NC0023973_staff comments_20070718

Clements, Trevor
From: Clements, Trevor
Sent: Wednesday, July 18, 2007 2:21 PM
To: 'Teresa Rodriguez'
Subject: RE: Confirmation of understanding of what dilution information DWQ is looking for from the
City of Wilmington
Teresa,
Thank you for the quick response. I understand that there must be no acute toxicity in 100 percent of the effluent
concentration, but I will make sure that this is reinforced in any discussion with the City of Wilmington.
Trevor
Original Message
From: Teresa Rodriguez [mailto:[e-mail]]
Sent: Wednesday, July 18, 2007 1:29 PM
To: Clements, Trevor
Subject: Re: Confirmation of understanding of what dilution information DWQ is looking for from the City of Wilmington
Trevor, I just want to clarify one issue, the way our mixing zone is
implemented we will apply the appropiate dilution from the diffuser to
implement chronic limits but acute limits are always implemented at
end -of -pipe. For acute impcats we don't consider the dilution. Otherwise
the apporach we discussed and you summarized here is acceptable.
Teresa
Clements, Trevor wrote:
> Hello Teresa,
>
> This email is intended to summarize my understanding of what we
> discussed this morning regarding the need for additional dilution
> information on the Wilmington Northside discharge. Please review my
> summary below and confirm whether it matches your understanding.
>
> * NC DWQ is requiring dilution information from Wilmington in
> order to establish an appropriate basis for calculating toxicity
> limitations. Past permitting decisions applied a default
> assumption of sufficient mixing such that chronic toxicity is
> not of concern and only a "no acute impact" requirement
> applies. Per EPA review comments, DWQ is requiring information
> to update the analysis with a sound scientific basis.
> * Per discussions with Teresa Rodriguez of the DWQ NPDES
> Permitting Program, DWQ understands that the City has invested
> substantial resources in modeling the impact of its Northside
> and Southside effluent discharges. It is not the intent of DWQ
> to require a specific application of CORMIX only that would
> require substantial additional effort by the City; rather DWQ
> needs to have modeling analysis information that is shown to be
> scientifically sound and in the terms necessary to support NPDES
> permitting decisions.
> * In the case of the Northside facility, DWQ will require
> information on the dilution at the end of pipe (defined as after
> port injection and plume rise to surface) and at the edge of the
> mixing zone (defined as a maximum of one third the width of the
> receiving water body at the point of discharge - Teresa
> estimates this at approximately 400 ft).
> * The City may work from the existing EFDC modeling study
1

> information (i.e., in lieu of a new dye study and CORMIX
• application) to build an analysis that is scientifically sound
• and produces dilution results at the outfall (for the expanded
• 16 MGD wasteflow) after immediate mixing and at the edge of the
> mixing zone region. Results should be presented in these terms
• to support acute and chronic toxicity permitting decisions.
> I look forward to hearing from you as to whether my understanding is
> correct, or regarding any additional considerations.
> Thank you,
> 'Trevor Clements* I *Director, Watershed Management Services*
> Direct: 919.485.8278 x100 I Fax: [phone]
> [e-mail]
> *Tetra Tech I Complex World, Clear Solutions
> *P.O. Box 14409 13200 Hwy 54, Suite 105 I Research Triangle Park, NC
> 27709 I www.ttwater.com
>
> PLEASE NOTE: This message, including any attachments, may include
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Teresa Rodriguez, P.E.
Division of Water Quality
EAST NPDES Program
919-733-5083 ext. 553