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S./ REGIONAL BOARD RESPONSE(STTRCB/0CC FILE A-1483) -7- PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS. RS-2002-0083 AND R5-2002-0084 CITY OF STOCKTON REGIONAL WASTEWATER CONTROL FACILITY contained in Attachment B of the Regional Board's 23 April 2002 Response to Comments (Administrative Record, Binder 1, Item 9). Another model was prepared by Hall & Associates for the City and is described in the report,Evaluation of Chronic Dilution in the San Joaquin River Under Drought Flow, dated 11 April 2001 (Administrative Record, Book 1, Item 3). The Report describes a simple model used to estimate the minimum monthly SIR flow at Stockton during drought flow conditions. Using RWCF effluent ammonia data and receiving water ammonia data, the Report attempts to estimate the minimum flow during drought conditions, which presumably occurred in the early 1990's. The Report uses a mathematical model to evaluate the flow based on the dilution of ammonia in the SJR. The Regional Board rejected the results due to uncertainty in the model that was not defined or quantified in the Hall &Associates report, and the fact that the model was not calibrated or validated. Detailed comments are contained in Attachment C of the Regional Board's 23 April 2002 Response to Comments (Administrative Record, Binder 1, Item 9). Based on the above models, the City proposes that the permit should include dilution credits of 1:1 for the acute condition and 4:1 for the chronic. However,models are only tools for estimating flow or dilution. Real-time river flow data has been measured at Stockton and shows very little flow available for dilution during above average wet years. The water quality models proposed by the City are poorly documented, have not been calibrated or validated, and technical peer reviews were not performed. They should not be used in lieu of real-time data. The evaluation of actual flow measurement at the Stockton USGS flow monitoring station during above average wet years, with reasonable assumptions for critical dry year conditions, is more dependable and protective, and was determined to be appropriate. Given the severely impaired condition of the SIR, the presence of threatened and endangered species in the vicinity of the discharge, and minimal flows in above average wet years, the Regional Board concluded that most permit limits should not include any dilution credit. The Regional Board must protect for worst-case conditions,rather than average conditions. The models did not evaluate worst-case conditions. See Transcript of 26 April 2002 at page 84, response of Dr. Brown, the City's representative. SECOND BASIS: PERMIT REQUIREMENTS PERTAINING TO AMMONIA Stockton requests that the State Board direct the to modify the ammonia effluent limitations, make modifications to the findings and conclusions, rescind Cease and Desist Order No. R5-2002- 0084, and remove Provision G.14,which requires an evaluation of nitrate levels. The Clean Water Act mandates that NPDES permits include effluent limitations that are as stringent as necessary to meet water quality standards established pursuant to state or federal law. Pursuant to 40 CFR section 122.44(d)(1)(i),NPDES permits must contain limits that control all pollutants that"are or maybe discharged at a level that will cause, have the reasonable potential to cause, or contribute to an excursion above any state water quality standard, including state narrative criteria for water quality." (the "reasonable potential analysis".) With respect to narrative objectives,the Regional Board must establish effluent limitations using one or more of three specified sources, including EPA's published water quality criteria. (40 CFR 122.44(d)(1)(vi)(A), (B), or(C). In addition, the Regional Board's Basin Plan