NC0001422_Renewal-Fact Sheet_20170810

DEQ/DWR
FACT SHEET FOR NPDES PERMIT DEVELOPMENT
RENEWAL
NPDES No. NC0001422
SUMMARY
This is a renewal of the NPDES wastewater permit for L.V. Sutton Energy Complex. 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.
The hermit is being re -noticed to incomorate a compliance boundary man into the permit. The
Daily Maximum limits for Arsenic (Outfall 001, Outfall 002, Outfall 004, and Outfall 008) are also
being reduced to 50.0 µg/L from 340.0 R/L in the previous Draft Permit. Duke Energy voluntarily
accepted these lower Arsenic limits despite recent change to the state Arsenic acute standard, which
would allow an increase in the Daily Maximum limits for Arsenic.
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.
Page 1 of 11
Facility Information
Applicant/FacilityName:
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:
Renewal Class
County:
New Hanover
Receiving Stream:
Miscellaneous
Cape Fear River
(001), Sutton Lake
002, 004, 008
Regional Office:
WiRO
Stream Classification:
C Sw (001)
C (002, 004, 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)
Datc:
August 10, 2017
Drainage Area mit :
Summer 7Q10 (cfs)
Tidally influenced
(Outfall 001);
Lake (Outfalls
002, 004, and 008
30Q2 cfs :
See above-
boveAvera
e Flow cfs :
Average
See above
IWC C/6):
100 all outfalls
Primary SIC Code:
SUMMARY
This is a renewal of the NPDES wastewater permit for L.V. Sutton Energy Complex. 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.
The hermit is being re -noticed to incomorate a compliance boundary man into the permit. The
Daily Maximum limits for Arsenic (Outfall 001, Outfall 002, Outfall 004, and Outfall 008) are also
being reduced to 50.0 µg/L from 340.0 R/L in the previous Draft Permit. Duke Energy voluntarily
accepted these lower Arsenic limits despite recent change to the state Arsenic acute standard, which
would allow an increase in the Daily Maximum limits for Arsenic.
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.
Page 1 of 11

4 4
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 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 mining 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.
Outfall 002 - wastewater associated with the old ash pond. May consist of low volume waste, yard
drains, oily waste treatment. Wastewater can be discharged to Sutton Lake or to Cape Fear River
through Outfall 001.
Outfall 004 - wastewater associated with the new ash pond. May consist of low volume waste, yard
drains, oily waste treatment. Wastewater can be discharged to Sutton Lake or to Cape Fear River
through Outfall 001.
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.
Internal Outfall 005 - wastewater from the Combined Cycle generation unit.
Internal Outfall 006 - wastewater from the Combined Cycle generation unit.
Internal Outfall 007 - stormwater/wastewater flows from the closure activities for coal-fired units.
Internal Outfall 009 - low volume wastes from a new simple cycle combustion turbine expected to
be online in 2017.
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 0 11 - 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 SWO01 - Runoff from the temporary laydown area and the parking lot.
Internal Outfall SWO02 - Runoff from the parking lot and Peaker Combustion Turbine area.
Internal Outfall SWO03 - Runoff from the parking lot.
Internal Outfall SWO04 - Pumped stormwater ft the 115 Electrical Switchyard area.
Internal Outfall SWO05 - Discharge from the sou wet detention basin.
Internal Outfall SWO06 - 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 SWO07 - Runoff from the potential rail loading yard, rail spur, and truck roads
installed to transport coal ash from the site.
Page 2 of 11

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.
REASONABLE POTENTIAL ANALYSIS(RPA)-OuTFars, 001, OUTFALL 002, OUTFAu 004.
OUTFA .L 008
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.
a) RPA for Decanting/Normal operation of Ash Pond (Outfall 001, 002, and 004).
The long term discharge data on the EPA Form 2C was used, it was supplemented by the
analysis of the free standing water in both ash ponds, landfill leachate analysis from similar
landfills, and groundwater sampling results. Since the highest available values for each
parameter was used, it is assumed that this RPA is applicable to all discharges that represent
coal ash contaminated water (outfalls 001, 002, 004). Calculations included: As, Be, Cd, Al,
Cr, Cu, F, Pb, Hg, Mo, Ni, Se, Ag, Zn, Ba, Sb and Tl (please see attached). The historic flow
of 12.84 MGD was used in the analysis, the groundwater pumping volume of 1.3 MGD and
landfill leachate volume of 0.1 MGD was added to the historic flow. The RPA indicated the
need for following the water -quality based limits: As, Cu, Ni, and Se.
b) RPA for Dewatering of Ash pond (Outfall 001 and Outfall 004).
To meet the requirements of the Coal Ash Management Act of 2014, the facility needs to
dewater ash ponds by removing the interstitial water. The facility's highest discharge rate
from the dewatering process will be 2.1 MGD. The facility submitted data for the standing
surface water in the ash ponds, interstitial water in the ash, and interstitial ash water that was
treated by filters of various sizes. To evaluate the impact of the dewatering on the receiving
stream the RPA was conducted for the wastewater that will be generated by the dewatering
process. To introduce the margin of safety, the highest measured concentration for a
particular parameter was used. The RPA was conducted for As, Be, Cd, Chlorides, Al, Cr,
Cu, F, Pb, Hg, Mo, Ni, Se, Ag, Zn, Ba, Sb and Tl (please see attached). The RPA indicated
the need for the following water -quality based limits: As, Chlorides, Al, Cu, F, Pb, Hg, Mo,
Ni, Se, and Zn.
c) RPA for Combined Cycle Unit (Outfall 008).
The RPA was also conducted for the Combined Cycle Unit. Calculations included: As, Be,
Cd, Al, Cr, Cu, Pb, Hg, Mo, Ni, Se, Ag, Zn, Ba, Sb and Tl (please see attached). The flow
volume of 211 MGD was used in the RPA. The RPA indicated the need for the following
water -quality based limits: As, Cu, and Se.
Page 3 of 11

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.
MERCURY EVALUATION (Outfall 001 normal operation)
The State of North Carolina has a state-wide mercury impairment. The TMDL has been developed
to address this issue in 2012. The TMDL included the implementation strategy, both documents were
approved by EPA in 2012.
The mercury evaluation was conducted in accordance with the Permitting Guidelines for Statewide
Mercury TMDL.
Year
2015
2016
Annual average
1.69
1.91
concentration n /L
Maximum sampling
3.43
9.8
result n /L
Allowable mercury concentration for this facility is 12.0 ng/L. All annual average mercury concentrations
are below the allowable level. All maximum sampling results are below the TBEL of 47.0 ng/L. Based on the
Permitting Guidelines for Statewide Mercury TMDL, the limits are not required.
INSTREAM MONITORING -OUTFALL 002
The permit required semi-annual upstream and downstream monitoring near the ash pond
discharge. These monitoring stations have been established through the Lower Cape Fear River
Program. The monitored parameters are: total arsenic, total selenium, total mercury (method
1631E), total chromium, dissolved lead, dissolved cadmium, dissolved copper, and dissolved zinc.
The results for all parameters are below detection level upstream and downstream of the Outfall
001. It is required that the monitoring of the instream stations will continue during the next permit
cycle.
CWA SECTION 316(a�
Since the Sutton Lake has been reclassified to the "waters of the State" on November 5, 2014, the
facility has to develop a strategy to meet the state temperature standard in Sutton Lake. In order to
obtain thermal variance/mixing zone for Lake Sutton/Cape Fear River the facility shall develop and
conduct comprehensive 316(a) studies. The 316(a) studies shall be performed in accordance with the
Division of Water Resources approved plan. The temperature analysis and the balanced and
indigenous study plan shall conform to the specifications outlined in 40 CFR 125 Subpart H and the
EPA's Draft 316(a) Guidance Manual, dated 1977, and the Region 4 letter to NCDENR, dated June
3, 2010.
CWA SECTION 316a
The permittee shall comply with the Cooling W
Division approved the facility request for an alt(
125.95(a)(2). The permittee shall submit all the i
application.
Current Requirement: Outfall 001, 002,
promelas
Recommended Requirement: Outfall 001, 002,
pmmelas
:r Intake Structure Rule per 40 CFR 125.95. The
tative schedule in accordance with 40 CFR
terials required by the Rule with the next renewal
008 — Acute P/F @ 90% using Pimephales
, 008 — Acute P/F @ 90% using Pimephales
This facility has passed all toxicity tests during Oe previous permit cycle, please see attached.
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For the purposes of the permitting, the long term average flow was used in conjunction with the
7Q10 summer flow to calculate the percent effluent concentrations to be used for WET.
COMPLIANCE SUMMARY
During the last 5 years, the facility has exceeded limit 3 times, please see attached. The limit violations were for
Oil and Grease (2 times - Outfall 005) and flow volume (Outfall 001), please see attached.
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 002, Outfall
004, Outfall 005, Outfall 006, Outfall 007, Outfall 008, Outfall 009, Outfall 010, and Outfall
011) are based on the requirements in 40 CFR 423.
• The pH limits (Outfall 001, Outfall 002, Outfall 004, Outfall 005, Outfall 006, Outfall 008,
Outfall 009, 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, Outfall 002, Outfall 004 and Outfall 008) is
based on the requirements of 15A NCAC 2B .0500.
• The Water Quality Based Effluent Limits for Total Arsenic, Total Selenium, Total Copper,
and Total Nickel (Outfall 001, Outfall 002, Outfall 004, and Outfall 008) are based on the
results of the Reasonable Potential Analysis.
• The Water Quality Based Effluent Limits for Total Aluminum, Total Lead, and Chlorides
(Outfall 001 — dewatering and Outfall 004 - dewatering) are based on the results of the
Reasonable Potential Analysis.
• The turbidity limit (Outfall 001 and Outfall 004 -dewatering) is based on North Carolina water
quality standards (15A NCAC 2B .0200).
• Mercury limit in the permit (Outfall 001 -dewatering) is based on the results of the
Reasonable Potential Analysis.
• The Water Quality Based Effluent Limits for Total Arsenic, Total Selenium, and Total
Copper (Outfall 008) are based on the results of the Reasonable Potential Analysis.
PROPOSED CHANGES
• A separate effluent page for the dewatering of the New Ash Pond (Outfall 004) was added to
the permit (Please see Special Condition A. (5)).
• Limits for Total Copper were added to the permit based on the results of the Reasonable
Potential Analysis (Outfall 001, Outfall 002, and Outfall 004).
• Limits for Total Nickel were added to the permit based on the results of the Reasonable
Potential Analysis (Outfall 001, Outfall 002, and Outfall 004).
• Limits for Total Iron were removed from the permit based on the updates to the North
Carolina standards (Outfall 001, Outfall 002, and Outfall 004).
• Limits for Total Cadmium were removed from the permit based on the results of the
Reasonable Potential Analysis (Outfall 001).
• Limits for Total Lead were removed from the permit based on the results of the Reasonable
Potential Analysis (Outfall 001 -normal operation).
• Limits for Chlorides were added to the permit based on the results of the Reasonable
Potential Analysis (Outfall 001 - dewatering).
• Monitoring for Hexavalent Chromium was added to the permit based on the results of the
Reasonable Potential Analysis (Outfall 001 - dewatering).
Page 5of11

• The daily maximum limit for Total Lead was increased based on the updates to the North
Carolina standards (Outfall 001 - dewatering).
• Limits for Total Mercury were removed from the permit based on the results of the Mercury
Evaluation (Outfall 001 -normal operation, Outfall 002 -normal operation, Outfall 004 -
normal operation).
• Limits for Total Arsenic, Total Copper, and Total Selenium were added to the permit based
on the results of the Reasonable Potential Analysis (Outfall 008).
• The Acute Toxicity monitoring frequency was reduced to Monthly (Outfall 002 and Outfall
004) to be consistent with other Duke permits.
• The Special Conditions Fish Tissue Monitoring near Ash Pond Discharge and Clean Water
Act Section 316(6) have been updated, please see A. (19.) and A. (21.).
• The Clean Water Act Section 316(a) Special Condition was added to the permit, please see
A. (20.).
• The Outfall 010 and 011 were added to the permit to accommodate discharges of
stormwater from the future coal ash landfill.
• The Ash Pond Closure Special Condition was removed from the permit since the facility
submitted Closure Plan in 2016.
• The Biocide Special Condition was updated to be consistent with other Duke permits, please
see A. (18.).
• The compliance schedule for Total Copper limit, Total Arsenic Limit, and Total Selenium
limit was added (Outfall 008), please see A. (29.).
• The compliance schedule for Total Copper limit, Total Lead limit, and Total Nickel limit
was added (Outfall 001).
• The compliance boundary map was added to the permit.
PROPOSED SCHEDULE
Draft Permit to Public Notice: August 14, 2017 (est.)
Permit Scheduled to Issue: September 29, 2017 (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].
Page 6of11

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 Chronic FW,
(Dissolved) µg/l
(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 213.0200 (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, /1
Cadmium, Acute
WER* { 1.136672-[ln hardness] (0.041838)1 e^{0.9151 [In hardness] -
3.1485}
Cadmium, Acute Trout
waters
WER* { 1.136672-[ln hardness] (0.041838)) e^ {0.9151 [ln hardness] -
3.6236)
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^ 10.9422[/n hardness] -1.700}
Page 7of11

Copper, Chronic
WER*0.960 • e^{0.8545[ln hardness] -1.702}
Lead, Acute
WER* { 1.46203-[ln hardness] (0.145712) } e^ { 1.273 [ln hardness] -
1.460}
Lead, Chronic
WER* { 1.46203-[ln hardness] (0.145712)) e^ { 1.273 [ln hardness] -
4.7051
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[/n hardness] -6.59}
Silver, Chronic
Not applicable
Zinc, Acute
WER*0.978 e^ (0.8473[/n hardness] +0.8841
Zinc, Chronic
WER*0.986 e^{0.8473[lnhardness] +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-Denendent 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 Divisipn may remove the monitoring requirement in the
reissued permit.
To perform a RPA on the Freshwater ha
compiles the following information:
• Critical low flow of the receiving
calculates the 1Q10 using the for
• Effluent hardness and upstream ]
• Permitted flow
• Receiving stream classification
metals the Permit Writer
n, 7Q10 (the spreadsheet automatically
1Q10 = 0.843 (s7Q10, cfs) 0.993
ess, site-specific data is preferred
Page 8 of 11

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,
mg/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.
4. The
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 = 1
Ctotal 1 + { [Kpo] [ss(l+a)] [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
Page 9 of 11

development for metals. For more information on conversion factors see the June, 1996
EPA Translator Guidance Document.
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 + QF) (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
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 duality 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 comp tred against water quality standards for chromium
III and chromium VI.
9. Effluent hardness sampling and instream 4ardness 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:
Page 10 of 11

Parameter
Value
Comments Data Source
Average Effluent Hardness (mg/L)
[Total as, CaCO3 or Ca+M
25.0
Default value
Average Upstream Hardness
(mg/L)
[Total as, CaCO3 or Ca+M
25.0
Default value
7Q10 summer cfs
0
Lake or Tidal
1 Q10 cfs
0
Lake or Tidal
Permitted Flow GD
2.1
For dewaterin
Page 11 of 11