Quality of Groundwater Used for Public Supply in the Continental United States: A Comprehensive Assessment

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 (PAs) that account for
84% of groundwater pumped for public supply in the U.S. (89.6 million
people on a proportional basis). Each PA was sampled across its lateral
extent using an equal-area grid, typically with 60 wells per PA. Samples
were analyzed for 502 constituents, of which 374 had either a regulatory
or nonregulatory human health benchmark (HHB). Nationally, elevated
concentrations (relative to HHBs) of geogenic constituents have a
larger effect than anthropogenic constituents, as indicated by three
metrics: detection frequency, 35% versus 8.1%; prevalence (based on
area), 41% versus 6.4%; and population potentially affected, 31.2
million versus 7.1 million. Prevalence of any constituent at elevated
concentrations was high40 to 75%in PAs comprising
unconsolidated sediment (eight PAs) and sandstone or interbedded sandstones
and carbonates (four PAs) in the West and Central Interior. Prevalence
was lower15 to 35%in PAs comprising sediment and sedimentary
rocks along the Gulf and Atlantic Coasts (four PAs), carbonates distributed
across the continental U.S. (seven PAs), and hard rock (two PAs).