NC0023965_Correspondence_19870506

NPDES DOCYNENT SCANNIMS COVER SMEET
NPDES Permit:
NCO023965
Wilmington — Northside WWTP
Document Type:
Permit Issuance
Wasteload Allocation
Authorization to Construct (AtC)
Permit Modification
Engineering Alternatives Analysis
201 Facilities Plan
Instream Assessment (67B)
1Corres,00ndencJ/,'re: IWC
Permit History
Date Range:
Document Date:
May 6, 1987
THIS DOCUMENT IS PRINTED ON REUSE PAPER - IGNORE ANY CONTENT ON THE
REVERSE SIDE

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State of North Carolina
Department of Natural Resources and Community Development
Division of Environmental Management
512 North Salisbury Street • Raleigh, North Carolina 27611
James G. Martin, Governor R. Paul Wilms
S. Thomas Rhodes, Secretary May 6, 1987 Director
Mr. John Bauer
Director of Public Utilities
P.O. Box 1810
Wilmington, NC 28402
Dear Mr. Bauer:
This letter is to confirm information that a member of my staff,
Trevor Clements, relayed to you by telephone. As you know, it is -dif-
ficult to estimate an instream waste concentration (IWC) for facilities
in the lower Cape Fear Basin due to the effect of tides on water move-
ment. In your case, calculation of the IWC for the two Wilmington
facilities is further complicated by stream regulation in the upper
basin and by diversion of water in the lower basin via the Brunswick
River. The best method for estimating an IWC for your two facilities
would involve the use of fluorescent dye studies to measure tidally -
averaged dilution of the effluent in the receiving waters. You may
want to consider performing this type of study sometime in the future.
For your immediate need regarding the headworks analyses for the
Northside and Southside plants, it is recommended that you use the
following information:
IWC = Qw x 100
7Q10 + Qw
where: Qw = Effluent Flow (cfs)
7Q10 = lowest 7 - consecutive day flow
w/ probability of occurring 1 in 10 years
IWC = Instream waste concentration (%)
Pollution Prevention Pays
P.O. Box 27687, Raleigh, North Carolina 27611-7687 Telephone [phone]
An Equal Opportunity Affirmative Action Employer

For the Northside facility:
Qw = 8 MGD = 12.4 cfs
reg. 7Q10 z 1121 cfs @ Navassa
- 448 cfs diversion Brunswick River (assumed 40%)
+ 26 cfs NE Cape Fear River
701 cfs
IWC = 12.4 x 100 = 1.7 percent
701 + 12.4
For the Southside facility:
Qw = 12 MGD = 18.6 cfs
reg. 7Q10 = 1121 cfs @ Navassa
+ 28 cfs NE Cape Fear River
IWC = 18.6 x 100 = 1.6 percent
1149 + 18.6
Therefore, our best estimates as to the IWC's for the Northside and
Southside Wilmington facilities are 1.7 and 1.6 percent, respectively.
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These estimates might be consideredAsince they do not account for tidal
mixing. However, they do include credit for all of the estimated freshwater
flow which is not conservative given the width of the river and the side -
stream location of each discharge. Therefore, these estimates should be
used with care and with full acknowledgement of their uncertainty. DEM is
recommending them for use in your headworks analysis. At this point in
time, we are not recommending them for use in determining whole -effluent
toxicity limits.
If you have any questions regarding this matter, please call either
Trevor Clements or myself (919/733-5083).
Sincerely,
Steve W. Tedder, Head
Technical Services Branch
SWT:gh
cc: Preston Howard
,Trevor Clements
Ken Eagleson