NCS000580_Minor Mod Request to Add SDO 002_20190107

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ENERGY®
File: Mayo l 2520-R
Ms. Bethany Georgoulias
North Carolina Division of Environmental uality
1628 Mail Service Center
Raleigh, NC 27699-1628
Subject: Duke Energy Progress, LLC.
Mayo Electric Generating Plant
NPDES Industrial Stormwater Permit NCS000580
Minor Modification Request
Dear Ms. Georgoulias:
Mayo Steam Plant
Duke Energy Progress
10660 Boston Road
Roxboro, NC 27574
December 11, 2018
NC Dept of Environmental ( Uality
DEC 2 0 2018
Raleigh Regional Offief-
6 ^ InrIr
DENR-LAND QUALITY
STORNMIA ER P,--R Vii i W!-O
As recently discussed at your office on August 23, 2018 with Lori Tollie, Duke Energy is submitting
herewith, supplemental information in support of a minor modification of NPDES permit NCS000580.
This modification will provide coverage for one newly created stormwater outfall discharging to Bowes
Branch, a class C surface water in the Roanoke River Basin.
This outfall will be created as a result of regulatory requirements associated with CAMA and the CCR
rule that require Duke Energy to cease all flows to the Mayo ash basin in early 2019.
Duke Energy identified four areas'requiring redirection of stormwater away from the ash basin. These
areas are shown on the attached map and include:
1. Absorber Area: Areas within the plant, including and immediately surrounding the Absorber and
associated structures (central portion of yellow area)
2. Southwest Area: Parking and storage areas southwest of the plant (southwestern portion of
yellow area) .
3. Northeast Area: Areas at the northeastern portion of the plant (northeastern portion of yellow
area)
4. Wesf Area: A wooded area at the west end of the site (Pink and green areas)
Natural and managed (pipes or swales) drainage patterns for the four areas were determined through
topographic mapping of the drainage areas and through investigation of catch basins and associated
piping.
The current drainage patterns for each of the four areas are as follows:
• The absorber area, southwest area and northeast area encompasses 34.2 acres that all drain
through catch basins and surface drainage to a low area north of the absorber area. At that
location, storm water is captured and flows to the northeast in a Swale that runs parallel to and

Ms. Bethany Georgoulias
Permit NCS000580 Modification Request
December 11, 2018
south of the railroad tracks. The swale drains into a culvert under the tracks and discharges into
the ash basin.
• The green portion of the wooded area encompasses 49.9 acres and drains through one 30-inch and
two 24-inch culverts under the railroad tracks. Both of these culverts discharge to a swale that
runs to the southwest, along the north side of the railroad tracks. That swale passes through a
culvert under Highway 501 and discharges to Bowes Branch.
• The pink portion of the wooded area encompasses 40.4 acres and drains through a 48-inch culvert
to discharge into the ash basin.
The future drainage patterns for each of the four areas will be as follows:
Stormwater drainage from the four identified areas will be rerouted to the new outfall to Bowes
Branch or to NPDES outfall 002. The entire yellow area (absorber area, northeast area and
southwest area) will continue to flow to the low area north of the absorber area. At that point,
stormwater will be directed to a new swale to be constructed that will run to the northeast, along
the south side of the railroad track. That stormwater will discharge to NPDES wastewater outfall
002 and finally to Mayo Reservoir.
• The green portion of the wooded area will continue to drain through the existing 24-inch and 30-
inch culverts into the swale north of the tracks and ultimately into Bowes Branch. The pink area
will continue to drain though the 48-inch culvert into a lined basin (basin 1) that is being built
north of the tracks. From basin 1, stormwater will be pumped through a force main into the swale
that leads to Bowes Branch.
Industrial activities in drainage areas discharging to Bowes Branch are as follows:
• The industrial activity and storage in the drainage area associated with the pink and green
portions of the map includes the railroad corridor for coal delivery to Mayo Steam Station, a
stockpile area and equipment storage for an active construction project, a temporary construction
trailer, a permanent facility warehouse, a permanent facility material stockpile, and equipment
staging areas.
Duke Energy took into consideration the possibility of a leak from the Absorber tank farm (central portion
of yellow area on map). The drainage from the catch basins in this area will be isolated and directed to a
new basin (basin 2) before discharge to the outfall 002 swale. This will provide safety mechanism to
capture any materials that may unexpectedly escape from the Absorber area and remove or reprocess
them.
Should you have any questions please contact Ms. Lori Tollie at [e-mail] or at
(336)-854-4916.
Sincerely,
om Copolo
General Manger — Mayo Steam Station