The presence of contaminants in a source water can constrain its suitability for drinking. The quality of groundwater used for public supply was assessed in 25 Principal Aquifers that account for 84 percent of groundwater pumped for public supply in the entire United States (89.6 million people on a proportional basis). Each Principal Aquifer was sampled across its lateral extent using an equal-area grid, typically with 60 wells per Principal Aquifer. Samples were analyzed for 502 constituents, of which 374 had either a regulatory or non-regulatory human-health benchmark. 

Nationally, elevated concentrations (relative to the human-health benchmarks) of geogenic constituents have a larger effect on the quality of groundwater used for public supply than anthropogenic constituents as indicated by three metrics: detection frequency; prevalence (based on area); and population potentially affected. In particular, elevated concentrations of unregulated geogenic constituents are broadly prevalent and, on a proportional basis, potentially affect 18.5 million people using groundwater from the 25 Principal Aquifers evaluated in this study. These Principal Aquifers account for 84 percent of the groundwater used for public supply in the United States. Additional attention to unregulated constituents may be warranted.  

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