall 008 is: Latitude: 34°17'29.27"; Longitude: 77°59'36.78".
Outfall 010 - non -contact stormwater from North Pond Emergency Spillway, the pond will receive
stormwater from the coal ash landfill after landfill is capped.
Outfall 011 — non -contact stormwater from South Pond Emergency Spillway, the pond will receive
stormwater from the coal ash landfill after landfill is capped.
Stormwater outfalls discharging to the effluent channel and then to Sutton Lake via Outfall 008:
Internal Outfall SW001 — Runoff from the temporary laydown area and the parking lot.
Internal Outfall SW002 — Runoff from the parking lot and Peaker Combustion Turbine area.
Internal Outfall SW003 — Runoff from the parking lot.
Internal Outfall SW004 — Pumped stormwater from the 115 Electrical Switchyard area.
Internal Outfall SW005 — Discharge from the south wet detention basin.
Internal Outfall SW006 — Discharge from the rip rap armored emergency spillway for the north
infiltration basin that treats stormwater from a parking lot and surrounding areas.
Internal Outfall SW007 — Runoff from the potential rail loading yard, rail spur, and truck roads
installed to transport coal ash from the site.
ASH POND DAMS
Seepage through earthen dams is common and is an expected consequence of impounding water
with an earthen embankment. Even the tightest, best -compacted clays cannot prevent some water
from seeping through them. Seepage is not necessarily an indication that a dam has structural
problems, but should be kept in check through various engineering controls and regularly monitored
for changes in quantity or quality which, over time, may result in dam failure.
Currently, no seeps have been detected at the site.
DILUTION MODEL
Geosyntec Consultants of NC has submitted a CORMIX model on behalf Duke Energy Progress,
LLC for the discharge of Sutton Lake water to the Cape Fear River (classified C, Swamp, PNA —
primary nursery area) at 0.35 and 3.5 MGD.
The CORMIX model was used in conjunction with a previously calibrated Environmental Fluid
Dynamics Code (EFDC) model to determine tidal inputs to the model. The outfall configuration
was updated with the latest changes to the outfall following the separation of the Cooling Pond
wastewater discharge to Sutton Lake. The model was completed in two parts. The first, the
Page 2 of 12

NC0001422
embayment stage, is a roughly uniform channel to the main river. The plume characteristics at the
end of the embayment model run are used as inputs to the river hydrodynamics portion of the
model. For reasons discussed below, mixing zone considerations were reserved to the embayment
channel.
The model results show a buoyant plume undergoing near -field mixing for approximately three
meters before contacting the surface of the water. Near -field mixing occurs along the surface
contact for another half meter before moving into passive ambient diffusion. The plume continues
with passive diffusion and contacts the nearest bank at 14 meters eventually ending approximately 8
meters wide and 0.25 meters deep from the surface. EPA guidance for sizing acute mixings zones
(section 4.3.3 of the Technical Support Document for WaterQuality-based Toxics Control, 1991) were
reviewed with the criteria of 50 times the discharge length scale (square root of the port cross -
sectional area) being the most restrictive of the three options, in this case 13.5 m with a dilution of
10.5. The Division believes that 13.5 m is too large of an acute mixing zone considering the
receiving water classification of Primary Nursery Area by the NC Division of Marine Fisheries, the
length of the passive buoyant plume, and that the dilution allowed at this distance too great to
protect downstream saltwater standards in accordance with 15A NCAC 2B .0203. To minimize the
mixing zone and the pollutant concentrations in the ambient spreading plume, the mixing zone will
be set where the plume contacts the water surface approximately 3 m from the outfall. The dilution
at this point is 4.5.
Dilution Chronic/Acute- 4.5:1
IWC%- 22%
Diffuser -Existing outfall structure.
Regulatory Mixing Zone- three meters downstream of outfall.
REASONABLE POTENTIALANALYSIS(RPA)-OuTFALL 001 AND Ou1r008
The Division conducted EPA -recommended analyses to determine the reasonable potential for
toxicants to be discharged at levels exceeding water quality standards/EPA criteria by this facility.
For the purposes of the RPA, the background concentrations for all parameters were assumed to be
below detections level. The RPA uses 95% probability level and 95% confidence basis in accordance
with the EPA Guidance entitled "Technical Support Document for Water Quality -based Toxics
Control." The RPA included evaluation of dissolved metals' standards, utilizing a default hardness
value of 25 mg/L CaCO3 for hardness -dependent metals. The 2007-2014 Triennial Review
standards adopted by NC in Nov. 2014 and approved by EPA in April 2016 were used to develop
the acute and chronic limits. The RPA spreadsheets are attached to this Fact Sheet.
RPA for Outfall 001 have been calculated based on the maximum daily discharge of 1.2 MGD from
the renewal application. Calculations included: As, Be, Cd, Chlorides, Al, Cr (VI), Cu, Pb, Hg, Ni,
Se, Ag, Zn, Sb, and Tl. The RPA indicated that limit for As must be maintained, and limits for Ni
and Se shall be removed (please see attached).
RPA for Outfall 008 have been calculated based on the results of the new Water Effect Ratio Study
that established following site -specific Copper standards for Sutton Lake: 71.4 µg/L (Chronic
Standard); 96.7 µg/L (Acute Standard).
Calculations included: As, Be, Cd, Cr (VI), Cu, Pb, Hg, Ni, Se, Ag, and Zn.
The highest projected flow of 211.0 MGD was used in the analysis. The RPA indicated that limit for
As, Ni, and Se shall be removed (please see attached).
Page 3of12

NC0001422
The proposed permit requires that EPA methods 200.7 or 200.8 (or the most current versions) shall
be used for analyses of all metals except for total mercury.
INSTREAM MONITORING-OuiPALL 002 AND OUTFALL 008
The permit requires for Cape Fear River Outfall 001 to be monitored upstream and downstream,
and for Sutton Lake Outfall 008 to be monitored at Bay 8 downstream of the discharge (there is no
upstream location).
Sampling for all three instream sites is conducted semi-annually and indicates that Arsenic,
Cadmium, Chloride (Lake Sutton site only), Chromium, Copper, Hardness, Lead, Mercury (method
1631E), Selenium, and Zinc are below detection level or well below the standard. There were 3
exceedances of the Chloride state standard at the Cape Fear downstream monitoring station, the
hardness level was also significantly elevated during these sampling events. This can be attributed to
the salt water intrusions because Cape Fear River at this location is tidally influenced.
The data shows that current operations and legacy impacts of the Sutton Plant do not negatively
influence water quality of Sutton Lake and Cape Fear River. The facility will continue to monitor the
receiving streams.
FISH TISSUE MONITORING
The permit requires fish tissue monitoring for As, Se, and Hg in Cape Fear River and Sutton Lake.
Largemouth Bass and Sunfish subspecies tissues were analyzed for these trace elements. All results
were below action levels for Se and Hg (10.0 µg/g — Se, 0.40 µg/g — Hg, NC) and screening value
for As (1.20 — µg/g, EPA).
CWA SECTION 316(a)
Since the Sutton Lake has been reclassified to the "waters of the State" on November 5, 2014, the
facility developed and conducted comprehensive 316(a) studies. The 316(a) study demonstrated that
Lake Sutton maintains Balanced and Indigenous Community. The DWR biologists concurred with
this conclusion.
CWA SECTION 316(b)
Sutton consists of a single highly efficient natural gas -fired combined -cycle unit, Unit 1, with a gross
generating capacity of 617 megawatts (MW). Sutton began commercial operation in November
2013, replacing the retired (and subsequently demolished) coal-fired L.V. Sutton Steam Electric
Plant.
Sutton withdraws water from the Cape Fear River using a make-up water intake structure (MWIS)
to support water elevations in the closed -cycle recirculating system (CCRS) cooling impoundment
known as Sutton Lake. Sutton Lake, which meets the definition of a CCRS as defined at 40 CFR
125.92(c)(2), was formed in 1972 by the impoundment of Catfish Creek, a tributary of the Cape
Fear River, to support operation of the three coal-fired units at the former L.V. Sutton Steam
Electric Plant.
The cooling impoundment was designed and operates as a CCRS. The MWIS is the point of
compliance from a Rule perspective as it provides make-up water to Sutton Lake. The MWIS is
equipped with fine -slot Rule -compliant cylindrical wedgewire screens which have a maximum
design through -screen velocity (TSV) of less than 0.5 feet per second (fps). Duke Energy is
required to submit each of the 5122.21 (r)(2)-(8) submittal requirements under the Rule as the
MWIS has a design flow of more than 2 million gallons per day (MGD). However, it also has an
Page 4of12

NC0001422
average flow of less than 125 MGD, which makes it ineligible for a compliance with an entrainment
requirements of the Rule.
Based on the existing design and operational data at Sutton, Duke Energy requested
concurrence that Sutton is compliant for impingement mortality (IM) reduction under Best
Technology available (BTA) Option 1 (CCRS Regulatory Determination). In addition, the
MWIS complies with IM BTA Option 2 and Option 3 with a design TSV and an actual TSV of
less than 0.5 fps.
IM BTA Option 1 CCRS Regulatory Determination
At §125.92(c), the Rule defines a CCRS as a system designed and properly operated using
minimized make-up and blowdown flows withdrawn from a Waters of the U.S. (WOTUS) to
support contact or non -contact cooling uses within a facility, and that passes cooling water
through the condenser and other cooling components of the cooling system and reuses the water
for cooling multiple times (USEPA 2014). A CCRS can also include a system designed to include
certain impoundments, with additional information requirements where the system includes
impoundments of WOTUS.
The Rule provides two criteria that impounded WOTUS must meet to be consistent with the
definition of a CCRS provided at S125.92(c):
• Criterion #1: The impoundment was constructed prior to October 14, 2014; and
• Criterion #2: The impoundment was "created for the purpose of serving as part of the
cooling water system" as documented in the CWA Section 404 permit or otherwise
demonstrated to the satisfaction of the National Pollutant Discharge Elimination System
(NPDES) Director (Director) (i.e., NCDEQ).
The construction of a cooling lake at Sutton (Sutton Lake) was authorized by the North Carolina
General Assembly approving the easement for development of the lake on May 28, 1971 and a
Permit for Impounding and Maintenance of Impounded Water issued by the North Carolina State
Board of Health on February 15, 1972. As such, Sutton Lake was created for the purpose of serving
as part of the cooling water system for Sutton. A summary of historical documents describing the
design and construction of Sutton Lake, acknowledging that it was permitted with the full
understanding by the permitting authorities that its sole purpose was to serve as an impoundment to
dissipate heat from Sutton. This summary includes the permit issued by the North Carolina State
Board of Health in February of 1972 for construction of a "cooling lake".
Sutton withdraws cooling water through a structure located on the southwest side of Sutton Lake.
Heated effluent is discharged to the southeast side of the lake via a 0.9-mile long effluent channel
(approximately 10 feet deep) and flows in a counter -clockwise direction around the lake back to the
intake structure (CP&T 1982). The main dike separates the effluent channel from the intake side of
the lake and there are six wing dikes that further separate the lake into smaller sections that create a
circuitous flow path. This system increases the circulation distance to maximize cooling of the water
before it is reused by the station. Therefore, consistent with the purpose of its creation as a CCRS,
Sutton Lake reduces condenser cooling water (CCW) withdrawals relative to an open -cycle system
that does not reuse cooling water. The average monthly MWIS flow during the 5-year period of
record evaluated (i.e., 2016-2020) was 11 MGD compared to the average monthly condenser
cooling water flow of 212 MGD representing an approximate 95 percent reduction in cooling water
flow.
Page 5 of 12

NC0001422
IM BTA Options 2 and 3 Design and Actual Maximum Intake TSV <0.5 fps
To maintain Sutton Lake water levels, make-up water is withdrawn through an MWIS on the Cape
Fear River and routed to Sutton Lake. The MWIS utilizes four 2 0-millimeter (mm) fine -slot
cylindrical wedgewire screens with a design and actual TSV of less than 0.5 fps, which meets IM
BTA Options 2 and 3 criteria and also minimizes entrainment given the orientation of the fine -slot
size (parallel to flow) and subsequent sweeping action across the screen face provided by the Cape
Fear River flows. The MWIS design pumping capacity is 69.1 MGD; however, the actual average
make-up water withdrawal over the 5-year period of record was 11.2 MGD, which is only 16
percent of the design pumping capacity.
The Division agrees with the conclusion of the Duke report that Sutton is compliant for
impingement mortality (IM) reduction under Best Technology available (BTA) Option 1
(CCRS Regulatory Determination). In addition, the MWIS complies with IM BTA Option 2
and Option 3 with a design TSV and an actual TSV of less than 0.5 fps. The concurrence is
based on the evaluation of the Environmental Sciences Section and the NPDES Industrial
Permitting Unit.
TOXICITY TESTING-OuTFALL 001 AND Ou1PALL 008
Current Requirement: Outfall 001 — Chronic P/F @ 22% using Ceriodaphnia dubia
Current Requirement: Outfall 008 — Acute P/F @ 90% using Pimepbales promelas
Recommended Requirement: Outfall 001 — Chronic P/F @ 22% using Ceriodaphnia dubia
Recommended Requirement: Outfall 008 — Acute P/F @ 90% using Pimepbales promelas
This facility has passed all toxicity tests during the previous permit cycle, please see attached.
COMPLIANCE SUMMARY
During the last 4 years, the facility has exceeded limit 4 times, please see attached. All the limit violations were
for exceeding Chloride limits in 2018 (Outfall 001 and Outfall 004). These violations can be attributed to the
coal ash excavation activities. There were no limit violations in 2019, 2020, 2021, and 2022 (January through
September).
PERMIT LIMITS DEVELOPMENT
• The temperature limits (Outfall 001 and Outfall 008) are based on the North Carolina water
quality standards (15A NCAC 2B .0200).
• The limits for Oil and Grease and Total Suspended Solids (Outfall 001, Outfall 008, Outfall
010, and Outfall 011) are based on the requirements in 40 CFR 423.
• The pH limits (Outfall 001, Outfall 008, Outfall 010, and Outfall 011) are based on the North
Carolina water quality standards (15A NCAC 2B .0200).
• The Whole Effluent Toxicity limit (Outfall 001 and Outfall 008) is based on the
requirements of 15A NCAC 2B .0500.
• The Water Quality Based Effluent Limits for Total Arsenic (Outfall 008) are based on the
results of the Reasonable Potential Analysis.
• The turbidity limit (Outfall 001) is based on North Carolina water quality standards (15A
NCAC 2B .0200).
Page 6ofl2

NC0001422
PROPOSED CHANGES
• Limits for Total Arsenic, Total Selenium, and Total Nickel have been removed from the
permit based on the results of the Reasonable Potential Analysis (Outfall 001).
• The monitoring frequency for the Total Mercury, Total Arsenic, Total Lead, Total Selenium,
Total Cadmium, Total Aluminum, Total Copper, Total Nickel, Total Zinc, Turbidity, and
Hardness was reduced from Weekly to Monthly based on the results of the Reasonable
Potential Analysis (Outfall 001).
• Monitoring for Total Iron has been removed from the permit since the water quality standard
for Iron has been eliminated in 2015.
• Limits for Total Arsenic and Total Nickel have been removed from the permit based on the
results of the Reasonable Potential Analysis (Outfall 008).
• The Section A. (19.) entitled Ash Settling Basin was removed from the permit since wastewater
is no longer discharged to the Ash Basin.
• The Special Condition A. (13.) entitled Clean Water Act Section 316(b) was updated based on
the approval of the 316(b) report.
• Fish tissue monitoring frequency has been reduced to twice per permit cycle (5 years) from
annually based on the review of the fish tissue data.
PROPOSED SCHEDULE
Draft Permit to Public Notice:
Permit Scheduled to Issue:
December 20, 2022 (est.)
February 20, 2023 (est.)
STATE CONTACT
If you have any questions on any of the above information or on the attached permit, please contact
Sergei Chernikov at [phone] or [e-mail].
CHANGES IN THE FINAL PERMIT
• The Total Arsenic limits have been removed from the permit to correct an error (Outfall
008).
Page 7 of 12

NC0001422
NPDES Implementation of Instream Dissolved Metals Standards — Freshwater Standards
The NC 2007-2015 Water Quality Standard (WQS) Triennial Review was approved by the NC
Environmental Management Commission (EMC) on November 13, 2014. The US EPA
subsequently approved the WQS revisions on April 6, 2016, with some exceptions. Therefore, metal
limits in draft permits out to public notice after April 6, 2016 must be calculated to protect the new
standards - as approved.
Table 1. NC Dissolved Metals Water Quality Standards/Aquatic Life Protection
Parameter
Acute FW, µg/1
(Dissolved)
Chronic FW,
µg/1
(Dissolved)
Acute SW, µg/1
(Dissolved)
Chronic SW,
µg/1
(Dissolved)
Arsenic
340
150
69
36
Beryllium
65
6.5
Cadmium
Calculation
Calculation
40
8.8
Chromium III
Calculation
Calculation
Chromium VI
16
11
1100
50
Copper
Calculation
Calculation
4.8
3.1
Lead
Calculation
Calculation
210
8.1
Nickel
Calculation
Calculation
74
8.2
Silver
Calculation
0.06
1.9
0.1
Zinc
Calculation
Calculation
90
81
Table 1 Notes:
1. FW= Freshwater, SW= Saltwater
2. Calculation = Hardness dependent standard
3. Only the aquatic life standards listed above are expressed in dissolved form. Aquatic life
standards for Mercury and selenium are still expressed as Total Recoverable Metals due to
bioaccumulative concerns (as are all human health standards for all metals). It is still
necessary to evaluate total recoverable aquatic life and human health standards listed in 15A
NCAC 2B.0200 (e.g., arsenic at 10 µg/1 for human health protection; cyanide at 5 µg/L and
fluoride at 1.8 mg/L for aquatic life protection).
Table 2. Dissolved Freshwater Standards for Hardness -Dependent Metals
The Water Effects Ratio (WER) is equal to one unless determined otherwise under
15A NCAC 02B .0211 Subparagraph (11) (d)
Metal
NC Dissolved Standard, µg/1
Cadmium, Acute
WER* {1.136672-[In hardness](0.041838)} • e^{0.9151 [In hardness] -
3.1485}
Cadmium, Acute
waters
Trout
WER* {1.136672- [ln hardness](0.041838)} • e^{0.9151[ln hardness] -
3.62361
Page 8 of 12

NC0001422
Cadmium, Chronic
WER* {1.101672-[In hardness](0.041838)} • e^{0.7998[ln hardness] -
4.4451 }
Chromium III, Acute
WER*0.316 • e^{0.8190[ln hardness]+3.7256}
Chromium III, Chronic
WER*0.860 • e^{0.8190[ln hardness]+0.6848}
Copper, Acute
WER*0.960 • e^{0.9422[ln hardness]-1.700}
Copper, Chronic
WER*0.960 • e^{0.8545[ln hardness]-1.702}
Lead, Acute
WER* {1.46203- [ln hardness] (0.145712)1 •
1.460}
e^ { 1.273 [ln hardness] -
Lead, Chronic
WER* {1.46203- [In hardness] (0.145712) } •
4.705 }
e^ {1.273 [In hardness] -
Nickel, Acute
WER*0.998 • e^{0.8460[ln hardness]+2.255}
Nickel, Chronic
WER*0.997 • e^{0.8460[ln hardness]+0.0584}
Silver, Acute
WER*0.85 • e^{1.72[ln hardness]-6.59}
Silver, Chronic
Not applicable
Zinc, Acute
WER*0.978 • e^{0.8473[ln hardness]+0.884}
Zinc, Chronic
WER*0.986 • e^{0.8473[ln hardness]+0.884}
General Information on the Reasonable Potential Analysis (RPA)
The RPA process itself did not change as the result of the new metals standards. However,
application of the dissolved and hardness -dependent standards requires additional consideration in
order to establish the numeric standard for each metal of concern of each individual discharge.
The hardness -based standards require some knowledge of the effluent and instream (upstream)
hardness and so must be calculated case -by -case for each discharge.
Metals limits must be expressed as `total recoverable' metals in accordance with 40 CFR 122.45(c).
The discharge -specific standards must be converted to the equivalent total values for use in the RPA
calculations. We will generally rely on default translator values developed for each metal (more on
that below), but it is also possible to consider case -specific translators developed in accordance with
established methodology.
RPA Permitting Guidance/WQBELs for Hardness -Dependent Metals - Freshwater
The RPA is designed to predict the maximum likely effluent concentrations for each metal of
concern, based on recent effluent data, and calculate the allowable effluent concentrations, based on
applicable standards and the critical low -flow values for the receiving stream.
If the maximum predicted value is greater than the maximum allowed value (chronic or acute), the
discharge has reasonable potential to exceed the standard, which warrants a permit limit in most
cases. If monitoring for a particular pollutant indicates that the pollutant is not present (i.e.
consistently below detection level), then the Division may remove the monitoring requirement in the
reissued permit.
Page 9of12

NC0001422
1. To perform a RPA on the Freshwater hardness -dependent metals the Permit Writer
compiles the following information:
• Critical low flow of the receiving stream, 7Q10 (the spreadsheet automatically
calculates the 1Q10 using the formula 1Q10 = 0.843 (s7Q10, cfs) 0.993
• Effluent hardness and upstream hardness, site -specific data is preferred
• Permitted flow
• Receiving stream classification
2. In order to establish the numeric standard for each hardness -dependent metal of concern
and for each individual discharge, the Permit Writer must first determine what effluent and
instream (upstream) hardness values to use in the equations.
The permit writer reviews DMR's, Effluent Pollutant Scans, and Toxicity Test results for any
hardness data and contacts the Permittee to see if any additional data is available for
instream hardness values, upstream of the discharge.
If no hardness data is available, the permit writer may choose to do an initial evaluation
using a default hardness of 25 mg/L (CaCO3 or (Ca + Mg)). Minimum and maximum limits
on the hardness value used for water quality calculations are 25 mg/L and 400 mg/L,
respectively.
If the use of a default hardness value results in a hardness -dependent metal showing
reasonable potential, the permit writer contacts the Permittee and requests 5 site -specific
effluent and upstream hardness samples over a period of one week. The RPA is rerun using
the new data.
The overall hardness value used in the water quality calculations is calculated as follows:
Combined Hardness (chronic)
_ (Permitted Flow, cfs *Avg. Effluent Hardness, mg/L) + (s7Q10, cfs *Avg. Upstream Hardness,
m L
(Permitted Flow, cfs + s7Q10, cfs)
The Combined Hardness for acute is the same but the calculation uses the 1Q10 flow.
3. The permit writer converts the numeric standard for each metal of concern to a total
recoverable metal, using the EPA Default Partition Coefficients (DPCs) or site -specific
translators, if any have been developed using federally approved methodology.
Page 10 of 12

4. The
NC0001422
EPA default partition coefficients or the "Fraction Dissolved" converts the
value for dissolved metal at laboratory conditions to total recoverable metal
at in -stream ambient conditions. This factor is calculated using the linear
partition coefficients found in The Metals Translator: Guidance for
Calculating a Total Recoverable Permit Limit from a Dissolved Criterion
(EPA 823-B-96-007, June 1996) and the equation:
Cdiss =
Ctotal
1
1 + { [Kpo] [SS'''] [10-6] }
Where:
ss = in -stream suspended solids concentration [mg/1], minimum of 10 mg/L
used, and
Kpo and a = constants that express the equilibrium relationship between
dissolved and adsorbed forms of metals. A list of constants used for each
hardness -dependent metal can also be found in the RPA program under a
numeric standard for each metal of concern is divided by the default partition coefficient (or
site -specific translator) to obtain a Total Recoverable Metal at ambient conditions.
In some cases, where an EPA default partition coefficient translator does not exist (ie.
silver), the dissolved numeric standard for each metal of concern is divided by the EPA
conversion factor to obtain a Total Recoverable Metal at ambient conditions. This method
presumes that the metal is dissolved to the same extent as it was during EPA's criteria
development for metals. For more information on conversion factors see the June, 1996
EPA Translator Guidance Document.
5. The RPA spreadsheet uses a mass balance equation to determine the total allowable
concentration (permit limits) for each pollutant using the following equation:
Ca = (s7Q10 + Qw) (Cwqs) — (s7Q10) (Cb)
Qw
Where: Ca = allowable effluent concentration (µg/L or mg/L)
Cwqs = NC Water Quality Standard or federal criteria (µg/L or mg/L)
Cb = background concentration: assume zero for all toxicants except NH3* (µg/L or
mg/L)
Qw = permitted effluent flow (cfs, match s7Q10)
s7Q10 = summer low flow used to protect aquatic life from chronic toxicity and human
health through the consumption of water, fish, and shellfish from noncarcinogens (cfs)
* Discussions are on -going with EPA on how best to address background
concentrations
Flows other than s7Q10 may be incorporated as applicable:
1Q10 = used in the equation to protect aquatic life from acute toxicity
QA = used in the equation to protect human health through the consumption of
water, fish, and shellfish from carcinogens
30Q2 = used in the equation to protect aesthetic quality
Page 11 of 12

NC0001422
6. The permit writer enters the most recent 2-3 years of effluent data for each pollutant of
concern. Data entered must have been taken within four and one-half years prior to the date
of the permit application (40 CFR 122.21). The RPA spreadsheet estimates the 95th
percentile upper concentration of each pollutant. The Predicted Max concentrations are
compared to the Total allowable concentrations to determine if a permit limit is necessary. If
the predicted max exceeds the acute or chronic Total allowable concentrations, the discharge
is considered to show reasonable potential to violate the water quality standard, and a permit
limit (Total allowable concentration) is included in the permit in accordance with the U.S.
EPA Technical Support Document for Water Quality -Based Toxics Control published in
1991.
7. When appropriate, permit writers develop facility specific compliance schedules in
accordance with the EPA Headquarters Memo dated May 10, 2007 from James Hanlon to
Alexis Strauss on 40 CFR 122.47 Compliance Schedule Requirements.
8. The Total Chromium NC WQS was removed and replaced with trivalent chromium and
hexavalent chromium Water Quality Standards. As a cost savings measure, total chromium
data results may be used as a conservative surrogate in cases where there are no analytical
results based on chromium III or VI. In these cases, the projected maximum concentration
(95th %) for total chromium will be compared against water quality standards for chromium
III and chromium VI.
9. Effluent hardness sampling and instream hardness sampling, upstream of the discharge, are
inserted into all permits with facilities monitoring for hardness -dependent metals to ensure
the accuracy of the permit limits and to build a more robust hardness dataset.
10. Hardness and flow values used in the Reasonable Potential Analysis for this permit included:
Parameter
Value
Comments (Data Source)
Average Effluent Hardness (mg/L)
[Total as, CaCO3 or (Ca+Mg)]
25.0
Default value
Average Upstream Hardness
(mg/L)
[Total as, CaCO3 or (Ca+Mg)]
25.0
Default value
7Q10 summer (cfs)
0
Lake or Tidal
1Q10 (cfs)
0
Lake or Tidal
Permitted Flow (MGD)
2.1
For dewatering
Page 12 of 12