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RESPONSE TO WRITTEN COMMENTS 11- City Of Stockton-Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting probably redundant because reasonable potential has already been shown under less than worst-case scenarios. Method 4: (Evaluation at critical conditions) Tite City claims, "...the high pH values cited in the Fact... .._._ Sheet correspond with instream ammonia levels below the level of toxicity (i.e., summer conditions not relevant to the timeframe when the City discharges higher ammonia levels)." The City further states, "The correct analysis would have been to statistically evaluate instream pH only when instream ammonia levels are elevated." First of all, it is irrelevant whether the ammonia levels were below the level of toxicity during the study period. The exercise was performed to determine the likely pH during low flow periods in order to calculate the acute and chronic ammonia toxicity criteria. However, using only the fall and winter pH data to determine design pH was considered by staff, since the RWCF discharge usually only has the potential for high effluent ammonia concentrations during this time and the critical flows in the SJR most likely occur during the winter. Only the fall and winter months were evaluated to determine the design pH. It was found that the 901h percentile and median pH values were essentially the same whether the spring and summer months were included. Using only the fall and winter months, the design pH was calculated to be 7.8 acute and 7.1 chronic. Using all months, the design pH was calculated to be 7.7 acute and 7.2 chronic. The tentative order has adequately demonstrated reasonable potential. Additionally, EPA Region IX, has reviewed the analysis and has provided written comments approving of the evaluation. A water quality-based effluent limitation must be developed for ammonia. Stockton Comment No. III.0 (2): Staff calculated ammonia effluent limits based on the assumption that no dilution was available at critically dry conditions. This assumption is plainly unsupported as demonstrated in our review of the proposed Findings on dilution. RWQCB Response: See Attachments A, B, and C. Stockton Comment No. III.0 (3): Section 10.9 of the Fact Sheet discusses ammonia effects on dissolved oxygen in the San Joaquin River. These Findings are largely irrelevant to the establishment of year-round ammonia limits for this permit, as acknowledged by the Board in prior permit findings. Ammonia toxicity is potentially an issue in winter months when no DO concerns exist. Moreover, the tentative ammonia limits are based on aquatic toxicity, which requires the build-up of ammonia in the receiving waters. Ammonia build-up only occurs when nitrification slows down or stops in the winter months due to low seasonal temperature and algal activity is at minimum. When nitrification slows down or stops, ammonia has no impact on dissolved oxygen levels in the river. Furthermore, a TMDL is in progress to address factors affecting dissolved oxygen in the San Joaquin River. The TMDL will determine whether ammonia discharges significantly affect dissolved oxygen levels and whether limitations are required. The Board's conclusion that the year-round limits on ammonia are necessary for DO protection is conclusory and not supported by any facts or data in the 3rd.