Groundwater quality in the Western San Joaquin Valley study unit, 2010: California GAMA Priority Basin Project

Abstract

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Groundwater quality in the western San Joaquin Valley, California

“…The Western San Joaquin Valley (WSJV) study unit covers approximately 5,600 square miles and includes the Delta-Mendota and Westside subbasins of the San Joaquin Valley groundwater basin (California Department of Water Resources, 2003;Fram, 2017). The freshwater aquifer system in the WSJV study unit primarily is composed of late Tertiary to Quaternary age alluvial and fluvial sediments derived from the Coast Ranges to the west and the Sierra Nevada to the east, and is divided into upper and lower zones by a clay-rich lacustrine unit, the Corcoran Clay (Faunt, 2009).…”

“…Benzene and tetrachloroethene (PCE) were the VOCs found at high concentrations. The likely source of the benzene was nearby natural petroleum deposits (Fram, 2017). PCE is a commonly used solvent.…”

“…Low concentrations include non-detections and values less than moderate concentrations. Methods for evaluating water quality are discussed by Fram (2017).…”

“…Factors that Affect Groundwater Quality The source of groundwater recharge is one of many factors that can affect groundwater quality. In the WSJV study unit, stable oxygen and hydrogen isotope ratios of water were used to distinguish between groundwater derived from recharge of runoff from the Sierra Nevada (delta oxygen-18 less than -9.5 per mil) and from the Coast Ranges (delta oxygen-18 greater than -8.5 per mil) (Dubrovsky and others, 1991;Fram, 2017). Groundwater from Sierra Nevada runoff generally has lower concentrations of total dissolved solids (TDS), and is found in some wells on the eastern side of the study unit (near the San Joaquin River and Fresno Slough).…”

“…Runoff from watersheds dominated by the Franciscan Complex (mostly metamorphic rock) generally has low TDS concentrations, whereas runoff from watersheds dominated by marine sedimentary deposits generally has high TDS and sulfate concentrations. The sulfate is derived from oxidation of biogenic pyrite in organic-rich, reduced marine shales (Presser and others, 1990;Fram 2017).…”