SITE INFORMATION AND CORRESPONDENCE_CASE 2

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REGIONAL WATER BOARD RESPONSE (SWRCB/OCC File A-1846(a) and A-1846(b)) -18- PETITIONS FOR REVIEW OF W = DISCHARGE REQUIREMENTS ORDER NO R5-2007-0036 (NPDtS NO. CA0079154)AND TIME SCHEDULE ORDER NO. R5-2007-0037 CITY OF TRACY, WASTEWATER TREATMENT PLANT CSPA — CONTENTION l: Failure to include an effluent limitation for dissolved oxygen violates federal regulations. This comment is based on the tentative Order issued in December 2005 and is no longer applicable. CSPA provided this emment in response to the December 2005 tentative Order, which did not include an effluent limitation for dissolved oxygen (DO). Regional Water Board staff agreed that a DO effluent limitation was necessary, due to low DO concentrations in the receiving water. At times the receiving water does not comply with the Basin Plan's water quality objective for DO. Since the discharge contains oxygen demanding substances (e.g. BOD, TSS, ammonia, and nitrogen), the effluent has a reasonable potential to cause, or contribute, to an in-stream excursion of the Basin Plan's water quality objective for DO. Order R5-2007-0036 includes an effluent limitation for DO, based on the Basin Plan's Delta numeric site-specific water quality objective for DO concentration. CSPA — CONTENTION J: The ammonia limitation is not protective of the Basin Plan's narrative toxicity objective and fails to employ a "worst case"scenario. To apply the Basin Plan's narrative toxicity objective, USEPA's 1999 Update of Ambient Water Quality Criteria for Ammonia provides the appropriate water quality criteria. The acute criterion, or criterion maximum concentrations (CMC), for ammonia is a function of pH, and is stated as a 1-hour average concentration, while the chronic criterion, or criterion continuous concentration (CCC), is a function of both pH and temperature. For ammonia, the CCC is stated as a 30-day average concentration, with the highest 4-day average within the 30-day average not to exceed 2.5 times the CCC. A worst-case scenario occurs when there is little to no dilution of the effluent by the receiving water. Therefore, for the acute and chronic criteria, water quality objectives need to be achieved in the effluent at the end-of-pipe. As allowed by the TSD", the CMC and CCC were calculated in Order R5-2007-0036 using critical conditions that are a combination of worst-case observations. The receiving water and effluent pH and temperature were evaluated to determine the critical criteria. Large datasets were used in the evaluation of the reasonable worst-case conditions for pH and temperature for establishment of the CCC and CMC. The evaluation was based on 280 receiving water pH observations from July 1998 — November 2003, 2,372 effluent pH observations from July 1998 — December 2004, and 2,347 effluent temperature observations during the same period. The effluent temperature was used in the evaluation, because it exceeds the receiving water temperature and the CCC is more stringent with higher temperatures. The maximum pH observation in the dataset was 9.3 on 15 August 2003. However, due to the variability of pH sampling, using the maximum pH may be over protective. Therefore, the 90th percentile of pH readings was used to determine the acute design pH. The 90th percentile was chosen for acute toxicity since it would be protective of the short-term spikes in ammonia 4 USEPA Technical Support Document for Water Quality-Based Toxics Control (EPA/505/2-90-001)