NC0000396_NOV2024LV0725 RESP_20240822

Keith Douthit
Plant General Manager
Asheville Combined Cycle Station
Duke Energy Progress
ASVL PLT | 46 Duke Energy Lane
Arden, NC 28704
o: 828-650-0620
f: 828-650-0701
[e-mail]

Delivered via email no hardcopy mailed

August 22, 2024

Andrew Moore, Regional Supervisor
Water Quality Regional Operations Section
Asheville Regional Office
Division of Water Resources, NCDEQ

Subject: Response to NOTICE OF VIOLATION
Tracking Number: NOV-2024-LV-0725
Permit No. NC0000396
Asheville Steam Electric Power Plant
Buncombe County

Dear Mr. Moore:

Duke Energy Progress, LLC (Duke Energy) appreciates the opportunity to provide this response and supporting
information to the subject Notice of Violation (NOV). Duke Energy has completed coal ash activities at the
Asheville Steam Electric Plant and closed the stations ash basin. Additionally, the coal fired units at the site were
retired in January 2020 with demolition completed in early 2023 and replaced by natural gas fired combined cycle
generation units commissioned in 2020.

The 1964 ash basin has been clean closed, and the dam breached. The only flow coming from the basin is from
the surfaced or emanating groundwater underdrains. Operational samples over a period demonstrated TSS from
the underdrains was non-detect. As a post closure activity, a new automated treatment system had been designed
and was under construction for pH adjustment only of the underdrains prior to being pumped to outfall 001. Based
off the data, there was no reason to believe that TSS treatment for the underdrain was necessary. On June 13,
2024, the temporary treatment system was shut down and cleaned, then restarted with only chemical treatment for
pH. Physical filtration still existed. It was discovered after 5 operational days, that bench TSS sampling of the
treatment system was increasing. On the day of the exceedance, the first bench TSS was 8 mg/L, elevated but well
within compliance. The second bench TSS sample taken during sampling 1 ½ hours later was 86, our outfall
sample returned 81.3.

Please see the timeline and actions taken below. We have returned TSS treatment and are redesigning a
permanent system to include TSS treatment. We believe this will ensure compliance going forward.

Outfall 001 TSS exceedance on 6/25/24 NC0000396

Background:
Outfall 001 is the Asheville Combined Cycle external discharge to the French Broad River. As of May 1, 2024,
after the 1964 dam breach, the discharge from Outfall 001 consists of underdrains pumped and treated thru the
former ash basin treatment system and co-mingled with combined cycle blowdown. Prior to May 1, surface water
remaining from ash basin closure was also treated in the water treatment facility. This system continuously
monitored and treated pH and TSS. Underdrains are typically ND for TSS.

Underdrain Background:
During the excavation and closure of the 1964 ash basin, groundwater was encountered. Operational data was
collected from groundwater that was daylighting in the basin from April 2022 until January 2023. Based on the
operational data, it was determined that only pH adjustment would be needed. The underdrains were constructed
to collect and direct groundwater in the 1964 basin. The decision was made to pump the basin underdrain to
Outfall 001 for discharge. A permanent pH treatment system was going to be installed to treat underdrain flows
during CC plant outages.

Event:
Since the dam breach on May 1, 2024, and in preparation for the future treatment system for pH only, TSS
treatment within the existing water treatment system was ceased due to underdrain operational sample data not
indicating need. The treatment system was shut down and cleaned on 6/13/24. Operation resumed on 6/15/24,
including daily operational testing at the treatment facility. Note, physical filters for TSS were still in place with
only chemical TSS treatment ceased. On 6/18/24, weekly sampling was taken and TSS was 2.6 mg/L. On
6/24/24, train 1 of the treatment system was taken out of service for maintenance. Train 2 remained operational.
Outfall 001 Weekly samples were collected on June 25, 2024, at 0945. Per the sample login report, the samples
were received by Pace Analytical Labs in Asheville for analysis on 6/25/2024 at 14:22. On June 26, 2024, the
sample analysis for Total Suspended Solids was completed and resulted in a TSS of 81.3 mg/L, which is an
exceedance of the daily max permit limit for TSS of 50 mg/L. After receiving results, ORC instructed treatment
plant operators to shutdown system operation until further notice to allow for investigation. Investigation showed
operational benchtop TSS that was collected and analyzed at 0830 on 6/25/24 was 8 mg/L. The operational
benchtop TSS that was collected and analyzed at 0945, same time as outfall sample collection, resulted at 86
mg/L. Visual inspection of the compliance sample by sampler did not give any indication of abnormal sample
conditions. TSS physical/chemical treatment was resumed and put back into service on 6/28/24. Additional TSS
operational bench samples continue to be taken daily during operation.

Preliminary findings:
Investigation has revealed that the chemical pH adjustment of the underdrains is resulting in precipitation of
solids. We have a team developing a water treatment design for the underdrains to include both pH and TSS
treatment. In the meantime, the existing treatment facility will remain and continue both pH and TSS treatment.

Should you have questions or need additional information regarding this notification, please contact Teresa
Williams, at [phone] or Don Safrit, at [phone].

Sincerely,

Keith Douthit
Asheville Plant Manager

cc: Mikal Wilmer, Asheville Regional Office (via email)
Jessica Bednarcik