CONSTRAINING NATURAL AND ANTHROPOGENIC CONTROLS ON BASE OF FRESHWATER AND UNDERGROUND SOURCE OF DRINKING WATER (USDW) IN NORTHERN SAN JOAQUIN VALLEY

Groundwater salinity trends in California’s Central Valley are poorly understood. This study identifies the shallowest elevation of brackish water (> 3,000 ppm or base of freshwater, BFW) and the shallowest elevation of saline water (>10,000 ppm or base of Underground Source of Drinking Water, USDW) in Fresno, Madera, Kings and Tulare counties of California’s San Joaquin Valley. Resistivity and lithology data from over 600 hydrocarbon well logs 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. North of this divide, rivers flow northwest towards the Sacramento/ San Joaquin delta. South of this divide, rivers flow into the Tulare Lake basin. In this southern area, BFW and USDW are deep, reaching up to 6,000 ft bgs on the eastern margin, but rise sharply to near surface elevation in the Tulare Lake Basin. North of the divide, BFW and USDW are shallower, with a central ridge of shallow groundwater about 1,500 ft higher than the margins. Along the western edge of the basin BFW and USDW elevations vary greatly, with documented salinity reversals. In general, the observed patterns across the study area are interpreted to result from freshwater recharge from the Sierra Nevada and Coast Ranges. BFW and USDW depths follow the basement along the eastern margin of the study area. We interpret the shallow depth of saline water in the Tulare Lake basin to result from a combination of the low permeability Corcoran clay limiting deep freshwater recharge and the natural filling and evaporation cycle of the closed basin. In general, USDW remains between 500 and 1,500 ft below BFW, except along the western edge of the study area in the Westside subbasin, where historical (pre-1960s) groundwater pumping was significant. We propose that historical groundwater pumping in the Westside subbasin has disrupted the natural BFW, but not USDW, as evidenced by an increased gap (~2,500 ft) between the two surfaces in this region. Shallow groundwater pumping is more likely to disrupt the BFW than USDW as low vertical permeability dampens the effect of shallow pumping at greater depths. The study supports the use of geophysical logs to determine possible locations for class VI wells since USDW is a fairly static interface that is less affected by groundwater pumping.