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-3- Free residual chlorine in the river was reported by Central Valley Reqional Water Quality Control Board (CVRWOCB) personnel as between 0.02 - 0. 08 ppm. Because chlorine is released to the atmosphere at the air/water interface , hiqher values are expected deeper in the water column. This is evidence that due to low river flow and hiqh SRWWTP 9 .scharge volumes , ' lution was not sufficient to reduce the chlorine residual below toxic conditions. Toxicity tests using channel catfish variously reported in the literature indicate LC50 96 hours (pH7 ) as 0. 156, and 0. 082, 0. 064 ppm. The discharge effluent pH was between 3. 1 and 5. 4 . Lethality increases as pH decreases, therefore, the effluent was more toxic than reported literature values. Conclusion A very high deqree of probability exists that a large population of channel catfish and other fish in the San Joaquin River were overcome by toxic levels of free residual chlorine present in effluent from SRWWTP. Estimates of the population affected are difficult because an unknown number would possibly not float, and an unknown number could have been removed by scavengers. Based on actual observations, Warden Lenihan conservatively estimated 5 ,000 fish were affected. Recommendation As a result of review of information reqarding the subject event, the DFG recommends: 1 . CVRWQCB should proceed with civil liability action against SRWWTP. 2. CVWQCB should order repairs at SRWWTP to prevent a repeat of toxic discharges. 3. CBWQCB should issue permit criteria calling for continuous chlorine and pH monitoring of the SRWWTP effluent to ensure 24 hour compliance with effluent limiations. 4 . Permit criteria should not permit SRWWTP waste discharges at low flow conditions and flow reversal conditions in the San Joaquin River.