NC0024406_24406 Fact Sheet Major Mod 2020_20210311

DEQ/DWR
FACT SHEET FOR NPDES PERMIT DEVELOPMENT
NPDES No. NC0024406
Facility Information
Applicant/Facility
Name:
Duke Energy Carolinas, LLC- Belews Creek Steam Station
Applicant Address:
3195 Pine Hall Road; Belews Creek, NC 27009
Facility Address:
3195 Pine Hall Road; Belews Creek, NC 27009
Permitted Flow
N/A
Type of Waste:
Industrial (- 100%); Domestic (< 1 %)
Prim.SIC Code: 4911
Facility/Permit
Status:
Class I /Active; Major Modification
County:
Stokes County
Miscellaneous
Receiving Stream:
Belews Lake (001,
005, 007), UT to
Dan River (003,
003A) , Dan River
(006)
Regional Office:
Winston-Salem
(WSRO)
Stream
Classification:
C (Belews Lake),
WS-IV (Dan River
and UT to Dan
River)
State Grid / USGS
Quad:
B18SE/Belews
Lake
303(d) Listed?
No
Permit Writer:
Sergei Chernikov,
Ph.D.
Subbasin:
03-02-01
Date:
12/ 11 /2020
Drainage Area (mi2):
501 (Dan River)
•
001: Lat. 36° 16' 49.5" N Long. 80° 03' 39.8" W
003: Lat. 36° 18' 22.0" N Long. 80° 04' 50.7" W
006: Lat. 36° 18' 34.8" N Long. 80° 04' 36.1" W
Summer 7Q10 (cfs)
80 (Dan River)
Winter 7Q10 (cfs):
160 (Dan River)
30Q2 (cfs)
195 (Dan River)
Avera e Flow cfs :
g ( )
576 Dan River
( )
IWC (%):
26.5
BACKGROUND
Duke Energy's Belews Creek Steam Station is a coal fired steam electric plant in Stokes
County. The facility is subject to the federal effluent guidelines 40 CFR Part 423.
In addition to NPDES Permit NC0024406, the facility also holds the following permits:
01983R12 (air permit), NCD000856591 (Hazardous wastes), and 85-03 (industrial
landfill) .
The facility applied for a Major Modification to the existing permit to incorporate recent
changes made by EPA to 40 CFR 423. The changes to the permit are listed on page 7
and page 8 of the Fact Sheet. The rest of the permit remains unchanged.
The facility operates the following outfalls:
Outfall 001: once through cooling water consisting of intake screen
backwash, recirculating cooling water, station equipment cooling water and
once -through cooling water, this Outfall discharges to Belews Lake.
Page 1 of 13

Outfall 003: ash basin discharge consisting of waste streams from the power
house and yard holding sumps, ash sluice lines, chemical holding pond,
coal yard sumps, stormwater, coal pile collection basins (collecting contact
stormwater from coal piles), remediated groundwater, emergency release of
anhydrous ammonia, seepage from coal ash basin, emergency overflow from
the retention basin, emergency overflows from the existing effluent channels,
and treated FGD wastewater from internal outfall 002. The wastewater from
this outfall discharges to Unnamed Tributary (UT) to Dan River.
• Internal outfall 002: FGD wastewater (discharging to ash pond)
• Outfall 003A/006. Upon completion of construction, discharge from the new
lined retention basin. Basin will accept wastes from holding basin, ash
contact water, various sumps, coal pile runoff, stormwater runoff, cooling
tower blowdown, FGD wastewater, and various low volume wastes such as
boiler blowdown, oily waste treatment, wastes/backwash from the water
treatment processes, coal pile collection basins (collecting contact
stormwater from coal piles), plant area wash down water, cooling tower
blowdown, equipment heat exchanger water, remediated groundwater,
emergency overflow (rain in excess of designed storm event), toe drain
(potential discharge to outfall 006 only), emergency release of anhydrous
ammonia, release of ammonia during quarterly testing, and treated domestic
wastewater. Outfall 003A discharges to UT to Dan River via the Outfall 003.
Upon completion of construction all waste streams previously discharged to
ash basin, will be re-routed to the new retention basin. During the transition
period, wastewater from the ash pond can also be discharged from Outfall
003. New Outfall 006 will be constructed for the lined retention basin within
2 years to replace Outfall 003A. Outfall 006 will discharge to Dan River, this
Outfall will be used for dewatering of the ash basin.
• Outfall 005. This is a former stormwater outfall SW002, consists of once
through non -contact chiller water and stormwater. The wastewater from this
outfall discharges to Belews Lake.
• Internal Outfall 006A-temporary internal outfall for dewatering of the ash
basin, it discharges through Outfall 006.
• Outfall 007 (lat. - 36016'51.604"; long. 80003'52.995"). This is an emergency
spillway for South Coal Basin. This outfall discharges to Belews Lake. The
spillway is designed for a flood greater than 100-year event. Sampling of this
spillway is waived due to unsafe conditions associated with sampling during
overflow event.
• Internal outfall 009. Domestic wastewater plant. The wastewater from this
outfall discharges to UT to Dan River via Outfall 003 or to Dan River via
Outfall 006.
• Toe Drain Outfall 111 (lat. - 36017'54.94"; long. - 80.04'32.57")- potentially
contaminated toe drain. This outfall discharges to UT to Dan River.
TOE DRAIN - OUTFALL 111
The facility identified 1 unpermitted toe drain from the ash settling basin.
The location of the toe drain is identified below and are depicted on the map attached
to the permit.
Table 1.
Discharge Coordinates and Assigned Outfall Numbers
Discharge ID
Latitude
Longitude
Outfall number
S-11
36017'54.94"
80.04'32.57"
111
Page 2 of 13

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.
FGD TREATMENT
Currently the facility uses the following treatment train for FGD wastewater
1) addition of lime, ferric chloride, and polymer in the multiple reaction tanks
2) 2 clarifiers
3) 3 filters
4) 8 first stage bioreactors
5) 8 second -stage bioreactors
However, this treatment is insufficient to consistently meet the new FGD limits for
selenium. For example, on 01 / 29 / 2015 and on 02 / 25 / 2015 the selenium
concentration in the wastewater discharged from Outfall 002 was 32.2 µg/L and 37.9
µg/L, respectively. This is substantially higher than the 23 µg/L, which is a selenium
limit in a newly promulgated effluent guideline. Therefore, Duke is proposing to install
an additional membrane ultrafiltration treatment. Installation and optimization of this
system would require time, it has been determined that November 1, 2020 is an
appropriate effective date for complying with the new FGD limits.
It is important to emphasize that Duke Energy's proposed combination of treatment
technology (adding membrane ultrafiltration to the existing chemical precipitation and
biological treatment system) is more advanced treatment than the BAT technology
basis for the ELGs (chemical precipitation and biological treatment). This treatment
has the potential to remove significant amounts of the arsenic, mercury and selenium
that remains following BAT -level treatment. Such would not be the case if a facility
were to propose adding less effective technology (such as sand filtration, which is
already part of the BAT technology basis) to the BAT technology. Furthermore, this
facility will be the first full-scale implementation of membrane ultrafiltration for
treating FGD wastewater.
REASONABLE POTENTIAL ANALYSIS(RPA)-AsH POND AND TOE DRAIN
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 RPA
spreadsheets are attached to this Fact Sheet.
a) RPA for the Lined Retention Basin (Outfall 006).
The RPA was conducted for decanting of Ash Pond, the calculations included: As,
Cd, Chlorides, Cr, Cu, F, Pb, Hg, Mo, Ni, Se, Zn, Ba, Sb, Sulfates, Al, and Tl
(please see attached). The design flow of 10.9 MGD was used for the analysis.
The discharge data on the EPA Form 2C, and DMR reports were used for the RPA,
the data was supplemented by the analysis of the free standing water in the ash
pond. The analysis indicates reasonable potential to violate the surface water
Page 3 of 13

quality standards or EPA criteria for the following parameters: Pb. The
appropriate limits were added to the permit.
b) RPA for ash pond decanting/normal operations was conducted based on the
assumption of the discharge to a zero -flow stream (current situation for Outfall
003 and Outfall 003A). The calculations included: As, Cd, Chlorides, Cr, Cu, F,
Pb, Hg, Mo, Ni, Se, Zn, Ba, Sb, Sulfates, Al, and Tl (please see attached). The
flow of 18.6 MGD was used for the analysis. The discharge data on the EPA Form
2C, and DMR reports were used for the RPA, the data was supplemented by the
analysis of the free standing water in the ash pond. The analysis indicates
reasonable potential to violate the surface water quality standards or EPA criteria
for the following parameters: Sulfates, Chlorides, Al, Cd, Se, Pb, and Tl. The
appropriate limits were added to the permit.
c) RPA for Dewatering of Ash pond (Outfall 006).
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 2.0 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 a margin of safety, the highest measured concentration for a
particular parameter was used. The RPA was conducted for As, Cd, Chlorides,
Cr, Cu, F, Pb, Hg, Mo, Ni, Se, Zn, Ba, Sb, Al, and Sulfates. The analysis indicates
reasonable potential to violate the surface water quality standards or EPA criteria
for the following parameters: Cu, Se, Mo, Al, and Pb. The appropriate limits were
added to the permit.
d) RPA for Toe Drain (Outfall 111).
The RPA calculations was conducted for Toe Drain, the calculations included: As,
Cd, Chlorides, Cr, Cu, F, Pb, Hg, Mo, Ni, Se, Zn, Ba, Sb, and Sulfates. The flow
volume for all seeps was measured at 0.2 MGD. However, the flow of 2.0 MGD
was used for RPA calculations to incorporate a safety factor, account for potential
new seeps that might emerge in the future or increase in flow volume at the
existing seeps. The analysis indicates reasonable potential to violate the surface
water quality standards or EPA criteria for the following parameters: As, Se, Cd,
Sulfates, Hg, and TDS. The appropriate limits were added to the permit.
e) RPA for Outfall 005.
The RPA was conducted for Outfall 005, the calculations included: As, Cd, Cr,
Cu, Pb, Hg, Mo, Ni, Se, Zn, Ba, Sb, and Tl (please see attached). The flow of 2.59
MGD was used for the analysis. The discharge data on the EPA Form 2C were
used for the RPA. The analysis indicates no reasonable potential to violate the
surface water quality standards or EPA criteria.
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 003 (ASH POND)
The State of North Carolina has a state-wide mercury impairment. A 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.
Page 4 of 13

Year
2011
2012
2013
2014
2015
2016
Annual
average
concentration
(ng/L)
1.83
2.15
1.47
1.57
2.97
2.6
Maximum
sampling
result (ng/L)
2.98
2.69
1.59
1.76
5.0
2.94
Number of
samples
2
4
4
6
9
2
The allowable mercury concentration for this facility is 45.3 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.
TEMPERATURE VARIANCE - OUTFALL 001
State of North Carolina (NC Board of Water and Air Resources) granted the facility a
temperature variance in 1970, which was prior to the 316(a) requirement of the CWA.
However, based on the biological study submitted in 2016, the Water Sciences Section
of the DWR concluded that the information provided in the latest report is insufficient
to determine existence of the Balanced and Indigenous population of fish and
macroinvertebrates in the receiving stream.
The facility will be provided a compliance schedule to develop and conduct a
comprehensive study of the Belews Lake and obtain a 316(a) Variance in accordance
with the 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 316(b)
The permittee shall comply with the Cooling Water Intake Structure Rule per 40 CFR
125.95. The Division approved the facility request for an alternative schedule in
accordance with 40 CFR 125.95(a)(2). The permittee shall submit all the materials
required by the Rule 3.5 years from the effective date of the permit. This is an
accelerated schedule that includes: 6 months for the development of the sampling
plan and obtaining approval from the Division, 2 years of sampling to adequately
characterize seasonal variation, and 1 year for the report development. Based on the
results of the studies, the Division may require an implementation of the additional
measures to reduce impingement and entrainment of the aquatic organisms.
Based on 40 CFR 125.92 (c) the Director has determined that operating and
maintaining the existing closed -cycle recirculating system meets the requirements for
a provisional BTA. The final determination will be made upon review of the materials
submitted by the permittee. This determination is consistent with the Region IV
decision regarding Robinson station and the Region III decision regarding North Anna
station.
Belews lake has a supplemental pumping station that provides water from the Dan
River during the drought years to maintain an operational level of water. During the
life of the station the pumping has been carried out 4 times, in 2002, 2008, 2009, and
2010. The Dan River intake minimizes impingement and entrainment by having an
approach velocity of < 0.5 feet/second at the inlet, it also equipped with a 2 mm fine
mesh travelling screen with a fish return system.
Page 5 of 13

INSTREAM MONITORING
In 2014, the facility provided instream sampling data for Oil & Grease, COD,
Chlorides, Fluoride, Sulfate, Mercury, Aluminum, Barium, Boron, Calcium, Hardness,
Iron, Magnesium, Manganese, Zinc, Antimony, Arsenic, Cadmium, Chromium,
Copper, Lead, Molybdenum, Nickel, Selenium, Thallium, TDS, TSS, pH, Temperature,
and Specific Conductance. The upstream monitoring station was located
approximately 6,000 ft. upstream of Outfall 003 and the downstream monitoring
station was located approximately 21,000 ft. downstream of the Outfall 003.
The following parameters were below detection level at both monitoring stations: Oil &
Grease, COD, Fluoride, Mercury, Zinc, Antimony, Arsenic, Cadmium, Chromium,
Copper, Lead, Molybdenum, Nickel, Selenium, and Thallium. All parameters were well
below water quality standards/EPA criteria.
The proposed permit will require a monthly monitoring for total arsenic, total
selenium, total mercury, total chromium, dissolved lead, dissolved cadmium, dissolved
copper, dissolved zinc, total bromide, total hardness (as CaCO3), temperature,
turbidity, and total dissolved solids (TDS).
FISH TISSUE MONITORING -NEAR OUTFALL 003/003A/006
The permit required fish tissue monitoring for As, Se, and Hg near the ash pond
discharge once every 5 years. This frequency is consistent with EPA guidance. Golden
Redhorse and Redbreast Sunfish tissues were analyzed for these trace elements. The
data was collected from one locations upstream of the discharge and two locations
downstream of the discharge. The results were below NC human consumption
advisory 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). Only one Golden Redhorse from one downstream location
had a mercury concentration of 0.40 µg/g, which is equal to NC human consumption
advisory level for Hg.
TOXICITY TESTING
Type of Toxicity Test:
Existing Limit:
Recommended Limit:
Recommended Limit:
dubia)
Recommended Limit:
Chronic P/F
003: Chronic P/F @ 19% (Ceriodaphnia dubia)
003/003A: Chronic P/F @ 90% (Ceriodaphnia dubia)
006A dewatering: Chronic P/F @ 3.7% (Ceriodaphnia
006: Chronic P/F @ 17.4% (Ceriodaphnia dubia)
This facility has passed all toxicity tests (22 out of 22) during the previous permit cycle,
please see attached.
The Division will increase the Instream Waste Concentration from 19% to 90% for
Outfalls 003/003A due to the reconsideration of the effluent channel status. For the
purposes of the permitting, the highest monthly average flow reported during the last 3
years in conjunction with the 7Q10 summer flow was used to calculate the percent
effluent concentration to be used for WET.
COMPLIANCE SUMMARY
During the last 5 years, the facility had 1 violations of the Copper limit (Outfall 003),
please see attached.
PERMIT LIMITS DEVELOPMENT
• The temperature limit in the permit (Outfall 001) is based on the North Carolina
water quality standards (15A NCAC 2B .0200).
Page 6 of 13

• The limits for Oil and Grease and Total Suspended Solids (Outfall 003, Outfall
003A, Outfall 006, Outfall 006A, and Outfall 111) were established in
accordance with the 40 CFR 423.
• The limits for BOD and Total Suspended Solids (Internal Outfall 009) were
established in accordance with the 40 CFR 133.102.
• The limit for Fecal Coliform (Internal Outfall 009) is based on the North
Carolina water quality standards (15A NCAC 2B .0200).
• The pH limits (Outfall 003, Outfall 003A, Outfall 006, Outfall 006A, and Outfall
111) in the permit are based on the North Carolina water quality standards (15A
NCAC 2B .0200).
• The turbidity limit in the permit (Outfall 003 and Outfall 006A-dewatering) is
based on the North Carolina water quality standards (15A NCAC 2B .0200).
• The Whole Effluent Toxicity limit (Outfall 003, Outfall 003A, Outfall 006A, and
Outfall 006) is based on the requirements of 15A NCAC 2B .0500.
• The Technology Based Effluent Limits for Total Arsenic, Total Mercury, Total
Selenium, Nitrate/nitrite as N, Oil and Grease and Total Suspended Solids
(Internal Outfall 002) are based on the requirements of 40 CFR 423.
• The Technology Based Effluent Limits for Total Iron and Total Copper (Outfall
003, Outfall 003A, and Outfall 006) are based on the requirements of 40 CFR
423.
• The Water Quality Based Effluent Limits for Chlorides, Sulfates, Total
Aluminum, Total Arsenic, Total Cadmium, Total Selenium, Total Lead, and
Total Thallium in the permit (Outfall 003 - normal operations/decanting) are
based on the North Carolina water quality standards (15A NCAC 2B .0200) and
EPA water quality criteria.
• The Water Quality Based Effluent Limits for Total Copper, Total Molybdenum,
Total Selenium, Total Aluminum, Total Lead, in the permit (Outfall 006A) are
based on the North Carolina water quality standards (15A NCAC 2B .0200) and
EPA water quality criteria.
• The Water Quality Based Effluent Limits for Total Selenium, Chlorides,
Sulfates, Total Cadmium, Total Arsenic, Total Lead, Total Aluminum, and Total
Thallium in the permit (Outfall 003A) are based on the North Carolina water
quality standards (15A NCAC 2B .0200) and EPA water quality criteria.
• The Water Quality Based Effluent Limits for Total Lead in the permit (Outfall
006) are based on the North Carolina water quality standards (15A NCAC 2B
.0200) and EPA water quality criteria.
• The Water Quality Based Effluent Limits for Total Mercury, Total Arsenic, Total
Cadmium, Total Selenium, TDS, and Chlorides (Outfall 111) are based on the
North Carolina water quality standards (15A NCAC 2B .0200) and EPA water
quality criteria.
• Ammonia limits in the permit (Outfall 003, Outfall 003A, and Outfall 006) are
based on the ammonia criteria (monthly average limit). The Division uses
ammonia criteria that were developed by EPA: 1 mg/L - summer; 1.8 mg/L -
winter.
PROPOSED CHANGES
• The Technology Based Effluent Limits for Total Arsenic, Total Mercury, Total
Selenium, and Nitrate/nitrite as N have been updated to reflect the latest EPA
changes to the Effluent Guidelines per 40 CFR 423 (Internal Outfall 002).
• The TSS limits at Internal Outfall 009 have been increased from 30 mg/L to 90
mg/L (Monthly Average) and from 45.0 mg/L to 135.0 mg/L (daily maximum) to
reflect the fact the facility is using a lagoon for treating domestic waste and
these limits meet the requirements of 15A NCAC 2B .406. In addition,
Page 7 of 13

monitoring for ammonia nitrogen, pH, and temperature have been add to
Internal Outfall 009 per 15A NCAC 2B .406.
• Leachate flow has been added to the Lined Retention Basin (Outfall 006) to
accommodate treatment of future leachate flows from the CCR landfill that will
be constructed at the site.
• Monitoring for Total Iron have been removed from Outfall 003 and Outfall 006a
since all the wastewater flows have been rerouted from the Ash Basin to the
Lined Retention Basin.
• A narrative condition for discharge of pollutants in bottom ash transport water
was added to Outfall 006.
PROPOSED SCHEDULE
Draft Permit to Public Notice:
Permit Scheduled to Issue:
December 22, 2020
February 14, 2021
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 8 of 13

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,
1.1,g/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 Trout
waters
WER*{1.136672-[ln hardness](0.041838)} • e^{0.9151[ln hardness] -
3.62361
Cadmium, Chronic
WER*{1.101672-[ln hardness](0.041838)} • e^ {0.7998 [ln hardness] -
4.4451}
Chromium III, Acute
WER*0.316 • e^{0.8190[In 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}
Page 9 of 13

Copper, Chronic
WER*0.960 • e^{0.8545[In 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- [ln hardness] (0.145712) } •
4.705 }
e^ {1.273 [ln hardness] -
Nickel, Acute
WER*0.998 • e^{0.8460[In hardness]+2.255}
Nickel, Chronic
WER*0.997 • e^{0.8460[In hardness]+0.0584}
Silver, Acute
WER*0.85 • e^{1.72[1n hardness]-6.59}
Silver, Chronic
Not applicable
Zinc, Acute
WER*0.978 • e^{0.8473[In hardness]+0.884}
Zinc, Chronic
WER*0.986 • e^{0.8473[In 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.
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
Page 10 of 13

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,
mg1L,
(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 =
Ctotal
1
1 + { [Kpo] [ss(1+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
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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
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)
Page 12 of 13

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
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