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RESPONSE TO WRITTEN COMMENTS -9- City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting RWQCB Response: See Attachments A, B, and C. III. Ammonia Limitations Stockton Comment No. III.0 (1): The Fact Sheet specifies that the pH selected for the ammonia toxicity analysis must reflect the mixture of the City's effluent with the receiving water. Each of the four methods identified by the Regional Board staff uses a critical pH that is unrelated to actual conditions observed in the San Joaquin River as affected by the City's discharge. These critical pH values cannot be applied to the development of ammonia limits because they violate the TSD guidance that the staff states it has followed to calculate appropriate ammonia limits. RWQCB Response: Staff showed under a variety of conditions that the City's discharge has the reasonable potential to cause or contribute to an exceedance of water quality objectives for ammonia in the receiving water. Responses to the specific allegations by the City are provided below: Method 1: (Ammonia toxicity in the effluent) The Discharger claims, "The rare high pH levels encountered in the discharge...correspond with short-term events that have no relevance to the one- hour average acute ammonia standard." Based on the data submitted by the City, there is no evidence to suggest that the maximum pH values are spikes that do not occur for at least one hour. The City also claims that"...these maximum pH values will have no relevance to actual instream pH..." Staff understands and expects that the effluent pH mixes with the receiving water pH (see Fact Sheet Section 10.2) and have developed three other methods to account for this mixing. Method 1 was developed in order to assess potential ammonia toxicity in the RWCF effluent (see excerpt below from EPA's 12 October 2000 letter to Mr. William T. Hall): "Question 5. If no mixing zone is authorized or available, should the model utilize the pH of the effluent or the pH of the up-stream water supply? "Response. ...If there is no mixing because of the absence of upstream dilution water, the effluent pH should be used. 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." The reasonable potential evaluation only needs to show that the discharge has the potential to cause or contribute to ammonia toxicity in the receiving water. It is not necessary to show actual instances of toxicity. The City only comments on the acute situation, but the Fact Sheet also shows that the effluent has shown ammonia toxicity under chronic conditions. Therefore, if there is evidence of ammonia toxicity in the effluent, it is easy to conclude that there is reasonable potential to cause or contribute to ammonia toxicity in the receiving water. Method " r �nia toxicity in th� :�ceiving water) Demonstrating actual toxici he receiving water can of ifficult. Therefore, much debate can be made whether staffs eve,i.ia on is