CONSTRAINING NATURAL AND ANTHROPOGENIC CONTROLS ON BASE OF FRESHWATER AND UNDERGROUND SOURCE OF DRINKING WATER IN CENTRAL SAN JOAQUIN VALLEY, CALIFORNIA

Groundwater salinity trends in California’s Central San Joaquin Valley were evaluated using geophysical logs from oil and gas wells to determine the shallowest elevation of base freshwater (BFW), <3,000 parts per million (ppm) total dissolved solids (TDS), and the shallowest elevation of base of underground source of drinking water (BUSDW), <10,000 ppm TDS. Resistivity and lithologic logs from over 600 oil and gas wells were analyzed and kriging was used for interpolation between wells. The results reveal distinct salinity trends north and south of the Kings River drainage divide. In this southern area, BFW and BUSDW are relatively deep, reaching up to 1,800 meters (m) (6,000 feet (ft)) below ground surface (bgs) on the eastern margin, but rise sharply to near surface elevation in the Tulare Lakebed basin. North of the divide, BFW and BUSDW are shallower, particularly along a central ridge where these surfaces are about 450 m (1,500 ft) higher than at the margins. Along the western edge of the basin, BFW and BUSDW elevations vary greatly. In general, the observed salinity patterns across the study area are interpreted to result from spatially variable freshwater recharge from the Sierra Nevada and Coast Ranges. BFW and BUSDW depths follow the basement along the eastern margin of the study area. We interpret the shallow depth of saline water in the Tulare Lakebed basin to result from a combination of the low permeability Tulare Lake sediments limiting freshwater recharge and the natural filling and evaporation cycle of the closed lake basin. In general, BUSDW remains between 152 and 450 m (500–1,500 ft) below BFW, except in the westside subbasin, where historical (pre-1960s) groundwater pumping was substantial. We propose that historical groundwater pumping of fresh groundwater in the westside subbasin has caused the BFW to become shallower, but not BUSDW, as evidenced by an increased gap ~750 m (~2,500 ft) between the two surfaces. Shallow groundwater pumping is more likely to result in upward movement of the BFW than BUSDW because these groundwater wells target only shallower freshwater zones and low vertical permeability inhibits upward flow of the deeper, more saline water. Study results suggest the date of geophysical logs is not especially critical when mapping BUSDW, which may change little over time.