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Establishing the base of underground sources of drinking water (USDW) using geophysical and chemical reports in the southern San Joaquin Basin, CA /
Recent concerns about well stimulation and oilfield disposal practices has resulted in the desire to learn more about the distribution of usable groundwater that might be impacted by these practices. Waters that require protection are classified by the United States Environmental Protection Agency as USDWs (Underground Sources of Drinking Water, defined by EPA regulations at 40 CFR 144.3). These waters have a concentration of 10,000 parts‐per‐million (ppm) total dissolved solids (TDS) and are not within an exempt aquifer. Direct sampling and chemical analyses of the water from oil and gas producing formations provide the most accurate values for the formation water salinities, but the data is scarce. There were 1,017 chemical reports recorded for the southern San Joaquin Basin. The method in this analysis uses open‐hole geophysical logs and Archie’s equation to calculate the salinity. The two methods used in the analysis are the spontaneous potential (SP) method that uses the spontaneous potential log and the mud and formation resistivity values to calculate a salinity, and the resistivity‐porosity (RP) method that uses the resistivity and porosity logs. The SP method was performed on 110 wells, and the RP method was performed on 51 wells. The low number of RP method calculations was due to the lack of porosity logs in the interested interval. The RP method was chosen to calculate the USDW due to its lower error (0.275) compared to the SP method (0.704) and better correlation between the tested and calculated salinities (0.807 for the RP method compared to 0.467 for the SP method). The USDW was calculated or 182 wells in the southern San Joaquin Basin. A deeper USDW exists on the eastern margin (> 5,000ft below sea level) near the Sierra Nevada Mountains that shallows westward toward the Coast Ranges where the USDW boundary is near surface. The shallowing of the USDW is more gradual along the Bakersfield Arch, although the USDW remains deep well into the Tejon Sub‐basin.
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