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RESPONSE TO WRITTEN COMMENTS � ;'j, City Of Stockton-Regional Wastewater Control Facility 10- NPDES No. CA0079138 26 April 2002 Board Meeting adequate. Staff has no reason to believe that data from the California Ammonia Company(Calamco) is inadequate. The daily average pH of Calamco's influent was 7.7 or 7.8 for 23 of the 30 days. There are three days where the pH dropped to 7.6 and one day where the average pH was 7.9. Only the last three days of the 30-day period did the average pH rise above 8.0, which had no affect on the 30-day average pH. The City states, "It is understood that the Board is pursuing potential violations associated with the elevated pH conditions." The violations referred to are a result of an ammonia leak that occurred in May 1998 and has no relevance in this evaluation The City claims,"Even if the Calamco data were not suspect, the tentative order inexplicably uses a small fraction of the available data to skew the chronic pH to a higher value than the entire data set indicate, and ignores the effect ofStockton's discharge on pH at relevant lower flow conditions." It is further stated, "The influent data include 12 observations per day, while the instream values are single grab samples. Table 10-1 gives equal weight to the daily average influent data (consisting of 12 observations) and the single grab samples." The average pH measured at Calamco's influent is 7.8 from 22 December 1999 to 20 January 2000, which is precisely the same as the average pH used in Table 10-1. Table 10-1 actually shows the calculation of the median value, not the average as indicated in the table. This was done so the high pH value from the instream grab sample taken at Calamco's R-4 would not skew the results. The City also states, "The data presented by staff does not reflect the average conditions or explain why a reading taken one-half mile from the discharge location under a flow condition much higher than 1/Q/10 of 30/Q/10 should be applied to the effluent, assuming low or no dilution." The readings were taken one and half miles from the discharge, so it is likely the ammonia concentrations are greater closer to the outfall. Also, Figure 10-3 of the tentative order clearly shows the affect the RWCF discharge has on the ammonia in the SJR. The Wine Slip is definitely affected by the City's discharge. Method 2 shows that a case could be made that ammonia toxicity has occurred in the receiving water and Figure 10-3 confirms that the City's discharge caused it. Finally, the City states, "Ifstaff is correct that no dilution is available, then the effluent pHshould be used to assess acute criteria compliance, as confirmed by EPA." Method 1 was developed to cover just that situation, as stipulated in EPA's 12 October 2000 letter to Mr. William T. Hall, "If a mixing zone is not authorized even though dilution is available, the analysis should still use the effluent pH, however, the analysis should also confirm that once mixing does physically occur, standards will still be achieved " (emphasis added). Methods 2 and 4 were developed to determine if standards would be achieved once mixing physically occurs. Method 3: (Combination of worst-case situations) The City states, "This approach violates the protocol established by EPA for applying ammonia criteria in the TSD, and cannot be used as the basis for any effluent limitations." This is a method widely used by the Regional Board to determine reasonable potential. The highest instream pH and the highest effluent ammonia have the possibility of occurring simultaneously, , and must be considered. It is another method for showing that there is potential for the discharge to cause or contribute to ammonia toxicity in the receiving water. If this worst-case method did not show reasonable potential, then there definitely would be none. It is