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City of Stockton -4- 15 August 1990 toxicity. This is expected to work for a period of a few years, until the midges genetically adapt and become resistant to the bacteria. Figure 2 plots 48 hour Ceriodaohnia survival in 100% City of Stockton effluent, as well as 7-test mean values. The significant toxicity which occurred due to midge control pesticides is shown by the data for May 1988 and May 1989 (Note: no bioassays were conducted during April , June or July 1988, when midge control pesticides were applied) . Additional significant acute invertebrate toxicity also exists in the City's winter effluent from other unidentified chemicals. This is shown by the data for the period March 1989, and October 1989-February 1990 (Note: no bioassays were conducted for the period August 1988-February 1989) . On 19 October, 1989, staff proposed that if mean Ceriodaohnia survival fell below 70% on any seven consecutive 48 hour tests the City should commence a TIE to determine the cause. Staff decided that a mean of 70% survival was a very liberal figure to define a toxicity problem, and should it be surpassed would be considered a serious toxicity problem. On 2 November, 1989, the City responded and agreed to the Regional Board staff's TIE trigger and proposed to use EPA's phase I TIE manual (EPA/600/3-88/034) and any additional references as required to complete the TIE. Board staff responded on 12 December, 1989, agreeing with the City's approach but requesting that it conduct phase I TIES on at least six different effluent samples collected over a representative period of the winter and spring to insure that effluent variability was properly characterized. Staff also requested a Technical Report at the completion of the TIES summarizing the finding and recommending a course of action during the next phase of the TRE. The City's 21 June 1990 Technical Report is incomplete as only two of the six requested phase I TIES (November and December, 1989) were performed. Both of these are considered only partially complete since the EDTA chelation, oxidation-reduction, and graduated pH test portions of the phase I TIE were not performed in an acceptable manner, as results were reported as inconclusive (see Table 4-1 and pages 4-3 of the City's report) . Nevertheless, results from the two phase I TIES suggest that nonpolar organics are responsible for a large portion of the invertebrate toxicity. The City concludes, "additional TIE tests. . .are necessary to determine the role of ammonia and nonpolar organics as causes of effluent toxicity. These tests should include additional TIE phase I analyses, using. . .Ceriodaohnia. . .followed by TIE phase II and phase III testing" (page 6-4) . We agree that these additional tests are necessary, but they should already have been completed using the past winter's toxic effluent. B. Recommendations 1 . As part of the Technical Report due 7 September 1990, the City of Stockton should submit an outline of its 1990/91 Ceriodaohnia TIE/TRE workplan. This plan should be separate from the Golden Shiner TIE/TRE plan. The overall goal of the TIE/TRE should be to eliminate all acute Ceriodaohnia toxicity (no death within 96 hours) from 100% effluent. This TIE/TRE should not be considered complete until the goal is accomplished. Subsequent TIE/TREs should work to remove any remaining chronic invertebrate toxicity, if still present. The objective of the 1990/91 TIE/TRE should be to determine by 1 May 1991, either the name(s) of the chemical (s) causing toxicity or to develop an implementable treatability option and demonstrate that it is capable of removing all acute invertebrate toxicity. At a minimum, the Technical Report must contain the following information: