NCS000598_2021 DMR_20210721

Align
an Air Uquide company
July 21, 2021
DEQ Winston-Salem Regional Office
Attention: DEMLR Stormwater Program
450 West Hanes Mill Road
Suite 300
Winston-Salem, NC 27105
Subject: Stormwater Permit Data Monitoring Report (DMR) for Period 1 in 2021
NPDES Stormwater Permit No. NCS000598
Airgas USA, LLC — Mebane ASU Facility
Mebane, Alamance County, North Carolina
To Whom It May Concern:
Airgas USA, LLC — Mebane Facility is submitting the enclosed Stormwater Discharge Outfall (SDO)
Monitoring Report as required in the National Pollutant Discharge Elimination System (NPDES) Permit
No. NCS000598. In accordance with the monitoring schedule in the NPDES Permit, the stormwater
discharge monitoring at Outfall 001 is required twice a year: Period 1 from January through June and
Period 2 from July through December. This report is for Period 1 in 2021.
Industrial stormwater runoff from the facility is first collected in the onsite wet pond and then discharged
through the wet pond outlet/effluent pipe (Outfall 001). The stormwater discharge sampling location is
Outfall 001 and the sampling occurs when there is a discharge flow at Outfall 001 during the monitoring
period. Due to the design of the wet pond, the stormwater discharge occurs at Outfall 001 when the water
level reaches the drawdown orifice of the riser structure. This drawdown orifice is approximatelyl0 feet
above the bottom of the wet pond. Thus, the stormwater discharge at Outfall 001 occurs if and when
there is a significant rainfall and does not occur immediately when it rains.
As shown in the enclosed SDO Monitoring Report, the required stormwater discharge monitoring could
not be conducted. It was discovered during an annual inspection that the wet pond leaked. Airgas
contacted the contractor who built the wet pond and subsequently the contactor scheduled and made the
repairs to the leak in February 2021. In the repair process, the contractor emptied the wet pond. Since the
repairs were completed, the water level in the wet pond has not reached the drawdown orifice of the riser
structure, which resulted in no flow at Outfall 001 during the remaining monitoring period. When there is
a significant rainfall in the future, there can be a discharge flow from the wet pond as designed and the
required stormwater discharge monitoring should be able to occur.
If you have any questions, please contact me at [phone] or at rodne .st.'ohn air as.com.
Sincerely,
r A
Rodn St.
Plant Manager
cc: File
Enclosures: As Noted

STORMWATER DISCHARGE OUTFALL (SDO) MONITORING REPORT FOR PERIOD 1 IN 2021

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