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SPX CORPORATION ORDER R5-2019-0018 SPX MARLEY COOLING TECHNOLOGIES GROUNDWATER CLEANUP NPDES NO. CA0081787 approximately October to April each year, the East Stockton Water District dams the Calaveras River at its fork with Upper Mormon Slough, diverting flows through Upper Mormon Slough and the Stockton Diverting Canal. The Discharger collects background receiving water data upstream at RSW-001 located 7500 ft upstream from discharge point, and downstream at station RSW-002 located 1450 ft from discharge point (Figure B-1). Flow data for the receiving water is obtained by the Discharger from the California Data Exchange Center, which reports flow data from a US Army Corps of Engineers monitoring station (Station MRS) located approximately 13 miles upstream from the discharge point (Figure B-2). With the agricultural use along the canal it is unlikely the flows measured 13 miles upstream of the discharge are representative of flows at the discharge. Therefore, conducting the RPA using the TSD mass balance approach, which considers flow and dilution is not feasible. Instead, RPA was conducted by evaluating quarterly TDS data measured in the canal at receiving water monitoring stations RSW-001 and RSW-002 to determine if the discharge is causing or contributing to an exceedance of the applicable water quality objectives that protect MUN and AGR beneficial uses. MUN Beneficial Use. Title 22, section 64449 of the California Code of Regulations requires that secondary MCLs for TDS be applied on an annual basis. The annual average concentrations of TDS in the receiving water downstream of the discharge point are consistently below the secondary MCL of 500 mg/L (see Table F-8 below). This demonstrates there is no reasonable potential for TDS based on the MUN beneficial use of the Stockton Diverting Canal. Table F-8. Downstream TDS Concentration (Annual Averages) Year Ann.Avg. Conc. (mg/L) 2017 91 2016 168 2015 (1) 2014 173 (1) No flow in the Stockton Diverting Canal. AGR Beneficial Use. From approximately April to October each year, flows are split between the Calaveras River and Upper Mormon Slough. A series of check dams are installed along the Calaveras River, Upper Mormon Slough, and the Stockton Diverting Canal to provide irrigation water for farmers adjacent to the Stockton Diverting Canal. The agricultural use of water from the Stockton Diverting Canal is by request only, meaning that in dry summer months the farmers can request water be let into the canal for their use. When this occurs, there is sufficient dilution and assimilative capacity for TDS in the canal. As shown in Table F-9 above, the TDS downstream of the discharge is typically below applicable objectives. In Table F-9 the only year where TDS levels were elevated was in 2015. During this period the Discharger reported no flow at RSW-001, so TDS was not measured at RSW-001. The downstream TDS concentrations during that period are similar to the ATTACHMENT F— FACT SHEET F-24