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California ''.egional Water Quality Corw•ol Board Central Valley Region QW; Karl E. Longley,ScD,P.E.,Chair a:% Linda Adams 11020 Sun Center Drive#200,Rancho Cordova,Califomia 95670-6 1 1 4 _ Arnold Secretaryfor Phone(916)464.3291 •FAX(916)464-4645 (=� $(chwavernegger Environmental hup//www waterboards.ca.gov/central valley Governor Protection MAR 1 0 2009 4 March 2009 ENVIRONMENT HEALTH PERMIT/SERVICES Nader Shareghi, P.E. Mountain House CSD 230 S. Sterling Drive, Suite 100 Mountain House, CA 95391 DETERMINATION ON NEED FOR WDRS, MOLWTAIN HOUSE CSD WATER TREATMENT PLANT, SAN JOAQUIN COUNTY, SAN JOAQUIN COUNTY On 14 September 2007, Mountain House Community Services District (CSD) submitted a Report of Waste Discharge (RWD), to apply for Waste Discharge Requirements (WDRs) for the newly constructed Mountain House Water Treatment Plant (WTP). The WTP is owned by the Mountain House CSD and is located at 18045 W. Kelso Road, Mountain House. The WTP is designed to produce up to 15 million gallons per day (mgd) of potable water and can be expanded to an ultimate capacity of 20 mgd. Wastewater is generated at a rate of approximately 5 percent of the chlorinated potable water production rate by back-flushing sand filters. At a potable water production rate of 20 mgd, approximately 1 mgd of wastewater will be generated. Wastewater is discharged to concrete lined settling lagoons. In addition to the backwash wastewater, groundwater can be discharged to the lined settling lagoons to prevent damage from high groundwater. The concrete lined settling lagoons act as the wastewater treatment system for the WTP. Solids are allowed to settle out of the wastewater and supernatant water is recycled back to the WTP where it is treated to produce potable water. If needed, the wastewater system allows wastewater to be discharged to the Mountain House domestic wastewater system. In normal operation, settled solids are dried in the concrete lined lagoons, removed, and disposed of at a landfill. The use of the concrete lined lagoons will not cause groundwater mounding, since the supernatant water is removed from the concrete lined lagoons regularly and there is limited leakage through the liners. The discharges to the concrete lined lagoons are not expected to cause trihalomethane impacts to groundwater because the low permeability of the concrete liners prevents a significant volume of wastewater from escaping into the subsurface. Depending on the season, the raw water delivered to the water treatment plant will contain a Total Dissolved Solids (TDS) concentration that will vary between 104 and 409 mg/L. That range of concentrations is significantly less than the upgradient TDS groundwater concentration (986 mg/L). The water treatment process will add insignificant amounts of dissolved solids to the raw water concentration. This indicates that the wastewater discharged to the concrete lined lagoons is unlikely to impact groundwater. California Environmental Protection Agency p�4 Rec}rled Paper