NC0023884_technical correction_20031210

A74
December 10, 2003
Mr. V. Randall Tinsley
Brooks, Pierce, McLendon, Humphrey & Leonard, L.L.P.
P.O. Box 26000
Greensboro, North Carolina 27420
Dear Mr. Tinsley:
Michael F. Easley, Governor
State of North Carolina
William G. Ross, Jr., Secretary
Department of Environment and Natural Resources
Alan W. Klimek, P.E., Director
Division of Water Quality
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Subject: Correction of IWC
Salisbury -Rowan WWTP
Permit NC0023884
Rowan County
Division personnel have reviewed your request for a correction of the instream waste concentration
(IWC) for Salisbury -Rowan WWTP. The decision to apply new mercury sampling requirements to
the above mentioned facility was based on dilution calculations using CORMIX, a mixing zone
model. The 7Q10 flow was only one of many factors that was used to determine dilution.
In order to revise the previously established IWC, the Division needs dilution modeling results
based on an updated CORMIX model that conclusively demonstrates validity of a new IWC. If
modeling results support your conclusion, the Division would modify your permit.
To the extent that stream flow statistics are used in this modeling, it has been a consistent practice
of the NPDES Unit to use 7Q10 data that was calculated by USGS. There are several methods for
calculating 7Q10 flows and results of these calculations depend on the method used. Meaningful
comparison between data can be done only if the same method was used for these calculations. The
City of Salisbury should obtain 7Q10 data for the point of interest directly from USGS.
If you have any questions concerning this response, please contact Sergei Chernikov at telephone
number [phone], extension 594.
cc: Central Files
Mooresville Regional Office/Water Quality Section
NPDES Unit
John Vest, City of Salisbury
Sincerely,
A.
David A. Goodrich
NPDES Unit Supervisor
North Carolina Division of Water Quality
1617 Mail Service Center
RalPinh Nnrth (Tarnlina 77F,QQ-1 r.17
[phone]
FAX [phone]