SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

REGIONAL BOARD RESPONSE (SYYRCB/OCC FILE A-1483) -10- PETITION FOR REVIEW OF WASTE DISCHARGE REQUIREMENTS ORDER NOS. R5-2002-0083 AND R5-2002-0084 CITY OF STOCKTON REGIONAL WASTEWATER CONTROL FACILITY Method 3 —Combination of Worst-Case Conditions: As allowed by the TSD, a worst-case scenario is to calculate the CMC using critical conditions that are a combination of worst-case observations. The highest receiving water pH of 8.9 was observed at R-2 on 17 September 1991. The calculated CMC for this condition is 1.0 mg/I ammonia as N. The effluent ammonia concentration on 17 September 1991 (16 mg/I ammonia as N) exceeded the CMC calculated for that day. In addition, during the fall and winter months, the effluent ammonia concentration exceeds the Method 3 CMC most of the time. A worst-case scenario was also used to calculate the CCC using critical conditions that are a combination of worst-case observations. The highest receiving water 30-day average pH was 8.2, observed at R-3 from 18 September 1990 to 17 October 1990. The calculated CCC for this condition is 1.8 mg/1 ammonia-N. The effluent 30-day average ammonia concentration during that time period (4.4 mg/l ammonia as N) exceeded the calculated CCC. In addition, the effluent ammonia concentration predominantly exceeds the Method 3 CCC during the fall and winter months. Therefore,Method 3 also demonstrates the effluent has the reasonable potential to cause or contribute to ammonia toxicity in the receiving water. Method 4—Evaluation at Critical Low Flow Conditions: Another method to evaluate reasonable potential is to calculate a CMC and CCC based on the expected pH that could occur under drought flow conditions at the point in the receiving water where the acute and chronic standards apply. This method also clearly demonstrates that the effluent has the reasonable potential to cause or contribute to ammonia toxicity in the receiving water. Effluent Limitations Based on the above discussion of reasonable potential, which identifies ammonia toxicity, it is apparent that maximum daily and average monthly effluent ammonia limitations are needed to protect aquatic organisms from ammonia toxicity. The TSD recommends that statistical permit limit derivations be used to develop chemical specific limitations for NPDES permits. Review of the effluent ammonia data reveals that the year-round concentrations do not produce a normal or log-normal distributed data set. This is because the seasonality the data exhibits, as a result of the effective nitrification of ammonia by the facultative ponds during the summer months, creates a bi-normal distributed data set. However,by selecting ammonia concentrations that exceed a set level, a normal distributed data set can be created. Defensible cutoff levels are the calculated CCC values determined in the above reasonable potential analyses. With the adjusted data set, a statistical derivation of the maximum daily and average monthly ammonia effluent limitations can be made using the TSD. Effluent limitations, based on each of the four methods above,were evaluated and are shown in Order R5-2002-0083 (Tables 10-2 through 10-5). The tables present a summary of the statistics and calculations used to derive the effluent limitations. Based on the calculations, the final daily maximum effluent limitation ranged from 1.0 mg/1 to 7.3 mg/L ammonia as N by Methods 3 and 4,respectively. The monthly average effluent limitation ranged from 0.5 mg/1 to 4.4 mg/L ammonia as N by Methods 3 and 4,respectively. As defined by the 1999 criteria, the 4-day average CCC ammonia concentration shall not exceed 2.5 times the value of the 30-day CCC. However, considering the maximum daily