SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

INFORMATION SHEET ORA NO. R5-2004-0028 20 CITY OF MANTECA, CITY OF LATHROP AND DUTRA FARMS WASTEWATER QUALITY CONTROL FACILITY SAN JOAQUIN COUNTY DHS must revise the current total THMs MCL in Title 22 CCR to be as low or lower than the USEPA MCL. Total Trihalomethanes (THMs) include bromoform, bromodichloromethane, chloroform, and dibromochloromethane. Chloroform does not have promulgated CTR criteria. The State Board, in WQO No 2003-0002, stated that the Drinking Water Standard primary MCL for Total THMs of 80 ug/1 could be applied to address chloroform in the discharge regulated in that Order. In addition, the Cal/EPA Office of Environmental Health Hazard Assessment(OEHHA)has published the Toxicity Criteria Database, which contains cancer potency factors for chemicals, including chloroform, that have been used as a basis for regulatory actions by the boards, departments and offices within Cal/EPA. This cancer potency factor is equivalent to a concentration in drinking water of 1.1 ug/1 (ppb) at the 1-in-a-million cancer risk level with the consumption of the drinking water over a 70-year lifetime. This risk level is consistent with that used by the Department of Health Services (DHS)to set de minimis risks from involuntary exposure to carcinogens in drinking water in developing MCLS and Action Levels and by OEHHA to set negligible cancer risks in developing Public Health Goals for drinking water. The one-in-a-million cancer risk level is also mandated by USEPA in applying human health protective criteria contained in the National Toxics Rule and the California Toxics Rule to priority toxic pollutants in California surface waters. MUN is a designated beneficial use of the receiving water. However, there are no known drinking water intakes on the San Joaquin River within several miles downstream of the discharge,and chloroform is a non-conservative pollutant. Therefore, to protect the MUN use of the receiving waters, the Regional Board finds that, in this specific circumstance, application of the USEPA MCL for total THMs for the effluent is appropriate, as long as the receiving water does not exceed the OEHHA cancer potency factor's equivalent receiving water concentration at a reasonable distance from the outfall (e.g.,before reaching the drinking water intakes). Effluent samples collected from January 2002 through December 2002 indicated that THMs were present with a maximum concentration of 17 ug/1 and an average concentration of 10 ug/l. Chloroform samples collected over the same period contained a maximum concentration of 12 ug/1 and an average concentration of 8 ug/l. Considering the available dilution based on the harmonic mean flow of the San Joaquin River,the discharge does not have a reasonable potential to cause or contribute to an in-stream excursion above the water quality objective for MUN use by causing an exceedance of the USEPA primary MCL for total THMs or the chloroform OEHHA cancer potency factor's equivalent receiving water concentration. Therefore, effluent limitations for total THMs and chloroform are not included in this Order. 11.2.2 Effluent Limitations for Human Carcinogenic Priority Pollutants The effluent limitation calculation procedures in Section 1.4 of the SIP allow for the granting of a dilution credit which, in this case, is 222-fold based on the harmonic mean flow of the San Joaquin River at Vernalis and the average discharge flow. However,the Regional Board finds that granting of this dilution credit would allocate an unnecessarily large portion of the River's assimilative capacity for these constituents and could violate the Antidegradation Policy. Instead, effluent limitations have been developed based on the amount of dilution that would be required, such that receiving water concentrations for these constituents would be met when effluent concentrations are at estimated maximum levels as determined by taking the mean plus 3.3-standard deviations or the maximum observed concentration, which ever is larger, for data sets with 10 or more values. For data sets with