Water Quality data compiled for Groundwater development leads to decreasing arsenic concentrations in the San Joaquin Valley, California

Groundwater arsenic concentrations in the San Joaquin Valley have varied over the decades from 1980 to 2019. This report was compiled to determine whether arsenic concentrations are increasing or decreasing and the mechanism controlling the trends. The San Joaquin Valley contains 4,979 wells with arsenic analyses and possible co-detections of any of the following constituents: dissolved oxygen, field-measured pH, iron, manganese, sulfate, nitrate, or water level. Water quality data comes from two sources: 3,302 wells from with California State Water Resources Control Board - Division of Drinking Water and 1,448 wells from the U.S. Geological Survey National Water Information System (California State Water Resources Control Board ? Division of Drinking Water, 2019; U.S. Geological Survey, 2020). There are an additional 229 wells with data from both sources. Other data compiled in addition to the constituents analysed are well type, water use, status, and depth. Well location in relation to the regions defined in the study unit, the El Nido and Pixley subsidence areas, and lateral position from the valley center were also collected (Hansen et al., 2018; Faunt and Sneed, 2015; Faunt, 2009; Voss et al., 2019). The co-detections of constituent trends with arsenic trends was used to determine possible mechanisms controlling arsenic variability in addition to the location and depth of the wells.