Laserfiche WebLink
WASTE DISCHARGE REQUIRIENTS ORDER NO. R5-2002-0213 • 11 <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 /> French Camp Slough, depending upon flow conditions and water quality characteristics of the <br /> Canal and the Slough. Considering the chemical constituent objective of the Basin Plan, the new <br /> human health criterion (MCL) for arsenic, flow conditions in the SSJID Canal, and the municipal <br /> and domestic supply beneficial use of the SSJID Canal, the discharge has the reasonable <br /> potential to exceed a water quality standard. Additional study of the flow quality and quantity <br /> characteristics of the SSJID Canal and French Camp Slough would be needed to determine if an <br /> alternative final 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 stated <br /> in the USEPA's March 1991 Technical Support Document for Water Quality Based Toxics <br /> 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 /> concern, mixing zones should not encroach on drinking water intakes. " Before establishing a <br /> mixing zone, the amount of receiving water available to dilute the discharge must be <br /> determined. Dilution credit is a numerical value associated with the mixing zone that <br /> accounts for the receiving water entrained in the discharge, and is a value used in the <br /> calculation of effluent limitations. For continuous discharges and/or year-round mixing <br /> zones, the mixing zone and dilution credit for human health criteria is developed considering <br /> the harmonic mean flow as the critical receiving water flow. <br /> At present, sufficient receiving water information regarding the quality and quantity of flows <br /> in the SSJID Canal and French Camp Slough is not available to adequately assess ambient <br /> dilution water and mixing zone characteristics needed to assess the need for, and calculate, if <br /> necessary, alternative final effluent limitations for arsenic. This Order includes provisions <br /> for completion of a Dilution/Mixing Zone Study that may be appropriate in developing <br /> alternative final effluent limitations for arsenic. Based upon the results of the study required <br /> by this Order,this Order may be reopened, and alternative final effluent limitations may be <br /> established to include appropriate human health criteria for arsenic considering pollutant <br /> attenuation provided by dilution and a mixing zone. <br /> b. Basin Plan Amendment Process <br /> Concurrent with the Dilution/Mixing Zone Study, the Discharger may provide information to <br /> the Regional Board to support a Basin Plan amendment to de-designate the municipal and <br /> domestic supply(MUI) beneficial use of the SSJID Canal based on the exceptions to <br /> SWRCB Resolution 88-63. If,through this process, the municipal and domestic supply <br />