NC0038377_Fact sheet major mod-2021_20211215

DEQ/DWR
FACT SHEET FOR NPDES PERMIT DEVELOPMENT
MAJOR MODIFICATION
NPDES No. NC0038377, Duke Energy Progress, LLC
Mayo Steam Electric Generating Plant
Facility Information
Applicant/Facility Name:
Duke Energy Progress/Mayo Steam Electric Generating Plant
Applicant Address:
10660 Boston Road, Roxboro, NC 27573
Facility Address:
(same)
Permitted Flow
Not limited
Type of Waste:
99.8 % Industrial, 0.2% - domestic
Facility/Permit Status:
Existing/Renewal
County:
Person
Miscellaneous
Receiving Stream:
Mayo Reservoir
and Crutchfield
Branch
Regional Office:
RRO
Stream Classification:
WS-V and C
Quad
A23SW
303(d) Listed?:
No
Permit Writer:
Sergei Chernikov, Ph.D.
Subbasin:
030205
(Roanoke)
Date:
July 14, 2021
Drainage Area (mi2):
N/A
wilimillir
Summer 7Q10 (cfs)
0
30Q2 (cfs):
0
Average Flow (cfs):
0
IWC (%):
100%
(assumed, no
modeling info.)
Primary SIC Code:
SUMMARY
This is a Minor Modification for the Mayo Electric Generating Plant. The facility is a coal-
fired electric generating plant with one unit rated at a maximum dependable capacity of
745 mw. The permit is being modified to make the following changes:
1. Include landfill leachate to the lined retention basin from the CCR landfill being
constructed to comply with terms of the settlement agreement. The leachate
volume generated by the new landfill will vary over time but is expected to be
approximately 50,000 gallons per day during the initial phases of development
and reducing over time. Only water conditioned ash will be placed in the new
CCR landfill.
2. Increase the flow limit from the dewatering system during dewatering activities
from 2.0 MGD to 3.0 MGD to accommodate flows from the storm events from the
large drainage area during rain events. The limit will be changed from Daily
Maximum to Monthly Average to be consistent with other Duke permits.
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
Page 2 NPDES No. NC0038377
3. Include ammonia conversion maintenance drainage as a contributing flow to the
lined retention basin. The Mayo Station has two Selective Catalytic Reactors
(SCR) for NOX control. An ammonia stripping column will be added per SCR
utilizing aqueous ammonia as the reagent. Periodic maintenance and or unit
shutdown periods, will require the column contents to be drained. Each column
could have a maximum of 740 gallons of water (1,480 gallons for both columns)
with ammonia concentration less than 1.0 mg/L with the pH being controlled in
the system with caustic. The column contents would be drained to the lined
retention basin for treatment discharging at internal Outfall 002A.
4. Reduce the sampling frequency for the domestic package plant (internal Outfall
011) from Weekly to Quarterly to be consistent with other Duke permits.
5. Include industrial stormwater from the new CCR landfill to be added as a
contributing flow to the lined retention basin before discharging to Mayo
Reservoir via Outfall 002.
All the remaining terms and conditions of the permit remain unchanged.
Water for plant uses is withdrawn from the Mayo Reservoir as required to make up
evaporative losses from the cooling tower, boiler water and drinking water needs. This
facility is subject to EPA effluent guideline limits per 40 CFR 423- Steam Electric Power
Generating Point Source Category. The facility has a closed cycle cooling system (cooling
tower), actual intake flow and design intake flow is less than 125 MGD. The facility has
a dry fly ash handling system, dry bottom ash handling system, and one ash pond.
The mixing zone for Chlorides was granted to the facility in December of 2007. The
daily maximum limit for Chlorides in the permit was an acute limit, monthly average
was allowed to exceed the state water quality standard in the mixing zone, it was set at
672.0 mg/L and was based on the modeling information. However, the Chloride
chronic standard was being met at the end of the mixing zone. The size of the mixing
zone was established in accordance with the model. The facility requested the removal
of the mixing zone with this renewal due to the installation of the Vapor Compressor
Evaporator for FGD wastewater. The request was granted.
The facility is located in the Lower Piedmont area of the state, the applicable state
water quality temperature standard is 32°C (89.6 F).
In response to North Carolina's Clean Air Initiative (Clean Smokestacks Bill of 2002),
which requires the reduction of SOx and NOx from air emissions, the company installed
Flue Gas Desulfurization (FGD) system. The FGD is essentially a scrubber system to
remove SOx by mixing flue gas with a limestone slurry.
The FGD blowdown generates a flow of approximately 0.254 MGD, with relatively
elevated concentrations of metals and chloride. Duke Energy Progress treats the FGD
blowdown via VCE (vapor compression evaporator) whose purpose is to evaporate the
majority of the waste water produced from the FGD scrubber system. The VCE became
operational in February, 2015. It produces two waste streams, both are utilized in the
plant processes. The concentrated wastewater is used for moisture conditioning of fly
ash prior to sending to the landfill. The second stream is a clean distillate that is
utilized to partially replace water withdrawal from Mayo Reservoir. The VCE system
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
Page 3 NPDES No. NC0038377
eliminates the FGD blowdown stream from Outfall 002, except during severe rain
events.
The ash pond dam has two toe drains that are designed for the stability of the dam.
The average discharge of both drains is approximately 11,000 gpd, the discharge is
routed to the Crutchfield branch. The Crutchfield Branch does not discharge to the
Mayo Reservoir.
The facility proposes to build a new Retention Basin to reroute all waste streams that
are currently discharged to the ash basin. This change is necessary to decommission
the existing ash pond and meet the requirements of Coal Ash Management Act. The
Retention Basin will have a cell where various vacuumed sediments and solids can be
decanted prior to disposal.
The facility is also constructing a new FGD settling basin, the waste from the basin will
be treated by VCE.
The facility operates the following outfalls:
• Outfall 001. Cooling Tower System (lat. - 36° 31'28" long. - 78° 52'56"). Less
than once per year the cooling towers and circulating water system are
drained by gravity and discharged directly to Mayo Reservoir.
• Outfall 002. Ash Pond Treatment System (lat. - 36° 32'03" long. - 78° 53'27").
Outfall 002 discharges directly to Mayo Reservoir. The ash pond receives
coal pile runoff, stormwater runoff, cooling tower blowdown, and various low
volume wastes such as boiler blowdown, oily waste treatment,
wastes/backwash from the water treatment processes including Reverse -
Osmosis (RO) wastewater, plant area wash down water, equipment heat
exchanger water, groundwater, yard sump overflows, occasional piping
leakage from limestone slurry and FGD system, and treated domestic
wastewater.
• Internal Outfall 008. Cooling tower blowdown is directly discharged to the
ash pond. Cooling tower blowdown is indirectly discharged to Mayo
Reservoir via the ash pond treatment system (Outfall 002).
• Internal Outfall 009. Discharge from the FGD blowdown treatment system.
FGD blowdown is indirectly discharged to Mayo Reservoir via the ash pond
treatment system (Outfall 002).
Outfall 002A. Upon completion of construction, discharge from the new lined
retention basin. The flows from the ash basin will be re -directed to the
retention basin when the construction of the retention basin is
completed. At that point, the ash basin will no longer accept any
wastewater. Retention basin will accept wastes from holding cell (vacuumed
sediments and solids), coal pile runoff, stormwater runoff, landfill leachate
from CCR landfill, industrial stormwater from CCR landfill, ammonia
conversion maintenance drainage, cooling tower blowdown, and various low
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
Page 4 NPDES No. NC0038377
volume wastes such as boiler blowdown, oily waste treatment,
wastes/backwash from the water treatment processes, including Reverse -
Osmosis (RO) wastewater, plant area wash down water, equipment heat
exchanger water, groundwater, occasional piping leakage from limestone
slurry and FGD system, chemical metal cleaning waste, and treated domestic
wastewater. The wastewater from this outfall discharges to Mayo Reservoir
via Outfall 002.
• Internal Outfall 002B. Yard sump overflows (contain all wastes routed to the
new retention basin). The wastewater from this outfall discharges to Mayo
Reservoir via Outfall 002.
• Internal outfall 011. Domestic wastewater plant. The wastewater from this
outfall discharges to Mayo Reservoir via Outfall 002A.
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)
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 Lined Retention Basin (LRB) (Outfall 002A).
The RPA was conducted for LRB, the calculations included: As, Be, Cd, Chlorides,
Total Phenolic Compounds, Cr, Cu, CN, F, Pb, Hg, Mo, Ni, Se, Ag, Zn, Ba, Sb, SO4,
and Tl (please see attached). The flow of 10.25 MGD (CCR leachate included) was
used for the analysis. The discharge data on the EPA Form 2C was used for the
RPA, it was supplemented by leachate data from the modification application. The
analysis indicates reasonable potential to violate the surface water quality
standards or EPA criteria for the following parameters: As, Chlorides, F, Ba, and
Sb. The appropriate limits were added to the permit.
b) RPA for Dewatering of Ash pond (Outfall 002).
To meet the requirements of the Coal Ash Management Act of 2014, the facility
needs to dewater two ash ponds by removing the interstitial water and excavate
the ash to deposit it in landfills. The facility's highest discharge rate from the
dewatering process will be 3.0 MGD (an increase from original 2.0 MGD). The
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
Page 5 NPDES No. NC0038377
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, SO4, and Tl. The analysis indicates
reasonable potential to violate the surface water quality standards or EPA criteria
for the following parameters: As, Cd, Cr (III), Cr (VI), Cu, Pb, Ni, Zn, Ba, and Tl.
The appropriate limits were maintained in the permit. The RPA indicated that new
limits were not necessary as the flow limit increased from 2.0 MGD to 3.0 MGD.
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 002 (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.
Year
2014
2015
2016
Annual average
concentration
(ng/L)
1.85
0.91
0.1
Maximum
sampling result
(ng/L)
7.05
1.18
0.1
Number of samples
28
43
13
The 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.
CWA SECTION 316(a) TEMPERATURE VARIANCE
This section is not applicable since the facility has a closed cycle cooling system, which
is considered a BAT. Effluent temperature is monitored daily at the Outfall 001, 002,
and 002A, and instream temperature is monitored semi-annually to assure compliance
with the state temperature standard.
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 with the next renewal application. The Actual Intake Flow and
Design Intake Flow for this station is less than 125 MGD.
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
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
Page 6 NPDES No. NC0038377
with §125.90(b) and 40 CFR 401.14. The existing closed -cycle system at Mayo is one of
the pre -approved compliance alternatives for impingement in accordance with
§ 125.94(c) (1). EPA also considered it as a pre -approved BTA for entrainment, but
excluded it from the rule due to the cost concerns. Based on this information the DEQ
has determined that the existing closed -cycle cooling system meets the requirements
for an interim BTA.
INSTREAM MONITORING- OUTFALL 002 (ASH POND)
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), turbidity, temperature, and
total dissolved solids (TDS).
TOXICITY TESTING-OUTFALL 002 (ASH POND)
Current Requirement: Outfall 002 - Acute P/F @ 90% using Pimephalis promelas
Recommended Requirement: Outfall 002 - Acute P/F @ 90% using Pimephalis promelas
This facility has passed all toxicity tests (22 out of 22) 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.
COMPLIANCE SUMMARY
During the last 5 years, the facility had 1 violations of the Fluoride limit (Outfall 002),
please see attached.
PERMIT LIMITS DEVELOPMENT
• The Free Available Chlorine limits, Total Chromium Limits, Total Zinc Limits,
and Priority Pollutant Limits (Outfall 001 and Outfall 008) were established in
accordance with the 40 CFR 423.
• The limits for Oil and Grease and Total Suspended Solids (Outfall 002, Outfall
002A, Outfall 002B, Internal Outfall 009, and Internal Outfall 011 (TSS only))
were established in accordance with the 40 CFR 423.
• The pH limits (Outfall 001, Outfall 008, Outfall 002, Outfall 002A, and Outfall
002B in the permit are based on the North Carolina water quality standards (15A
NCAC 2B .0200).
• The turbidity limit in the permit (Outfall 002) is based on the North Carolina
water quality standards (15A NCAC 2B .0200).
• The Whole Effluent Toxicity limit (Outfall 002) is based on the requirements of
15A NCAC 2B .0500.
• The BOD and Fecal Coliform limits (Outfall 011) were established in accordance
with the 40 CFR 133.
• The Technology Based Effluent Limits for Total Arsenic, Total Mercury, Total
Selenium, and Nitrate/nitrite as N (Outfall 009) are based on the requirements
of 40 CFR 423.
• The Water Quality Based Effluent Limits for Total Beryllium, Total Chlorides,
and Total Fluoride in the permit (Outfall 002 - decanting) are based on the North
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
Page 7 NPDES No. NC0038377
Carolina water quality standards (15A NCAC 2B .0200) and EPA water quality
criteria.
• The Water Quality Based Effluent Limits for Total Arsenic, Total Cadmium, Total
Chlorides, Chromium (III), Chromium (VI), Total Copper, Total Lead, Total
Nickel, Total Zinc, Total Barium, and Total Thallium in the permit (Outfall 002 -
dewatering) 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 Arsenic, Total Chlorides, Total
Fluoride, Total Barium, and Total Antimony in the permit (Outfall 002A - lined
retention basin) are based on the North Carolina water quality standards (15A
NCAC 2B .0200) and EPA water quality criteria.
PROPOSED CHANGES
1. Landfill leachate from the CCR landfill was added as a contributing flow to the
lined retention basin.
2. The flow limit for Outfall 002 was increased from 2.0 MGD to 3.0 MGD to
accommodate large storm events. The limit was changed from the Daily
Maximum to the Monthly Average to be consistent with other Duke permits.
3. Ammonia conversion maintenance drainage was added as a contributing flow to
the lined retention basin.
4. The sampling frequency for the domestic package plant (Internal Outfall 011)
was changed from Weekly to Quarterly to be consistent with other Duke permits.
5. Industrial stormwater from the CCR landfill was added as a contributing flow to
the lined retention basin.
6. The Water Quality Based Effluent Limits for Total Arsenic, Total Chlorides, Total
Fluoride, Total Barium, and Total Antimony were added to the permit (Outfall
002A - lined retention basin) based on the results of RPA.
PROPOSED SCHEDULE
Draft Permit to Public Notice:
Permit Scheduled to Issue:
July 27, 2021
September 24, 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].
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
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NPDES No. NC0038377
NPDES Implementation of Instream Dissolved Metals Standards — Freshwater Standards
The NC 2007-2015 Water Quality Standard (WQS) Triennial Review was approved by the NC
Environmental Management Commission (EMC) on November 13, 2014. The US EPA
subsequently approved the WQS revisions on April 6, 2016, with some exceptions. Therefore, metal
limits in draft permits out to public notice after April 6, 2016 must be calculated to protect the new
standards - as approved.
Table 1. NC Dissolved Metals Water Quality Standards/Aquatic Life Protection
Parameter
Acute FW, µg/1
(Dissolved)
Chronic FW,
µg/1
(Dissolved)
Acute SW, µg/1
(Dissolved)
Chronic SW,
µg/1
(Dissolved)
Arsenic
340
150
69
36
Beryllium
65
6.5
Cadmium
Calculation
Calculation
40
8.8
Chromium III
Calculation
Calculation
Chromium VI
16
11
1100
50
Copper
Calculation
Calculation
4.8
3.1
Lead
Calculation
Calculation
210
8.1
Nickel
Calculation
Calculation
74
8.2
Silver
Calculation
0.06
1.9
0.1
Zinc
Calculation
Calculation
90
81
Table 1 Notes:
1. FW= Freshwater, SW= Saltwater
2. Calculation = Hardness dependent standard
3. Only the aquatic life standards listed above are expressed in dissolved form. Aquatic life
standards for Mercury and selenium are still expressed as Total Recoverable Metals due to
bioaccumulative concerns (as are all human health standards for all metals). It is still
necessary to evaluate total recoverable aquatic life and human health standards listed in 15A
NCAC 2B.0200 (e.g., arsenic at 10 µg/1 for human health protection; cyanide at 5 µg/L and
fluoride at 1.8 mg/L for aquatic life protection).
Table 2. Dissolved Freshwater Standards for Hardness -Dependent Metals
The Water Effects Ratio (WER) is equal to one unless determined otherwise under
15A NCAC 02B .0211 Subparagraph (11) (d)
Metal
NC Dissolved Standard, µg/1
Cadmium, Acute
WER*{1.136672-[In hardness](0.041838)} • e^{0.9151 [In hardness] -
3.1485}
Cadmium, Acute
waters
Trout
WER* {1.136672- [ln hardness](0.041838)} • e^{0.9151[ln hardness] -
3.62361
Cadmium, Chronic
WER*{1.101672-[In hardness](0.041838)} • e^{0.7998[ln hardness] -
4.4451 }
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Duke Energy Progress - Mayo
NPDES No. NC0038377
Chromium III, Acute
WER*0.316 • e^{0.8190[ln hardness]+3.7256}
Chromium III, Chronic
WER*0.860 • e^{0.8190[ln hardness]+0.6848}
Copper, Acute
WER*0.960 • e^{0.9422[ln hardness]-1.700}
Copper, Chronic
WER*0.960 • e^{0.8545[ln hardness]-1.702}
Lead, Acute
WER* {1.46203- [In hardness] (0.145712) } •
1.460}
e^ {1.273 [In hardness] -
Lead, Chronic
WER* {1.46203- [In hardness] (0.145712) } •
4.705 }
e^ {1.273 [In hardness] -
Nickel, Acute
WER*0.998 • e^{0.8460[ln hardness]+2.255}
Nickel, Chronic
WER*0.997 • e^{0.8460[ln hardness]+0.0584}
Silver, Acute
WER*0.85 • e^{1.72[ln hardness]-6.59}
Silver, Chronic
Not applicable
Zinc, Acute
WER*0.978 • e^{0.8473[ln hardness]+0.884}
Zinc, Chronic
WER*0.986 • e^{0.8473[ln hardness]+0.884}
General Information on the Reasonable Potential Analysis (RPA)
The RPA process itself did not change as the result of the new metals standards. However,
application of the dissolved and hardness -dependent standards requires additional consideration in
order to establish the numeric standard for each metal of concern of each individual discharge.
The hardness -based standards require some knowledge of the effluent and instream (upstream)
hardness and so must be calculated case -by -case for each discharge.
Metals limits must be expressed as `total recoverable' metals in accordance with 40 CFR 122.45(c).
The discharge -specific standards must be converted to the equivalent total values for use in the RPA
calculations. We will generally rely on default translator values developed for each metal (more on
that below), but it is also possible to consider case -specific translators developed in accordance with
established methodology.
RPA Permitting Guidance/WQBELs for Hardness -Dependent Metals - Freshwater
The RPA is designed to predict the maximum likely effluent concentrations for each metal of
concern, based on recent effluent data, and calculate the allowable effluent concentrations, based on
applicable standards and the critical low -flow values for the receiving stream.
If the maximum predicted value is greater than the maximum allowed value (chronic or acute), the
discharge has reasonable potential to exceed the standard, which warrants a permit limit in most
cases. If monitoring for a particular pollutant indicates that the pollutant is not present (i.e.
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
Page 10 NPDES No. NC0038377
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
2. In order to establish the numeric standard for each hardness -dependent metal of concern and
for each individual discharge, the Permit Writer must first determine what effluent and
instream (upstream) hardness values to use in the equations.
The permit writer reviews DMR's, Effluent Pollutant Scans, and Toxicity Test results for any
hardness data and contacts the Permittee to see if any additional data is available for instream
hardness values, upstream of the discharge.
If no hardness data is available, the permit writer may choose to do an initial evaluation using
a default hardness of 25 mg/L (CaCO3 or (Ca + Mg)). Minimum and maximum limits on
the hardness value used for water quality calculations are 25 mg/L and 400 mg/L,
respectively.
If the use of a default hardness value results in a hardness -dependent metal showing
reasonable potential, the permit writer contacts the Permittee and requests 5 site -specific
effluent and upstream hardness samples over a period of one week. The RPA is rerun using
the new data.
The overall hardness value used in the water quality calculations is calculated as follows:
Combined Hardness (chronic)
_ (Permitted Flow, cfs *Avg. Effluent Hardness, mg/L) + (s7Q10, cfs *Avg. Upstream Hardness,
m L
(Permitted Flow, cfs + s7Q10, cfs)
The Combined Hardness for acute is the same but the calculation uses the 1Q10 flow.
3. The permit writer converts the numeric standard for each metal of concern to a total
recoverable metal, using the EPA Default Partition Coefficients (DPCs) or site -specific
translators, if any have been developed using federally approved methodology.
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NPDES PERMIT FACT SHEET
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4. The
Duke Energy Progress - Mayo
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EPA default partition coefficients or the "Fraction Dissolved" converts the
value for dissolved metal at laboratory conditions to total recoverable metal
at in -stream ambient conditions. This factor is calculated using the linear
partition coefficients found in The Metals Translator: Guidance for
Calculating a Total Recoverable Permit Limit from a Dissolved Criterion
(EPA 823-B-96-007, June 1996) and the equation:
Cdiss =
Ctotal
1
1 + { [Kpo] [SS'''] [10-6] }
Where:
ss = in -stream suspended solids concentration [mg/1], minimum of 10 mg/L
used, and
Kpo and a = constants that express the equilibrium relationship between
dissolved and adsorbed forms of metals. A list of constants used for each
hardness -dependent metal can also be found in the RPA program under a
numeric standard for each metal of concern is divided by the default partition coefficient (or
site -specific translator) to obtain a Total Recoverable Metal at ambient conditions.
In some cases, where an EPA default partition coefficient translator does not exist (ie. silver),
the dissolved numeric standard for each metal of concern is divided by the EPA conversion
factor to obtain a Total Recoverable Metal at ambient conditions. This method presumes that
the metal is dissolved to the same extent as it was during EPA's criteria development for
metals. For more information on conversion factors see the June, 1996 EPA Translator
Guidance Document.
5. The RPA spreadsheet uses a mass balance equation to determine the total allowable
concentration (permit limits) for each pollutant using the following equation:
Ca = (s7Q10 + Qw) (Cwqs) — (s7Q10) (Cb)
Qw
Where: Ca = allowable effluent concentration (µg/L or mg/L)
Cwqs = NC Water Quality Standard or federal criteria (µg/L or mg/L)
Cb = background concentration: assume zero for all toxicants except NH3* (µg/L or
mg/L)
Qw = permitted effluent flow (cfs, match s7Q10)
s7Q10 = summer low flow used to protect aquatic life from chronic toxicity and human
health through the consumption of water, fish, and shellfish from noncarcinogens (cfs)
* Discussions are on -going with EPA on how best to address background
concentrations
Flows other than s7Q10 may be incorporated as applicable:
1Q10 = used in the equation to protect aquatic life from acute toxicity
QA = used in the equation to protect human health through the consumption of
water, fish, and shellfish from carcinogens
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NPDES PERMIT FACT SHEET Duke Energy Progress - Mayo
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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)
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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