SITE INFORMATION AND CORRESPONDENCE_2005-CURRENT

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Reclamation, but does not require those agencies to implement the 0.7 mmhos/cm EC objective until April 1, 2005. The City's monitoring reports from January 1998 through December 2002 show that the average EC of its effluent was 1099 micromhos per centimeter(µmhos/cm), the lowest monthly average was 819 µmhos/cm and the highest monthly average was 1300 µmhos/cm.16 The EC data collected at receiving water sample location R1 (located 100 feet upstream from the point of discharge) from January 2002 through December 2002 show that EC in the receiving water ranged from 380 µmhos/cm to 1100 µmhos/cm and averaged 686 µmhos/cm in 12 sampling events. Hourly EC data from the downstream Department of Water Resources Mossdale monitoring station (RSAN087) located near the City's outfall show that, from December 2000 through September 2002, the conductivity of the San Joaquin River ranged from 299 µmhos/cm to 1131 µmhos/cm and averaged 721 µmhos/cm. The EC data from the Vernalis station upstream of the City's discharge for between 1985 and 1998 showed frequent exceedances of EC water quality objectives. Based on the data cited, there is a reasonable potential for the City's effluent to contribute to a violation of the EC water quality objectives, and the Regional Board correctly determined that the City's permit should include EC effluent limitations. The Regional Board order establishes effluent limitations based on the 1995 Delta Plan water quality objectives for EC in the southern Delta. Until April 1, 2005, the year-round EC effluent limitation is 1000 µmhos/cm. On April 1, 2005,the EC effluent limitation for April I through August 31 is reduced to 700 µmhos/cm and it remains at 1000 µmhos/cm for the remaining months. At the time of the Regional Board proceeding, the City relied entirely upon groundwater sources for the water it delivers. The record establishes that the Cityis in the process of replacing a portion of its groundwater supplies with lower salinity surface water from the South San Joaquin Irrigation District.° The City estimates that the changes in its water supply will reduce the EC of the effluent it discharges to the river to an average of between 825 and 1026 µmhos/cm. The City further estimates that implementation of other source control 16 The effluent limitations for EC in the permit are expressed inµmhos/cm and the water quality objectives for EC in the Basin Plan are expressed in mmhos/cm. (1000 µmhos/cm are equal to 1.0 mmhos/cm) " The EC of the surface water supply is 100 µmhos/cm. 11.