Quality of Groundwater Used for Public Supply in the Basin and Range Basin-Fill Aquifers, Southwestern U.S.

The Basin and Range basin-fill aquifers underlie a large area of the southwestern U.S. and are extensively and increasingly used for public supply. The aquifers consist primarily of thick unconsolidated to semi-consolidated gravel, sand, silt, and clay deposits in sediment-filled alluvial basins bounded by mountain ranges. In 2013, the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey began a sampling effort focused on the quality of groundwater used for public supply in principal aquifers across the nation. The Basin and Range basin-fill aquifers rank fourth among the nation’s principal aquifers for groundwater withdrawal for public supply. NAWQA sampled 78 public-supply wells in the Basin and Range basin-fill aquifers in Arizona, California, Idaho, Nevada, and Utah in 2013. Samples were analyzed for a comprehensive suite of water-quality constituents including major and trace elements (including hexavalent chromium), nutrients, pesticides, volatile organic compounds, radionuclides, microbial indicators, pharmaceuticals, hormones, and groundwater age tracers. The most common exceedances of water-quality benchmarks were for salinity-related constituents: 35% of samples had high concentrations of dissolved solids, chloride, and (or) sulfate, relative to secondary standards. The most common exceedance of human-health benchmarks for drinking water was for arsenic (13%). Exceedances for uranium (3%) and nitrate (0%) were low; these results contrast with previous NAWQA studies of shallow groundwater in these aquifers, where exceedances of human-health benchmarks for constituents such as arsenic, uranium, and nitrate occurred more frequently. Results are being evaluated to assess the relation of water quality with natural and human-related factors. Most constituents that have exceedances of water-quality or human-health benchmarks in the Basin and Range basin-fill aquifers are derived from geologic sources, and generally are observed at higher concentrations in older, more geochemically evolved groundwater.