NC0036269_Speculative Limits_20241001

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a`"HATE a
ROY COOPER
Governor
MARY PENNY KELLEY
Secretary
RICHARD E.ROGERS,JR. NORTH CAROLINA
Director Environmental Quality
October 1, 2024
Mr. Chad VonCannon,P.E.
Engineering Director
Water& Sewer Authority of Cabarrus County
232 Davidson Hwy
Concord,NC 28027
Subject: Rocky River Monitoring&Modeling Report
Review Speculative Effluent Limits
NPDES Permit No.NCO036269
Rocky River Regional WWTP
Cabarrus County
Dear Mr. VonCannon:
DWR reviewed the modeling report and associated files submitted to support evaluation of Rocky River
Regional WWTP(RRRWWTP)expansion(NPDES Permit No.NC0036269). The modeling package,
submitted by Tetra Tech on behalf of the Water and Sewer Authority of Cabarrus County(WSACC),was
initially received by the DWR Modeling and Assessment Branch(MAB) staff on April 24,2024. Over
the course of the review,numerous communications were made between the staff and Tetra Tech,which
resulted in several rounds of revision and resubmission of the work:
- During the review of the initial submission, it was noted that the 7Q 10 low flows for boundary
conditions used in the Rocky River QUAL2K model application were different from the flow
values that had been approved by the NCDWR prior to the start of the critical condition analysis.
A revised modeling package addressing this issue was submitted by Tetra Tech on June 7,2024,
along with the supplementary information detailing the flow adjustment steps to ensure the
reproducibility of the work.
- An inquiry was made to Tetra Tech regarding some headwater/tributary flows in the modeling
files(for Long Creek headwater,Little Long Creek tributary, Little Bear Creek tributary, and Big
Bear Creek tributary)that were still seemingly inconsistent with those in the revised report. In
response, Tetra Tech submitted the updated modeling report on July 8,2024,expanded with
clarifying text and a new table.
These updates helped enhance the consistency and reproducibility of the work. The adjustment of the
7Q10 flows applied in this analysis is a two-step process of. (1) estimating the unweighted interim flows
based on the unit drainage area approach and(2) adjusting them to obtain weighted interim flows using
the USGS gages as anchor points for water balancing.
As it has been noted before,Rocky River is under high development pressure. Several new and expanding
discharge requests are anticipated that are likely to be evaluated using the Rocky River QUAL2K model.
The Rocky River monitoring and calibration studies,which led to the development of the model, showed
that Rocky River is a highly complex system that may experience periodic low DO conditions susceptible
to sporadic events like stormwater slugs moving through the system.
North Carolina Department of Environmental Quality I Division of Water Resources
D E Q 512 North Salisbury Street 11617 Mail Service Center I Raleigh,North Carolina 27699-1611
NORTH C of Envl A 919.707.9000
Dnpenmem m Envlronmentel Uualiry

Docusign Envelope ID: BE014344-3150-42B2-8F85-B2DBDFC01DFA
Receiving Stream. The Rocky River is located within the Yadkin-Pee Dee River Basin. The Rocky River
has a stream classification of C. The Summer minimum 7Q10 is 20.7 cfs,the Winter minimum 7Q10 is
31.9 cfs,the minimum 30Q2 is 44.5 cfs,Annual 7Q2 is 32.9 cfs. The Average CFS is 229 cfs. The Rocky
River at the outfall location [stream segment 13-17b3] is currently listed as an impaired waterbody on the
2022 North Carolina 303(d)Impaired Waters List, impaired for turbidity and benthos.
Threatened or Endangered Species: Based upon a review of information available from the North
Carolina Natural Heritage Program Online Map Viewer,there are no Federally Listed threatened or
endangered aquatic species identified within a 5-mile radius of the proposed discharge location. If there
are any identified threatened/endangered species, it is recommended that the applicant discuss the
proposed project with the US Fish and Wildlife Service to determine whether the proposed discharge
location might impact such species.
Speculative Effluent Limits.Based on the revised materials provided,NPDES concludes that an expanded
discharge of 40.0 MGD or 50.0 MGD would include the following permit limits (in Table 1 and Table 2).
A complete evaluation of these limits,the appropriate design flow from the EAA assessment and
monitoring requirements for metals and other toxicants, as well as potential instream monitoring
requirements,will be addressed upon receipt of a complete NPDES permit application.
Every applicant shall also submit documentation of any additional pollutants for which there are certified
methods with the permit application if their discharge is anticipated. These pollutants may be found in 40
CFR Part 136, if there are additional pollutants with certified methods to be reported,please submit the
Chemical Addendum to NPDES Application and, if applicable, list the selected certified analytical
method used.
TABLE 1. Speculative Limits Rocky River Regional WWTP Discharge(40.0 MGD)
EFFLUENT LIMITS
EFFLUENT
CHARACTERISTICS Monthly Weekly Daily
Average Average Maximum
Flow 40.0 MGD - -
CBOD5,20°C—Summer' 10.0 mg/L 15.0 mg/L -
CBOD5,20°C—Winter' 20.0 mg/L 30.0 mg/L -
Total Suspended Solids(TSS) 30.0 mg/L 45.0 mg/L -
NH3 as N—Summerl 1.5 mg/L 3.75 mg/L -
NH3 as N—Winterl 3.4 mg/L 8.5 mg/L -
Fecal coliform(geo.Mean) 200/100 mL 400/100 mL -
Total Nitrogen M&R M&R -
Total Phosphorus M&R M&R -
Total Residual Chlorine - - 24 ug/L
Dissolved Oxygen(minimum) Daily average>6.0 mg/1
Chronic Toxicity Pass/Fail 90%(Quarterly)
PFAS Monitoring(EPA Method M/R(Quarterly)
1633
' Summer:April 1—October 31 *Winter:November 1—March 31
North Carolina Department of Environmental Quality I Division of Water Resources
D E Q 512 North Salisbury Street 11611 Mail Service Center I Raleigh,North Carolina 27699-1611
NORTH C of Envi A 919.707.9000
Deparimem of Envlronmenml Dual

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TABLE 2. Speculative Limits cky River Regional WWTP Discharge(50.0 MGD)
EFFLUENT LIMITS
EFFLUENT
CHARACTERISTICS Monthly Weekly Daily
Average Average Maximum
Flow 50.0 MGD -
BOD5,20°C—Summer' 10.0 mg/L 15.0 mg/L -
BOD5,20°C-Winter' 20.0 mg/L 30.0 mg/L -
TSS 30.0 mg/L 45.0 mg/L -
NH3 as N-Summer' 1.5 mg/L 3.75 mg/L -
NH3 as N-Winter' 3.2 mg/L 8.0 mg/L -
Fecal coliform(geo.mean) 200/100 mL 400/100 mL -
Total Nitrogen M&R M&R -
Total Phosphorus M&R M&R -
Total Residual Chlorine - - 24 ug/L
Dissolved Oxygen(minimum) Daily average>6.0 mg/1
Chronic Toxicity Pass/Fail 90%
(Quarterly)
PFAS Monitoring(EPA Method M/R(Quarterly)
1633
' Summer:April 1—October 31 *Winter:November 1—March 31
Engineering Alternatives Analysis(EAA). Please note that the Division cannot guarantee that an NPDES
permit for a new or expanding discharge will be issued with these speculative limits. Final decisions can
only be made after the Division receives and evaluates a formal permit application for the expanded
discharge.
In accordance with North Carolina Administrative Code 15A NCAC 2H.0105(c),the most
environmentally sound alternative should be selected from all reasonably cost-effective options.
Therefore, as a component of all NPDES permit applications for new or expanding flow includes a
detailed engineering alternatives analysis (EAA)that must be prepared.
The EAA must justify the requested design flow and provide an analysis of potential wastewater
treatment alternatives. A copy of the Division guidance for preparing EAA documents is attached and/or
can be found online at: hgps://files.nc.gov/ncdeq/Surface%2OWater%2OProtection/NPDES/permits/eaa-
guidance-20140501-dwr-swp-Mdes_13.pdf.
Summary of PFAS Requirements: Also,to identify PFAS concentrations throughout the State,monitoring
of PFAS chemicals would be added to the permit. As the Rocky River WWTP accepts influent
wastewater from several industrial facilities that are potential sources of PFAS via the approved
pretreatment program, effluent PFAS monitoring would be added to the permit at a quarterly frequency
using Method 1633.
EPA finalized Method 1633 in January 2024 but has not yet published the method in the Federal Register
as a 40 CFR 136 method.Upon evaluation of laboratory availability and capability to perform the draft
analytical method, it was determined that the sampling may be conducted using the 3rd or more recent
Draft Method 1633 or the January 2024 Final Method 1633. Sampling would take effect the first full
calendar quarter following 6 months after the effective date of the permit to provide WSACC time to
North Carolina Department of Environmental Quality I Division of Water Resources
D E Q 512 North Salisbury Street 11611 Mail Service Center I Raleigh,North Carolina 27699-1611
NORTH C of Envi A 919.707.9000
Deparimem of Envlronmenml Dual

Docusign Envelope ID: BE014344-3150-42B2-8F85-B2DBDFC01DFA
select a laboratory, develop a contract, and begin collecting samples. Effective 6 months after EPA has a
final wastewater method in 40 CFR Part 136 published in the Federal Register,WSAAC would then
conduct effluent monitoring using Method 1633. In addition to monitoring at the wastewater management
facility,WSACC would also identify and monitor SIUs suspected of discharging PFAS compounds
within 6 months of the permit effective date. Additional details can be obtained from the Pretreatment
Staff. This information is provided for awareness as part of the major NPDES permit modification
process.
State Environmental Policy Act(SEPA)EA/EIS Requirements.A SEPA EA/EIS document may be
required for projects that: 1) involve$10 Million or more of state funds; or 2)will significantly and
permanently impact 10 or more acres of public lands.Please check with the DWR SEPA coordinator
(David Wainwright, david.wainwrightkdeq.nc.gov, or 919-707-9045)as to whether your project requires
SEPA review. For projects that are subject to SEPA,the EAA requirements discussed above will need to
be folded into the SEPA document. If required, a FONSI must be issued prior to issuance of the NPDES
permit for the expansion. Details related to FONSI can be found on-line at: https://deq.nc.goy/permits-
regulations/sepa/review-process and at: h!Ws:Hdeg.nc.gov/about/divisions/water-
infrastructure/documents/cdbgi-fonsiea
We understand this expansion project is moving forward rapidly;however,we would expect that any
request for NPDES permitting action(major permit modification) for the expansion at either flow noted
be taken within a five-year period from the date of this letter, otherwise the noted speculative limits in
Tables 1 and 2 would be subject to reassessment and review.
Should you have any questions about the monitoring and modeling report review, speculative limits, or
NPDES permitting requirements,please feel free to contact Anjali Orlando at [e-mail]
or Michael Montebello at michael.montebello(acr�deq.nc.gov.
Respectfully,
nnA,D,ocuSigned by'.
`1•h'w.''`
C464531431644FE...
Michael Montebello
NPDES Program Branch Chief
ec: NPDES Files [Laserfiche]
Tetra Tech/Hillary Yonce[[e-mail]]
WSACC/Michael Wilson[[e-mail]];Chad VonCannon[[e-mail]]
Black&Veatch/Mike Osborne[[e-mail]];Lee Campbell[[e-mail]]
DWR/Mike Montebello [[e-mail]];Karen Preston[[e-mail]];
Anjali Orlando[[e-mail]];Pam Behm[[e-mail]];
Adugna Kebede [[e-mail]];Bongghi Hong[[e-mail]];
Derek Denard[[e-mail]];Nick Coco [[e-mail]];
North Carolina Department of Environmental Quality I Division of Water Resources
D E Q 512 North Salisbury Street 11611 Mail Service Center I Raleigh,North Carolina 27699-1611
NORTH C of Envi A 919.707.9000
Deparimem of Envlronmenml Dual