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INFORMATION SHEET • • 13 <br /> WASTE DISCHARGE REQUIRMENTS ORDER NO.R5-2002-0213 <br /> U.S.DEPARTMENT OF DEFENSE <br /> DEFENSE LOGISTICS AGENCY <br /> DEFENSE DISTRIBUTION DEPOT SAN JOAQUIN <br /> SHARPE GROUNDWATER REMEDIATION SYSTEM <br /> SAN JOAQUIN COUNTY <br /> Arsenic <br /> Previous Order No. 95-258 included a monthly median effluent limitation for arsenic of 40 µg/L <br /> (ppb) and a daily maximum effluent limitation of 50 µg/L(ppb) to avoid potential health impacts <br /> from discharge to an empty canal. The Basin Plan states that waters shall not contain chemical <br /> constituents in concentrations that adversely affect beneficial uses. At a minimum, water <br /> designated for use as domestic and/or municipal supply shall not contain concentrations of <br /> chemical constituents in excess of maximum contaminant levels (MCLs). The new USEPA <br /> primary MCL for arsenic is 10 pg/L (ppb). The compliance date for water purveyors to meet the <br /> new MCL is 23 January 2006. Federal regulations at 40 CFR Section 122.44(d)(1)(vi)(A) allow <br /> the state to establish effluent limitations using an explicit state policy interpreting its narrative <br /> objectives. Use of the USEPA Primary MCL is appropriate to implement the narrative chemical <br /> constituent objective. As shown in Attachment C, results of effluent monitoring of the three <br /> GWTPs indicates concentrations of arsenic in the final effluent range from 13 µg/L(ppb) to a <br /> maximum recorded value of 21 µg/L(ppb) (North Balloon GWTP). <br /> As noted previously, the SSJID Canal, absent the discharge, is at times a seasonal and/or <br /> ephemeral waterbody. The effluent flow from the DDJC-Sharpe outfall to the SSJID Canal may <br /> represent, at times, most of or all of the flow in the SSJID Canal. Considering these flow <br /> conditions and the results of effluent monitoring,the SSJID Canal at times likely contains <br /> concentrations of arsenic that exceed the new USEPA primary MCL of 10 µg/L (ppb), potentially <br /> impacting the municipal and domestic supply beneficial uses of the SSJID Canal and French Camp <br /> Slough, depending upon flow conditions and water quality characteristics of the Canal and the <br /> Slough. Considering the chemical constituent objective of the Basin Plan, the new human health <br /> criterion(MCL) for arsenic, flow conditions in the SSJID Canal, and the municipal and domestic <br /> supply beneficial use of the SSJID Canal, the discharge has the reasonable potential to exceed a <br /> water quality standard. Additional study of the flow quality and quantity characteristics of the <br /> SSJID Canal and French Camp Slough would be needed to determine if an alternative final <br /> effluent limitation for arsenic may be appropriate. <br /> Regarding compliance with water quality standards for arsenic, three options are available to the <br /> Discharger: <br /> a. Dilution/Mixing Zone Study <br /> In establishing and determining compliance with effluent limitations for applicable human <br /> health criteria, the Regional Board may consider mixing zones and dilution credits. As <br /> stated in the USEPA's March 1991 Technical Support Document for Water Quality Based <br /> Toxics Control (TSD), criteria or guidance consist of three components, including pollutant <br /> magnitude, in-stream concentration duration, and the frequency of criteria exceedance. <br /> Regarding the application of a mixing zone for protection of human health, the TSD states: <br /> ...the presence of mixing zones should not result in significant health risks, when evaluated using <br /> reasonable assumptions about exposure pathways. Thus, where drinking water contaminants are a <br />