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RESPONSE TO WRITTEN COMMENTS 18- a City Of Stockton- Regional Wastewater Control Facility NPDES No. CA0079138 26 April 2002 Board Meeting rather than modify the effluent limitation, the receiving water limitation was adjusted to be consistent with the Basin Plan. The 6.5-8.5 pH range change must be applied instantaneously, whereas the 0.5 pH change can be applied over an averaging period. This is because the effects of change of pH within the optimum range have been shown to have a negligible impact on aquatic life as long as the- optimal range is maintained. Stockton Comment No. IV.G (Chronic Whole Effluent Toxicity): The proposed order seeks to impose a condition that would require the City to demonstrate no chronic toxicity in 100% effluent using various dilution waters. The applicable test runs over a seven-day period. The provision is based upon the presumption that zero dilution is available in the receiving waters. This provision is unnecessarily restrictive as all data confirm that dilution does exist over a seven-day average period (see, discussion on dilution and Finding 45). This provision should be revised to allow for dilution and require that the ITUc limit be applicable in at most 25% effluent. The requirement that the City confirm compliance in both receiving water and laboratory dilution water is also inappropriate. RWQCB Response: Responses concerning the lack of dilution are discussed above. The purpose for requiring the use of receiving water, as dilution water, is to determine the additive or synergistic effects of the discharge on the receiving water. A discussion of the rationale for the use of different types of dilution water based on the objectives of the study is provided in Section 7 of US EPA Short- Term Methods for Estimating the Chronic Toxicity of Effluents and Receiving Water to Freshwater Organisms, Third Edition. Section 7.1.1.3 states, "If the objective of the test is to determine the additive or mitigating effects of the discharge on already contaminated receiving water, the test is performed using dilution water consisting of receiving water collected immediately upstream or outside the influence of the outfall. A second set of controls, using culture water, should be included in the test." The testing requirements in.the proposed permit are consistent with this guidance. Stockton Comment No. IV.H (Organics): The City object to the manner in which the limitations were calculated using a lower adjustment factor rather than the statistical procedures contained in the SIP. The Board is bound to use the adopted guidance for calculating permit limitations and thus should recalculate the limits using the applicable procedures. Moreover, as discussed in the dilution section, the actual SIP harmonic mean flow dilution is 20:1, not 10:1. Thus, the Board needs to revise all of the applicable final limits. RWQCB Response: In October 1995, the Public Advisory Task Force to the State Water Resources Control Board developed proposed guidance to the State Board regarding development of the Inland Surface Waters Plan and the Enclosed Bays and Estuaries Plan. The guidance proposed direction for developing interim maximum limitations. Staff incorrectly applied the guidance in the development of the interim performance-based effluent limitations for organics. Staff finds that application of the statistical TSD approach may over estimate the maximum concentration due to the large coefficient of variation produced by a limited and varied data set. However, the TSD approach is an approved method for calculating performance-based interim limits, therefore, the tentative permit has been modified to incorporate new interim limitations for organics, as well as the inorganic constituents