NC0000396_Fact Sheet_20181108

DEQ/DWR
FACT SHEET FOR NPDES PERMIT DEVELOPMENT
NPDES No. NC0000396, Duke Energy Progress, LLC
Asheville Steam Electric Plant
Facility Information
Applicant/Facility Name:
Duke Energy Progress, Inc./ Asheville Steam Electric Plant
Applicant Address:
200 CP&L Drive, Arden, NC 28704
Facility Address:
same
Permitted Flow
Not applicable
Type of Waste:
100 % Industrial
Facility/Permit Status:
Renewal
County:
Buncombe
Miscellaneous
Receiving Stream:
French Broad River
(001); UT to Frenche
Broad River (101);
Lake Juhan 002
Regional Office:
ARO
Stream Classification:
B (French Broad River
and UT to French
Broad River)
C(Lake ulian
Quad
F8NE
303(d) Listed?:
No
Permit Writer:
Ser ei Chernikov, Ph.D.
Subbasin:
040302 (French
Broad
Date:
May 3, 2018
Drainage Area (mi2):
655 (French Broad
River discharge, 001)
Summer 7Q10 (cfs)
Winter 7Q10 (cfs)
306
409 winter
30Q2 (cfs):
631
Average Flow (cfs):
1769
IWC (%):
Est., 1.75%
(Based on a flow of
3.52 MGD
Primary SIC Code:
4911
SUMMARY
This is a renewal of the NPDES wastewater permit for Asheville Steam Electric Plant (Asheville
Plant). The Asheville Plant is a coal fired steam electric generating plant (2 Units; 2 internal combustion
turbines). This facility is subject to EPA effluent guideline limits per 40 CFR 423 - Steam Electric
Power Generating Point Source Category. The facility is located in the Mountain area of the state; the
applicable state water quality temperature standard is 29°C (84.2 F).
The site has 1 new ash pond (built in 1982), 1 old ash pond (build in 1964) and a cooling pond (Lake
Julian). The facility has excavated all ash from the 1982 pond, the ash is currently being removed from
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NPDES PERMIT FACT SHEET NPDES No. NCO000396
the 1964 pond. The state law requires all the ash to be removed by August 1, 2022. Lake Julian is a
320-acre waterbody constructed in 1963 by CP&L (original owner of the facility) to serve as a cooling
water source, it is classified as "waters of the State". Discharge from Lake Julian to the French Broad
River is extremely rare and any occurrence would be during periods of heavy rainfall. These discharges
are routed through a spillway.
The facility has installed a new Reverse Osmosis (RO) system in 2010, it will be used in conjunction
with alum coagulation/filtration to provide water for various plant processes. The reject stream
from the RO unit is sent to the ash pond.
The EPA's contractor conducted a review of the stability of the ash pond dams at the site in 2010.
The contractor determined that the embankment and spillway systems appeared to be structurally
sound for both dams (old and new ash ponds). However, the contractor recommended that some
studies be conducted on the 1964 dam, including a slope stability analysis. After conducting these
studies, Progress Energy (owner of the facility at the time) decided to make the enhancements to the
1964 dam. In order to accommodate these enhancements, the facility had to move the existing
Outfall 001 approximately 3,000 ft. downstream. Permission for this relocation was granted by
DWQ on May 13, 2011. All the enhancements to the 1964 dam were completed in 2012.
The facility is being converted to the combined cycle plant powered by natural gas, the new plant is
expected to begin power production by the end of 2019, the coal-fired units will be shut -down by
January 31, 2020.
Outfall 001.
Outfall 001 discharges directly to the French Broad River. The ash pond/rim ditch receives ash
transport water, coal pile runoff, storm water runoff, various low volume wastes (such as boiler
blowdown, backwash from the water treatment processes, ash hopper seal water, plant drains), air
preheater cleaning water and chemical metal cleaning wastewater discharged from Internal Outfall
004 (potentially). It also might contain some stormwater and groundwater seepage from an old ash
pond. Historically, this flow has been allowed to evaporate and/or infiltrate. The enhancements to
the old pond that were completed in 2011 require that this water level be managed and maintained
below a certain elevation. Therefore, it may be necessary to periodically pump water from the old
ash pond to the combined, relocated settling basin and Outfall 001.
Outfall 002.
Once through non -contact cooling water system. This waste stream is discharged directly to Lake
Julian via Outfall 002.
Internal Outfall 004.
Chemical Metal Cleaning Treatment System. This waste stream may occasionally be discharged via
Internal Outfall 004 to the ash pond treatment system or to the old ash pond (with prior DWR
approval). Generally chemical metal cleaning wastes are treated by evaporation in boilers.
Internal Outfall 005.
Flue Gas Desulfurization (FGD) wet scrubber wastewater treatment system. Currently, FGD
wastewaters are discharged to the Buncombe County Water Reclamation Facility.
Outfall 101.
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NPDES PERMIT FACT SHEET
NPDES No. NCO000396
1964 pond toe drains (combined outfall for 3 toe drains). This outfall discharges to an Unnamed
Tributary (UT) to French Broad River.
TOE DRAINS- OUTFALL 101.
The facility identified 3 unpermitted toe drains from the 1964 ash settling basin, wastewaters from
the toe drains are collected in the same structure and discharged through a single pipe. The location
of Outfall 101 is identified below and depicted on the map attached to the permit.
Table 1. Dischar e Coordinates and Assigned Outfall Numbers
Seep ID
Latitude
Longitude
Outfall number
64 EO-01, 64 EO-02,
and 64 EO-03
35.468
-82.549
Outfall 101
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.
REASONABLE POTENTIAL ANALYSIS(RPA)- OUTFALL 001 AND OUTFALL 101
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 detection levels. 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 Dewatering of Ash Pond (Outfall 001).
The RPA was conducted for dewatering of Ash Pond, the calculations included: As, Be, Cd, Chlorides,
Cr, Cu, Pb, Hg, Mo, Ni, Se, Zn, Al, Sb, and Tl (please see attached). The renewal application listed
3.52 MGD as a current flow. The discharge data on the EPA Form 2C was used for the RPA, it was
supplemented by the analysis of the free standing water in the ash pond. The analysis indicates no
reasonable potential to violate the surface water quality standards or EPA criteria.
b) RPA for Toe Drain (Outfall 101).
The RPA calculations was conducted for 3 toe drains and the worst -case scenario was used to establish
the permit limits. The calculations included: As, Cd, Chlorides, Cr, Cu, Pb, Hg, Mo, Ni, Se, Zn, Sb,
Tl, and Al. The flow volume for 64EO-01 toe drain was measured at 0.33 MGD. However, the flow
of 1.0 MGD was used for RPA calculations to incorporate a safety factor and account for a potential
increase in flow volume. The flow volume for this 64EO-02 toe drain was measured at 0.039 MGD.
However, the flow of 0.1 MGD was used for RPA. The flow volume for 64EO-03 toe drain was
measured at 0.001 MGD. However, the flow of 1.0 MGD was used for RPA
The analysis indicates that the limits for Chlorides, Ni, Se, and Zn are necessary to protect the receiving
stream. The appropriate limits were added to the permit.
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NPDES PERMIT FACT SHEET
NPDES No. NC0000396
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
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
2008
2009
2010
2011
2012
Annual average
53.3
24.1
17.6
21.1
11.4
concentration n /L
Maximum sampling
173
56.4
83.2
49.2
43.3
result (n /L)
The Water Quality -based allowable mercury effluent discharge concentration for this facility is 680.6
ng/L. All annual average mercury concentrations are below allowable. However, there are values that
exceed TBEL of 47.0 ng/L. Based on the Permitting Guidelines for Statewide Mercury TMDL, the
TBEL limit of 47.0 ng/L will be added to the permit.
CWA SECTION 316(a) TEMPERATURE VARIANCE AND FISH TISSUE MONITORING—
OuTFALL 001, OuTFALL 002
There is a current Section 316(a) thermal variance for Lake Julian pending renewal. The facility
conducts biological monitoring of Lake Julian to meet the requirements of the Section 316(a) CWA.
The Water Sciences Section (WSS) of the DWR reviewed the biological monitoring report that was
submitted to the DWQ in 2010. The WSS determined that "the fisheries community in Lake Julian
currently meets the balanced and indigenous population (BIP) definition of Section 316(a) of the Clean
Water Act". The WSS also imposed the additional monitoring requirements for the future BIP studies.
Lake Julian has been monitored since 1973. Monitoring encompasses water quality, water chemistry,
phytoplankton, zooplankton, benthic macroinvertebrates, and fish. Prior to 2001 the monitoring was
conducted annually; currently it is conducted triennially.
The 2007-2008 Monitoring Report also investigated fish tissue. Fish tissue of bass and channel
catfish collected from Lake Julian were analyzed for As, Cd, Cu, Hg, and Se. All trace element
concentrations in fish tissue were below detection level with an exception of Se. However, all Se
values were well below the NC consumption advisory threshold of 50 µg/g dry weight.
In addition to Lake Julian, the facility conducts fish tissue sampling at 3 stations in French Broad River
to determine potential impacts of the discharge. The latest fish tissue report was submitted to WSS in
2012. The WSS concluded: "2012 data demonstrates that there were no selenium issues noted at any
of the three stations on the French Broad River and all selenium levels remain well below the North
Carolina Department of Health and Human Services (NCDHHS) fish consumption advisory criteria
of 10 ppm". The WSS also concluded: "In terms of mercury, a total of 91 total fish were collected
from the three stations. From this total, seven individuals (8%) exceeded the 0.4 ppm NCDHHS
mercury advisory criteria."
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NPDES PERMIT FACT SHEET NPDES No. NC0000396
The WSS also reviewed that fish tissue data for 2014 and 2016. The WSS stated: "The review of the
2014 data demonstrates that there were no selenium issues noted at any of the three stations on the
French Broad River and all selenium levels remain well below the North Carolina DHHS fish
consumption advisory criteria of 10 mg/kg. For 2014, as was the case in previous years, there were
exceedances of 0.4 mg/kg mercury criteria measured at all three monitoring locations (upstream,
adjacent to the discharge and below the discharge). However, based on the Duke Energy data
presented in the 2014 NPDES report (and past reports), we agree that these levels are not attributable
to the discharge from the Asheville plant." The 2016 review states that "there have been no changes
in Mercury and Selenium levels since the FGD system went operational".
INSTREAM MONITORING— OUTFALL 001, OUTFALL 002
The 2007-2008 Environmental Monitoring Report for Lake Julian concludes that — "Except for
copper, all mean trace element mean concentrations surface waters remained below their respective
reporting limits during 2007. All measured copper concentrations were relatively low and less than
the North Carolina water quality standard". The same report provides data for Copper, Selenium,
and Arsenic that were measured in two locations at Lake Julian, and in the French Broad River
below the discharge from ash pond. The vast majority of samples were below detection level, the
remaining samples were well below water quality standards. The only exception is one Se
measurement in French Broad River (5.1 ug/L) that slightly exceeded the water quality standard of
5.0 µg/L.
The update to the permit renewal application submitted in 2014 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, Specific
Conductance, and Turbidity. The upstream monitoring station was located 5,500 ft. upstream of
Outfall 001 and the downstream monitoring station was located 2,900 ft. downstream of the Outfall
001.
The following parameters were below detection level at both monitoring stations: Oil & Grease,
COD, Fluoride, Mercury, Boron, Antimony, Arsenic, Cadmium, Chromium, Copper, Lead,
Molybdenum, Nickel, Selenium, and Thallium. The rest of the parameters did not indicate a
significant difference between the upstream and the downstream monitoring locations except for
Specific Conductance.
The proposed permit will require 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).
CWA SECTION 316(2)
The permittee shall comply with the Cooling Water Intake Structure Rule per 40 CFR 125.95. The
permittee shall submit all the materials required by the 316(b) Rule 180 days before the planned
commencement of cooling water withdrawals for the operation of the new unit.
The rule requires the Director to establish interim BTA requirements in the permit on a site -specific
basis based on the Director's best professional judgment in accordance with §125.90(b) and 40 CFR
401.14. However, the facility is undergoing a conversion to a combined cycle plant with the closed -
cycle cooling system. The final design of the new facility is being finalized and will be provided to
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NPDES PERMIT FACT SHEET
NPDES No. NC0000396
the DEQ before the end of 2018. The new facility will utilize the closed -cycle cooling system, which
is one of the pre -approved technologies to satisfy the impingement requirements. The actual intake
flow of the new plant will be less than 125 MGD. In addition, if the owner of an existing facility
plans to retire the facility before the current NPDES permit expires, per 5 122.21(r) (1) (ii) (F)) the
submittal requirements listed in 5122.21 (r) (1) (ii) (A), (B), and (C) do not apply.
TOXICITY TESTING-OUTFALL 001
Current Requirement: Outfall 001 — Chronic P/F @ 1.8% using Ceriodaphnia Dubia
Recommended Requirement: Outfall 001 — Chronic P/F @ 1.8% using Ceriodaphnia Dubia.
This facility has passed all toxicity tests during the previous permit cycle, please see attached.
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 tests for
each facility.
COMPLIANCE SUMMARY
During the last 5 years, the facility had no violations of the permit limits, please see attached.
PERMIT LIMITS DEVELOPMENT
• The temperature limits (Outfall 002) are based on the North Carolina water quality standards
(15A NCAC 2B .0200) and 316(a) Thermal Variance.
• The Total Residual Chlorine Limit and Time of Chlorine Addition Limit (Outfall 002) were
established in accordance with the 40 CFR 423.
• The limits for Oil and Grease and Total Suspended Solids (Outfall 001 and Outfall 101) were
established in accordance with the 40 CFR 423.
• The pH limits (Outfall 001, Outfall 002, and Outfall 101) in the permit are based on the North
Carolina water quality standards (15A NCAC 2B .0200).
• The turbidity limit in the permit (Outfall 001) is based on the North Carolina water quality
standards (15A NCAC 2B .0200).
• The mercury limit in the permit (Outfall 001) is based on the North Carolina water quality
standards (15A NCAC 2B .0200) and implementation strategy for Statewide Mercury
TMDL.
• The Technology Based Effluent Limits for Total Arsenic, Total Mercury, Total Selenium,
and Nitrate/nitrite as N (Outfall 005) are based on the requirements of 40 CFR 423.
• The Whole Effluent Toxicity limit (Outfall 001) is based on the requirements of 15A NCAC
2B .0500.
• The limits for Total Copper and Total Iron (Outfall 004) were established in accordance with
the 40 CFR 423.
• The limits for Chlorides, Total Nickel, Total Selenium, and Total Zinc in the permit (Outfall
101) are based on the North Carolina water quality standards (15A NCAC 2B .0200).
PROPOSED CHANGES
• The Technology Based Effluent Limit for Total Mercury was added to the permit (Outfall
001) and is based on the requirements of 40 CFR 125.3(a) , 40 CFR 122.44(a)(1); 40 CFR
125.3(c) and (d).
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NPDES PERMIT FACT SHEET
NPDES No. NC0000396
• The Technology Based Effluent Limits for Total Mercury, Total Arsenic, Total Selenium,
and Nitrate/nitrite as N were added to the permit (Outfall 005) and are based on the
requirements of 40 CFR 423.
• The turbidity limit was added to the permit (Outfall 001) to meet the state turbidity standard
per 15A NCAC 2B .0211(3) (k).
• The daily maximum TSS limit for Outfall 001 was reduced from 100 mg/L to 50 mg/L to
meet the requirements of 40 CFR 423.
• An effluent page for Outfall 001 was modified to incorporate dewatering.
• The Toe Drains Outfall (Outfall 101) was added to the permit.
• The Clean Water Act Section 316(a) Thermal Variance Special Condition was updated to
reflect the new regulations. Please see Special Condition A. (7.).
• The Clean Water Act Section 316(b) Special Condition was added to the permit. Please see
Special Condition A. (8.).
• The Structural Integrity Inspections of Ash Pond Dam Special Condition was added to the
permit. Please see Special Condition A. (9.).
• The Instream Monitoring Special Condition was added to the permit to monitor the impact
of the facility on the receiving stream. Please see Special Condition A. (10.).
• The Applicable State Law Special Condition was added to the permit to meet the requirements
of Senate Bill 729 (Coal Ash Management Act). Please see Special Condition A. (11.).
• The Fish Tissue Monitoring Near Ash Pond Discharge Special Condition was updated. Please
see Special Condition A. (13.).
• Federal regulations require electronic submittal of all discharge monitoring reports (DMRs)
and program reports. The final NPDES Electronic Reporting Rule was adopted and became
effective on December 21, 2015. The requirement to begin reporting discharge monitoring
data electronically using the NC DWR's Electronic Discharge Monitoring Report (eDMR)
internet application has been added to your final NPDES permit. [See Special Condition A.
(14.)]
For information on eDMR, registering for eDMR and obtaining an eDMR user account,
please visit the following web page: http://deq.nc.gov/about/divisions/water-
resources/edmr.
For more information on EPA's final NPDES Electronic Reporting Rule, please visit the
following web site:
https: / /www.federalregister.gov/documents /2015 /10 /22/2015-24954/national-pollutant-
discharge-elimination-system-npdes-electronic-reporting-rule
• The Additional Conditions and Definitions Special Condition was added to the permit. Please
see Special Condition A. (15.).
• Monitoring for Total Beryllium, Total Cadmium, Chlorides, Total Chromium, Fluoride,
Total Lead, Total Manganese, Total Nickel, Total Silver, and Total Zinc was removed from
the permit (Outfall 005) to be consistent with the requirements of 40 CFR 423.
• The stormwater outfalls were deleted from the permit. The facility received a separate
stormwater permit.
• The Biocide Special Condition A. (16.) was added to the permit to be consistent with the
permitting procedure for power plants.
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NPDES PERMIT FACT SHEET
NPDES No. NC0000396
• The Special Condition entitled Compliance Boundary was added to the permit. Please see
Special Condition A. (17.).
• The monitoring frequency for Total Mercury, Total Arsenic, Total Selenium, Total Beryllium,
Total Cadmium, Chlorides, Total Chromium, Total Copper, Total Fluoride, Total Lead, Total
Nickel, Total Silver, Total Thallium, Total Zinc, and Chronic Toxicity was increased to Weekly
from Quarterly (Outfall 001).
• The monitoring for Total Manganese was eliminated due to the removal of the Manganese
state standard during the latest tri-annual review (Outfall 001).
• The Selenium speciation special condition was removed from the permit.
PROPOSED SCHEDULE
Draft Permit to Public Notice: July 22 2018 (est.)
Permit Scheduled to Issue: August 31, 2018 (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
• Monitoring for bromides, hardness and TDS was added to Outfall 001 and 101.
• The TSS limits were added to Outfall 005.
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NPDES PERMIT FACT SHEET
NPDES No. NC0000396
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
Emits 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/l
(Dissolved)
Chronic FW,
µg/l
(Dissolved)
Acute SW, µg/l
(Dissolved)
Chronic SW,
µg/l
(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/l 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, µ /l
Cadmium, Acute
WER*11.136672-[ln hardness](0.041838)1 e^10.9151 [ln hardness]-
3.14851
Cadmium, Acute Trout
WER*{1.136672-[ln hardness](0.041838)1 e^10.9151[ln hardness] -
waters
3.62361
Cadmium, Chronic
WER*11.101672-[In hardness](0.041838)1 e^10.7998[ln hardness]-
4.44511
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NPDES PERMIT FACT SHEET
NPDES No. NC0000396
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]-1.7001
Copper, Chronic
WER*0.960 e^{0.8545[ln hardness]-1.7021
Lead, Acute
WER* { 1.46203- [ln hardness] (0.145712)1 • e^ { 1.273 [ln hardness] -
1.460 }
Lead, Chronic
WER* { 1.46203- [ln hardness] (0.145712)) • e^ { 1.273 [ln hardness] -
4.705 }
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[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/WOBELs 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 Division may remove the monitoring requirement in the
reissued permit.
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NPDES PERMIT FACT SHEET
NPDES No. NCO000396
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) ""
• 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 1 Q10 flow.
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.
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NPDES PERMIT FACT SHEET
NPDES No. NC0000396
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 = I
Ctotal I + { [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
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 + Qw) (Cwas) — 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* (tg/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 12 of 13

NPDES PERMIT FACT SHEET
NPDES No. NC0000396
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.
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+M )]
25.0
Default value
Average Upstream Hardness
(mg/L)
otal as, CaCO3 or Ca+M
25.0
Default value
7Q10 summer (cfs)
0
Lake or Tidal
1Q10 (cfs)
0
Lake or Tidal
Permitted Flow (MGD)
2.1
For dewaterin
Page 13 of 13