Predicted arsenic concentrations in Southwest basin-fill aquifers

Predicted concentrations of arsenic in groundwater exceed the Maximum Contaminant Level (MCL) of 10 μg/L in 43 percent of the areal extent of the four Southwest basin-fill aquifers. The Basin and Range basin-fill aquifers in Nevada, Utah, southern California, and western Arizona encompass the largest area where arsenic concentrations are predicted to exceed the MCL, largely due to the presence of volcanic rocks, the arid to semiarid climate, and many closed basins (basins with no surface-water outflow).  As published in USGS Circular 1360 (Water quality in principal aquifers of the United States, 1991-2010) and modified from Anning and others, 2012.