SITE INFORMATION AND CORRESPONDENCE_1985-2004

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REGIONAL BOARD RESPONSE (SWRCB/OCC FILE A-1634) - 9 - PETITION 9 - PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS. R5-2004-0028 AND R5-2004-0029 CITY OF MANTECA WATER QUALITY CONTROL FACILITY to modify, Order No. R5-2004-0028 to include effluent limitations for arsenic calculated using the harmonic mean dilution credit of 222:1, and Order No. R5-2004-0029 to account for the recalculation of the effluent limitations for arsenic. The discharge of arsenic, a pollutant that produces human health effects and is a known carcinogen,was found to have a reasonable potential to cause or contribute to an in-stream excursion above the Basin Plan numerical water quality objective. As such, an effluent limitation for arsenic is required and was included in Order No. R5-2004-0028. The validity and applicability of the Basin Plan numeric objective for arsenic and the use of a federal MCL as a criterion to protect water quality were previously addressed in the 19 March 2004 Response to Comments,pages 5-6 and 8-9. Please also refer to the response to the first basis, above. Regarding the request by the Discharger to utilize harmonic mean dilution, the Regional Board considered the overlapping plume with a downstream discharger, and the lack of assimilative capacity in the receiving water for arsenic. The evaluation of dilution was presented in Finding 31 of the permit and in Section 2 of the Information Sheet, and was further explained in pages 16 and 18 of the 19 March 2004 Response to Comments. The Regional Board properly adopted effluent limitations for arsenic based on the numerical water quality objective in the Basin Plan. All effluent limitations are supported by substantial evidence in the record. Please refer to permit finding 34, information sheet section 11.1, permit tables 9 and 11 and the 19 March 2004 response to comments document(pages 3 and 16-18, Item No. 4, Binder No. 1) for more information. SEVENTH BASIS: IMPROPER IMPOSITION OF EFFLUENT LIMITATIONS FOR CYANIDE Manteca requests that the State Board modify, or order the Regional Board to modify, Order No. R5-2004-0028 to include effluent limitations for cyanide based upon NTR criteria as incorporated by the CTR and Order No. R-2004-0029 to account for the recalculation of the effluent limitations for cyanide. In addition,Manteca requests that the State Board modify, or order the Regional Board to modify, Order No. R5-2004-0028 to allow dilution for acute toxicity in its recalculation of the effluent limitations for.cyanide. The discharge of cyanide was found to have a reasonable potential to cause or contribute to an in- stream excursion above the Basin Plan numerical water quality objective. There is also a CTR criterion for cyanide,but consistent with the CTR, the Basin Plan objective applies, and not the