SITE INFORMATION AND CORRESPONDENCE

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INFORMATION SHEET OR*NO. R5-2004-0028 .. 19 CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY hardness under design flow conditions (see Figure 1). The minimum flow at Vernalis is approximately 1000 cfs which is the flow that the U.S. Bureau of Reclamation maintains at Vernalis to meet the 1995 Water Quality Control Plan salinity objective of 1000 umhos/cm. Hardness data was then evaluated in the range of 800 to 1,200 cfs. The receiving water hardness generally ranged from 150 to 250 mg/1 as CaCO3 with the lowest observed receiving water hardness under these conditions being 108 mg/l CaCO3. At a hardness of 108 mg/l,the chronic criterion, or criterion continuous concentration(CCC), for copper is 9.6 ug/l. Effluent hardness values ranged from 170 mg/1 to 190 mg/1 during the period from March 2002 to December 2002. Because no dilution is allowed for effluent limitations based on acute criteria,the minimum effluent hardness value of 170 mg/1 was used for calculating effluent limitations. Using the minirnum effluent hardness, the acute criterion,or.criterion maximum concentration(CMC), for copper is 22.2 ug/1 as dissolved, based on the SIP. However, the hardness dependent SIP criterion exceeds the Basin Plan site-specific objective of 10 ug/1 as dissolved. Therefore, the copper effluent limits were calculated using a CMC of 10 ug/1 as dissolved. Effluent limitations,which are expressed as total recoverable, are somewhat higher after the application of a 0.96 translator. There have been no approved studies by the Discharger to evaluate discharge-specific metal translators for copper; therefore, the default USEPA translators within the CTR were used in the calculation of the final effluent limitations. The final effluent limitations were calculated using a steady-state model method described in Section 1.4 of the SIP. Section 5.4.4 of the TSD was utilized to determine the monthly average limit for arsenic. Water quality-based effluent limitations are included in this Order based on the Basin Plan chemical constituents objective. The data are included in Table 9 and the effluent limitation calculations in Table 11. 11.2 Human Carcinogens There were five (5)human carcinogenic compounds present in the WQCF effluent. As summarized in Table 10, dibromochloromethane,bromodichloromethane, 2,4,6-trichlorophenol, and bis(2- ethylhexyl)phthalate were determined to present reasonable potential to exceed a one-in-a-million incremental human cancer risk criteria for water and/or organism consumption. Chloroform does not show reasonable potential to exceed the primary MCL. None of these constituents were detected in the receiving water. 11.2.1 Total Trihalomethanes and Chloroform Information submitted by the Discharger indicate that the effluent contains trihalomethanes (THMs) including chloroform. The Basin Plan contains the "Chemical Constituent" objective that requires, at a minimum,that waters with a designated MUN use not exceed California MCLS. In addition,the Chemical Constituent objective prohibits chemical constituents in concentrations that adversely affect beneficial uses. The California's Drinking Water Standard primary MCL for total THMs is 100 ug/1. The USEPA primary MCL for total THMs is 80 ug/l, which was effective on 1 January 2002 for surface water systems that serve more than 10,000 people. Pursuant to the Safe Drinking Water Act,