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REGIONAL BOARD RESPONSE (DERC&OCC FILE A-1483) -9- PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS. R5-2002-0083 AND R5-2002-0084 CITY OF STOCKTON REGIONAL WASTEWATER CONTROL FACILITY The reasonable potential evaluation shows the RWCF effluent has reasonable potential to cause or contribute to an exceedance of ammonia water quality criteria in the receiving water. This has been demonstrated using four separate methods: (1) identifying toxicity in the RWCF effluent using "real- time" data (ammonia,pH, and temperature occurring simultaneously), (2) identifying toxicity in the receiving water using"real-time" data, (3) showing reasonable potential based on critical conditions that are a combination of worst-case observations, and (4) evaluation based on the expected receiving water pH and temperature occurring under drought flow conditions (10-year return flow). A brief summaryof the methods is presented below. For a detailed description of these methods,please refer to the Fact Sheet(Section 10) of Order No. R5-2002-0083. Method 1 —Effluent Toxicity: In Method 1, the acute criterion for ammonia was determined by evaluating the maximum effluent pH at the end of the pipe. There were eleven(11) days over the 1,583 days in the period from 1995-2000, when effluent ammonia concentrations exceeded the calculated ammonia criterion maximum concentration (CMC) value. For the chronic condition, the average monthly pH was used to calculate the criterion continuous concentration (CCC) for each month from January 1995 through June 2000. The RWCF effluent exceeded the CCC in 37 of the 66 months evaluated. Generally, the RWCF effluent consistently exceeded the CCC during the fall and winter months. Therefore, Method 1 clearly demonstrates that the effluent has the reasonable potential to cause or contribute to acute and chronic ammonia toxicity in the receiving water. Method 2—Receiving Water Toxicity: In Method 2,receiving water pH and ammonia concentrations were evaluated to determine if acute and chronic ammonia toxicity has been observed in the receiving water. On 19 January 2000 the receiving water pH, as measured by the California Ammonia Company (Calamco), '/<mile upstream of the DWSC (approximately 1.5 miles downstream of the outfall), was 8.7, resulting in a CMC of 1.5 mg/I ammonia as N. The ammonia concentration at the same monitoring station was 4.8 mg/1 ammonia as N, demonstrating acute ammonia toxicity in the receiving water. For the chronic condition, an evaluation of receiving water data at the nearest receiving water sample locations upstream (R-2, 1 mile upstream from outfall) and downstream(R-3, 1.5 miles downstream of outfall) for ammonia, pH, and temperature, showed that in January 2000 there may have been conditions when the potential to exceed the chronic ammonia criterion occurred. The City collected only one receiving water sample (12 January 2000) during the entire month. Therefore, data collected by the Calamco, downstream of the discharge,has been used to provide a more robust dataset for the evaluation. Downstream ammonia data, from 22 December 1999 through 20 January 2000, has been organized in Table 10-1 of Order R5-2002-0083. The average downstream ammonia concentration was 3.3 mg/L ammonia as N. The average pH was 7.8 resulting in a CCC of 3.1 mg/L ammonia as N. Therefore, the receiving water ammonia concentration exceeded the calculated CCC. Method 2 demonstrates that the effluent has the reasonable potential to cause or contribute to acute and chronic ammonia toxicity in the receiving water.