NC0004979_Fact Sheet_20210728

NCDEQ/DWR
FACT SHEET FOR NPDES PERMIT Major Modification
Duke Energy Carolinas, LLC — Allen Steam Station
NC0004979
Facility Information
Applicant/Facility Name:
Duke Energy Carolinas, LLC — Allen Steam Station
Applicant Address:
P.O. Box 1006, Charlotte, North Carolina 28201
Facility Address:
253 Plant Allen Road, Belmont, North Carolina 28012
Permitted Flow
No limit
Type of Waste:
—100% industrial
Primary SIC Code: 4911 — Electric Services
Facility/Permit Status:
Class I/Active; Major Modification
County:
Gaston County
Miscellaneous
Receiving Stream:
Catawba River (11-
123.5), South Fork
Catawba River
(Lake Wylie)
Regional Office:
Mooresville
Stream Classification:
WS-IV B (Catawba
River) and WS-V B
(South Fork
Catawba River)
State Grid / USGS Quad:
G14 NE
303(d) Listed?
Yes (Catawba River
Only)
Permit Writer:
Sergei Chernikov,
Ph.D.
Subbasin:
03-08-34
Date:
October 30, 2020
Drainage Area (mi2):
635 (Catawba
River)
adallik,
001: Lat. 35° 11' 23" N Long. 81° 00' 45" W
002: Lat. 35° 10' 30" N Long. 81° 00' 23" W
002A: Lat. 35° 11' 34" N Long. 81° 00' 22" W
002B: Lat. 35° 11' 36" N Long. 81° 00' 30" W
004: Lat. 35° 11' 35" N Long. 81° 00' 22" W
006: Lat. 35° 11' 35" N Long. 81° 00' 22" W
007: Lat. 35° 11' 55" N Long. 81° 00' 36" W
008: Lat. 35° 10' 55" N Long. 81° 00' 24" W
Summer 7Q10 (cfs)
95
Winter 7Q10 (cfs):
95
30Q2 (cfs)
314
Average Flow (cfs):
2470
IWC CM:
4.7 (Outfall 002)
5.4 (Outfall 006)
SUMMARY
Duke Energy requested a Major Modification for Allen Steam Station to incorporate latest changes to
the Effluent Guidelines, expedite ash pond dewatering operations, and facilitate a long term facility
closure plan.
Duke Energy's Allen Steam Station is a coal fired steam electric plant in Gaston County. Units 1, 2,
and 3 are scheduled to retire by December 31, 2024 and Units 4 and 5 by December 31, 2028.
The facility began dry fly ash handling in 2009. The dry bottom ash disposal work was completed in
March 2019.
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NPDES Permit Fact Sheet NC0004979
The facility is subject to the effluent guidelines - 40 CFR 423.12. The facility is also subject to the
Cooling Water Intake Structure Rule (316(b) Rule) per 40 CFR 125.95 and Coal Ash Management
Act (State Law).
The facility built a new Retention Basin in 2018 and all waste streams that have previously discharged
to the Ash Pond are now re-routed to the Retention Basin. This change was necessary to
decommission the existing Ash Pond and meet the requirements of Coal Ash Management Act.
Below is description of the existing outfalls:
• Outfall 001 is comprised of once through, non -contact condenser cooling water. This outfall
discharges to the South Fork Catawba River.
• Outfall 002 is the ash basin discharge. This outfall includes domestic wastewater,
stormwater from the coal pile area, miscellaneous stormwater flows, ash sluice, wastewater
from turbine non-destructive testing, a yard drain sump, water treatment filter backwash,
extracted groundwater, laboratory wastes, and the power house sump at Unit 5. The
domestic waste is pre-treated by a septic tank. Outfall 002 wastewater is treated using
chemical coagulation, settling, and pH neutralization. This outfall discharges to the Catawba
River. Upon construction of the new Retention Basin, the waste streams currently
discharging through Outfall 002 will be rerouted to the new Retention Basin (Outfall 006).
This outfall discharges to the Catawba River. Outfall 002 and Outfall 006 might be
operational at the same time during the transition period.
• Outfall 002A is an intermittent discharge of emergency overflows from coal yard sump
(discharge from coal handling and storage areas). This outfall discharges to the Catawba
River.
• Outfall 002B is an intermittent discharge of emergency overflows from power house sump
(floor wash water, boiler blowdown, water treatment waste, condensates, equipment cooling
water, sealing water and miscellaneous leakage). This outfall discharges to the Catawba River.
• Outfall 003 is miscellaneous once through non -contact cooling water and seal water. This
outfall discharges to the South Fork Catawba River.
• Outfall 004 is also miscellaneous once through non -contact cooling water. This outfall
includes a small amount of intake screen backwash and car wash rinse water. This outfall
discharges to the Catawba River.
• Outfall 005 is internal, discharging to the Retention Basin. It is comprised of flue gas
desulfurization wastewater. The treatment facilities at this outfall consist of flow
equalization, pH stabilization using lime addition, chemical precipitation, clarification, gravity
filtration, biological selenium removal, aerated sludge holding, and a sludge filter press.
• Outfall 006 is the new Retention Basin discharge. This outfall discharges to the Catawba
River. Outfall 002 and Outfall 006 might be operational at the same time during the
transition period. This outfall includes domestic wastewater, stormwater from the coal pile
area, miscellaneous stormwater flows, ash sluice, wastewater from turbine non-destructive
testing, a yard drain sump, water treatment filter backwash, extracted groundwater,
wastewater from the Ash Pond dewatering process, bottom ash purge from the submerged
flight conveyers (purge volume not to exceed 10% of the water systems volume), laboratory
wastes, CCR landfill leachate, and the power house sump at Unit 5. The domestic waste is
Page 2 of 13

NPDES Permit Fact Sheet NC0004979
pre-treated by a septic tank. Outfall 002 wastewater is treated using chemical coagulation,
settling, and pH neutralization. This outfall discharges to the Catawba River.
• Outfall 007 is the emergency spillway of the new Retention Basin. 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. This outfall discharges to the
Catawba River.
• Outfall 008 is the emergency spillway of the retired Ash Pond. 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. This outfall discharges to the
Catawba River.
• Toe Drain Outfalls 103, 104, 108, and 108B - 4 potentially contaminated toe drains. These
outfalls discharge to the Catawba River, except for outfall 104, which discharges to unnamed
tributary to Catawba River.
On December 15, 2016 the station personnel observed a minor drainage from a corrugated metal
pipe along Catawba River, the coordinates of the pipe are 35°11'20.6" N, 81°0'27.55" W. The facility
inspected the pipe and determined it is clogged by debris 10 feet from the opening. The flow from
this pipe is extremely small, only about 114 GPD. The analysis of the water indicate that most
parameters of concern are either below detection level or below water quality standards. Based on the
analysis of the sample and on the flow amount it is likely a groundwater infiltrating into the pipe.
The facility has grouted the pipe to eliminate discharge of the groundwater.
TOE DRAINS —OUTFALLS 103, 104, 108, and 108B
The facility identified 4 unpermitted toe drains from the ash settling basin.
The locations of the toe drains are identified below and are depicted on the map attached to the
permit.
Table 1.
Discharge Coordinates and Assigned Outfall Numbers
Discharge ID of toe
drains
Latitude
Longitude
Outfall number
S-3
35°10.512'
81°0.360'
103
S-4
35°10.541'
81°0.364'
104
S-8
35°10.710'
81°0.384'
108
S-8B
35°10.689'
81°0.391'
108B
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NPDES Permit Fact Sheet NC0004979
Each outfall discharges through its own effluent channel meeting the requirements in 15A NCAC 2B
.0228.
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 002 AND OuTFALL 006
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°A) probability level and 95°A) 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 002).
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 increased from 1.0 MGD to 3.0 MGD to assure timely
closure. The facility submitted data for the standing surface water in the ash ponds,
interstitial water in the ash, and interstitial ash water that was treated by filters of various
sizes. To evaluate the impact of the dewatering on the receiving stream the RPA was
conducted for the wastewater that will be generated by the dewatering process. To introduce
the margin of safety, the highest measured concentration for a particular parameter was
used. The RPA was conducted for As, Cd, Chlorides, Cr, Cu, F, Pb, Mo, Hg, Ni, Se, Ag, Zn,
Sulfate, Ba, Sb, Tl, B, and Al (please see attached). The flow of 3.0 MGD was used in the
RPA.
b) RPA for New Retention Basin (Outfall 006).
The Retention Basin will be receiving all the waste streams that were previously discharged
to Ash Pond. However, the RPA that was conducted for Outfall 002 was modified for
Outfall 006 by adding interstitial concentration data provided by Duke to evaluate impacts
from the CCR landfill leachate. The flow volume was also increased from 3.3 MGD to 3.5
MGD based on the latest estimates that incorporate flor from the CCR landfill. The RPA
was conducted for As, Cd, Chlorides, Cr, Cu, F, Pb, Mo, Hg, Ni, Se, Ag, Zn, Sulfate, Ba, Sb,
Tl, B, and Al,(please see attached). The flow of 3.5 MGD was used in the RPA. The RPA
indicated a need for water -quality based limits for As.
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.
FGD TECHNOLOGY BASED EFFLUENT LIMITS -INTERNAL OuTFALL 005
The latest update to the Effluent Guidelines (40 CFR 423) allows the facilities that retire early to
eliminate Technology Based Effluent Limits (1BELs) for As, Se, Hg, and Nitrate/nitrite and
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NPDES Permit Fact Sheet NC0004979
substitute them with limits for TSS and Oil and Grease. This change will be implemented in this
permit modification. The narrative standard of proper operation and maintenance of the FGD
treatment system will also be added to the permit modification.
The existing FGD system for the facility is unable to meet the numerical As and Se limit with 100%
consistency. Review of the FGD discharge data from 1/1/2011 through 8/2/2016 indicates that:
a) Se daily maximum limit would have been violated 5 times out of 141 if it was implemented during
the last renewal.
b) As daily maximum limit would have been violated 17 times out of 68 if it was implemented during
the last renewal.
MERCURY EVALUATION- OUTFALL 002
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
2010
2011
2012
2013
2014
Annual average
concentration (ng/L)
1.6
1.18
6.9
0.93
0.83
Maximum sampling
result (ng/L)
2.4
1.8
22.6
1.6
1.0
Number of samples
4
4
4
4
3
The allowable mercury concentration for this facility is 50.9 ng/L. All annual average mercury
concentrations are below the allowable level. All maximum sampling results are below the 1BEL 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 — OUTFALL 001
The facility has a temperature variance. In order to maintain the variance the facility has to conduct
annual biological and chemical monitoring of the receiving stream to demonstrate that it has a
balanced and indigenous macroinvertebrate and fish community. The latest BIP (balanced and
indigenous population) report was submitted to DWR in November of 2014. The DWR has
reviewed the report and concluded that the receiving stream near Allen Steam Station has a balanced
and indigenous macroinvertebrate and fish community.
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 following materials shall be submitted:
- §122.21(r)(2) Source Water Physical Data
- §/22.21(r)(3) Cooling Water Intake Structure Data
- §122.21(r)(4) Source Water Baseline Biological Characterization Data
- §/22.21(r)(5) Cooling Water System Data
- §122.21(r)(6) Chosen Method(s) of Compliance with the Impingement Mortality
Standard (the chosen method will be defined after the site -specific BTA
determination is made)
- §122.21(r)(7) Entrainment Performance Studies
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NPDES Permit Fact Sheet NC0004979
- §122.21(r) (8) Operational Status
- §122.21(r) (9) Entrainment Characterization Study
- §122.21(r)(10) Comprehensive Technical Feasibility and Cost Evaluation Study
- §122.21(r)(11) Benefits Valuation Study
— 5122.21(r)(12) Non -water Quality and Other Environmental Impacts Study
Currently, the facility withdraws a water for all five units through a single CWIS located in a small
embayment on the west bank of Lake Wylie. The CWIS is flush with the shoreline and is
approximately 247.2 feet long. The CWIS is divided into 15 screens bays, three per unit. Each screen
bay is 14 feet wide and equipped with a trash rack and traveling water screen.
The trash racks prevent large debris from entering the CWIS protects the traveling screens from
damage. Typically, debris loading on the trash racks is minimal Top elevation of the trash racks is
580 feet with an overall height of 34.5 feet. The width of each trash rack is approximately 14 feet.
The trash rack bars are made of steel bars that are 4 inches by 0.375 inch spaced 3-inches on center,
which provides 2.6-inch clear bar spacing.
After water flows through the trash racks, it then passes through an isolation gate. The dimensions
for the isolations gates are as follows:
• Units 1 and 2: six feet by eight feet
• Units 3, 4, and 5: eight feet by eight feet
Downstream of the isolation gates, are 15 traveling screens - three traveling screens per unit. Each
screen is ten feet wide with an invert elevation of 544.5 feet. The intake bays constrict down to 11
feet 2 inches wide at the traveling screens compared to the approximately 14 feet wide at the trash
racks. The traveling screens consist of #12 gauge wire mesh with 3/8 inch square mesh openings. A
high-pressure front wash spray system is used to remove impinged fish and debris from the traveling
screens. The fish is returned through front trough that travels —421 ft south of CWIS where it joins a
72 inch corrugated metal drain pipe and travels an additional —374 ft back to the lake.
Each unit has two circulating water pumps located downstream of the traveling screens.
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.
The modified traveling screens with the fish return system is one of the pre -approved compliance
alternatives for impingement. The rule requires to make a site specific determination for entrainment.
In the absence of the comprehensive information required by the 316(b) rule the DEQ has
determined that the existing traveling screen system which includes a fish handling return system
meets the criteria of the interim BTA.
INSTREAM MONITORING-OuTFALL 002
The permit required semi-annual upstream and downstream monitoring near the ash pond discharge.
The upstream site is approximately 250 meters upstream of Outfall 006 and downstream location is
approximately 250 meters downstream of Outfall 006. The monitored parameters are: As, Cd, Cr,
Cu, Hg, Pb, Se, Zn, turbidity, and Total Dissolved Solids (TDS). The majority of the results are
below detection level (Hg, As, Cd, Cr, Pb, Se), the rest of the results are below water quality
standards (Cu, Zn, TDS). No parameter demonstrated any increase in the concentration at the
monitoring stations below the discharge.
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NPDES Permit Fact Sheet NC0004979
It is required that the monitoring of the instream stations will continue during the next permit cycle.
It is also required that the facility uses low level method 1631E for all Hg analysis.
FISH TISSUE MONITORING -NEAR OuTFALL 002
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. Redear sunfish and largemouth bass
tissues were analyzed for these trace elements. The results were below action levels for Se and Hg
(10.0 µg/g — Se, 0.4 µg/g — Hg, NC) and screening value for As (1.2 — µg/g, EPA). These results are
consistent with the previous monitoring results.
TOXICITY TESTING-OuTFALL 002 AND OuTFALL 006
Current Requirement: Outfall 002 dewatering — Chronic P/F @ 1.6% using Ceriodaphnia
Recommended Requirement: Outfall 002 dewatering — Chronic P/F @ 4.7% using Ceriodaphnia
Current Requirement: Outfall 006 — Chronic P/F @ 5.1% using Ceriodaphnia
Recommended Requirement: Outfall 006 — Chronic P/F @ 5.4% using Ceriodaphnia
This facility has passed all toxicity tests during the previous permit cycle (20 out of 20).
For the purposes of the permitting, the long term average flow was used in conjunction with the 7Q10
summer flow was used to calculate the percent effluent concentrations to be used for WET.
COMPLIANCE SUMMARY
During the last permit cycle, the facility has exceeded limits 4 times, please see attached. Three limit
violations were for temperature (Outfall 001) and one limit violation was for Fe (Outfall 002).
PERMIT LIMITS DEVELOPMENT
• The temperature limits (Outfall 001) are based on the North Carolina water quality standards
(15A NCAC 2B .0200) and 316(a) Thermal Variance.
• The limits for Oil and Grease and Total Suspended Solids (Outfalls 002, 002A, 002B, 004
(oil and grease only for 004), 005 (TSS only for 005), 006, 103, 104, 108, and 108B) were
established in accordance with 40 CFR 423.
• The BOD limits (Outfalls 002, 002A, and 006) were established in accordance with 40 CFR
122.123.
• The pH limits (Outfalls 002, 002A, 002B, 006, 103, 104, 108, and 108B) in the permit are
based on the North Carolina water quality standards (15A NCAC 2B .0200).
• The fecal coliform limits (Outfall 002, 002A, and Outfall 006) in the permit are based on the
North Carolina water quality standards (15A NCAC 2B .0200).
• The limits for Total Copper and Total Iron (Outfalls 002, 002A, 002B, and 006) were
established in accordance with 40 CFR 423.
• The turbidity limit in the permit (Outfall 002) is based on the North Carolina water quality
standards (15A NCAC 2B .0200).
• The total silver limits in the permit (Outfall 002) are based on the North Carolina water
quality standards (15A NCAC 2B .0200).
• The total silver and total arsenic limits in the permit (Outfall 006) are based on the North
Carolina water quality standards (15A NCAC 2B .0200).
• The Whole Effluent Toxicity limits (Outfall 002 and Outfall 006) are based on the
requirements of 15A NCAC 2B .0500.
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NPDES Permit Fact Sheet NC0004979
PROPOSED CHANGES
• The Bottom Ash Purge was added to the list of waste streams discharged to the Retention
Basin.
• The wastewater generated by the dewatering process and extracted groundwater were added
to the to the Retention Basin to achieve timely closure of the Ash Pond and provide greater
operational flexibility.
• The flow limit for dewatering operations was increased from 1.0 MGD to 3.0 MGD to
achieve timely closure of the Ash Pond.
• The Upstream and Downstream monitoring locations were moved to monitor discharge
from the Retention Basin.
• The pH limits at the Internal Outfall 005 were removed to be consistent with the
requirements in 40 CFR 423.
• The Technology Based Effluent Limits for Total Arsenic, Total Mercury, Total Selenium,
and Nitrate/nitrite as N were changed (Outfall 005 — FGD wastewater) based on the latest
update to the 40 CFR 423.
• The Technology Based Effluent Limits for TSS and Oil and Grease were added to the
permit (Outfall 005) based on the latest update to the 40 CFR 423.
• The RPA for Retention Basin was conducted by adding the highest reported concentration
of the contaminants in the interstitial wastewater. The flow volume was also increased from
3.3 MGD to 3.5 MGD based on the latest estimates that incorporate flor from the CCR
landfill. The limits for As were added as a result of this RPA (Outfall 006).
• The RPA for Ash Basin dewatering phase was conducted by increasing flow from 1.0 MGD
to 3.0 MGD. No new limits were added as a result of this RPA (Outfall 002).
• The Special Condition that requires electronic submittal of all discharge monitoring reports
was changed based on the latest update to the NPDES Electronic Reporting Rule (See
Special Condition A. (26.)).
• The Toxicity Test Instream Waste Concentrations for dewatering and Retention basin have
been adjusted (Outfall 002 dewatering and Outfall 006) to account for the increased flow
volume.
PROPOSED SCHEDULE
Draft Permit to Public Notice:
Permit Scheduled to Issue:
April 6, 2020 (est.)
May 31, 2021 (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
• Early retirement exemption has been postponed and earlier FGD limits have been re-
instated until Duke submits the Notice of Planned Participation.
• The ELG re -opener clause was added to the permit.
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NPDES Permit Fact Sheet
NC0004979
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. Calculatiol = 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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NPDES Permit Fact Sheet
NC0004979
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. consistently below detection level), then the Division may remove the
monitoring requirement in the reissued permit.
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NPDES Permit Fact Sheet NC0004979
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, mg/L)
(Permitted Flow, cfs + s7Q10, cfs)
The Combined Hardness for acute is the same but the calculation uses the 1Q10 flow.
3. The permit writer converts the numeric standard for each metal of concern to a total
recoverable metal, using the EPA Default Partition Coefficients (DPCs) or site -
specific translators, if any have been developed using federally approved
methodology.
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NPDES Permit Fact Sheet
NC0004979
4. The
EPA default partition coefficients or the "Fraction Dissolved" converts the value for
dissolved metal at laboratory conditions to total recoverable metal at in -stream ambient
conditions. This factor is calculated using the linear partition coefficients found in The
Metals Translator: Guidance for Calculating a Total Recoverable Permit Limit from a
Dissolved Criterion (EPA 823-B-96-007, June 1996) and the equation:
Cdiss = 1
Ctotal 1 + { [Kpo] [sso+a)I [10-6] }
Where:
ss = in -stream suspended solids concentration [mg/11, 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 sheet labeled DPCs.
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* Olg/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
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NPDES Permit Fact Sheet NC0004979
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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