SITE INFORMATION AND CORRESPONDENCE_2005-CURRENT

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Mr. Phil Govea -2- 21 September 2009 A when comparing the MEC and the maximum background receiving water silver concentration to the criteria. These criteria are presented in dissolved concentrations (refer to previous section IV.C.2.c). (b) RPA Results. For the effluent, the applicable silver acute criterion (maximum 1-hour average concentration) is 0.7 Ng/L, as total recoverable, based on the minimum observed upstream ambient hardness of 36 mg/L. The CTR does not contain a chronic criterion. Since the Facility was upgraded to provide tertiary-level treatment in September 2007, only one sample was obtained for analysis (April 2008), which indicated that concentrations of silver were not detectable at the method detection level (MDL) of less than 0.12 pg/L. However, Regional Water Board staff determined that one sample is insufficient data to perform an RPA. Out of the five effluent samples obtained since April 2004, one sample indicated a MEC of silver at 0.86 pg/L, which is above the applicable criterion of 0.7 pg/L. All five receiving water samples obtained since April 2004 indicated silver concentrations values below the water quality criteria at a MDL of less than 0.1 pg/L. Based on this information, the discharge exhibits reasonable potential to cause or contribute to an in-stream excursion of the CTR criterion for silver. (c) WQBELs. As described in detail in section IV.C2.c above, the applicable effluent concentration allowances for total recoverable silver is 2.2 pg/L for the acute (maximum 1-hour average concentration). Using the procedures for calculating WQBELs in the Section 1.4 of the SIP, results in final effluent limitations for total recoverable silver are 1.1 pg/L and 2.0 pg/L, as the AMEL and MDEL, respectively_ (d) Plant Performance and Attainability. Analysis of the effluent data shows that the MEC of 0.86 pg/L is less than the applicable WQBELs. The Regional Water Board concludes, therefore, that immediate compliance with these effluent limitations is feasible. The Proposed NPDES Permit is scheduled for consideration of adoption at the 7/8 October 2009 Central Valley Water Board hearing. Although the public comment period on this item has closed, Central Valley Water Board staff will accept written comments on the above changes only, until 5:00 p.m. on 1 October 2009. Anyone having questions on the proposed correction, or on the Proposed NPDES Permit, should contact Ms. GGaayleen Perreira at (916) 464-4824. Diana Messina, Chief Point Source Permitting Section