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San Joaquin River Hydrologic Region California's Groundwater San Joaquin Valley Groundwater Basin Bulletin 118 Groundwater Budget(Type C) There is insufficient published data available to provide a groundwater budget for this subbasin. Groundwater Quality Characterization. In general,the northern.part of the subbasin is characterized by a sodium water type and the southern part of the subbasin is characterized by calcium-sodium type water(Sorenson 1981). The northern part of the subbasin is also characterized by a wide range of anionic water types including: bicarbonate; chloride; and mixed bicarbonate-chloride types. Major anions in the southern part of the subbasin include sulfate-chloride and bicarbonate-chloride. Dissolved solids concentrations in well water sampled in San Joaquin and Contra Costa Counties ranged from 50 to 3,520 mg/L, with a mean of 463 and medium of 269(Sorenson 1981). The highest TDS values were found in the central and western portion of the USGS study area which,in general, corresponds with the limits of the Tracy Subbasin. Based on analyses of 36 water supply wells in the subbasin,TDS ranges from 210 to 7,800 mg/L and averages about 1,190 mg/L. Impairments. Areas of poor water quality exist throughout the subbasin. Areas of elevated chloride occur in several areas including: along the western side of the subbasin; in the vicinity of the City of Tracy; and along the San Joaquin River. Areas of elevated nitrate occur in the northwestern part of the subbasin and in the vicinity of the City of Tracy. Areas of elevated boron occur over a large portion of the subbasin from south of Tracy and extending to the northwest side of the subbasin. Water Quality in Public Supply Wells . . Constituent Group Number of Number of wells with a . . . wells sampledZ concentration above an MCLS Inorganics-Primary 34 4 Radiological 39 2 N itrates 36 2 Pesticides 36 2 VOCs and SVOCs 37 0 Inorganics-Secondary 34 . . 18 'A description of each member in the Constituent groups and a generalized discussion of the relevance of these groups are included in California's Groundwater -Bulletin 418 by DWR(2003). Represents distinct number of wells sampled as required under DHS Title 22 program from 1994 through 2000. Each well reported with a concentration above an MCL was confirmed with a second detection above an MCL.. This information is intended as an indicator of the types of activities that cause contamination in a given basin. It represents the water quality at the sample location. It does not indicate the water quality delivered to the consumer. More detailed drinking water quality information can be obtained from the local water purveyor and its annual Consumer Confidence Report. Last update 1120106