The USGS National Water Quality Assessment program, in collaboration with the California Groundwater Ambient Monitoring and Assessment Program, recently published a comprehensive assessment of the quality of untreated groundwater used as a source for public supply in the continental United States. The national study found that naturally occurring chemical elements were present in drinking water sources at concentrations above regulatory thresholds more often than anthropogenic (human-induced) contaminants. California principal aquifers were included in the national study, the Central Valley aquifer, the Basin and Range aquifer, and the California Coastal Basins aquifer. 

A similar result was found in an earlier statewide analysis of California’s groundwater quality completed in 2015 for the Groundwater Ambient Monitoring and Assessment program (GAMA), a collaborative effort by the USGS and the California State Water Resources Control Board. 

When raw groundwater is above regulatory thresholds, it requires treatment—which can be expensive—or mixing with cleaner sources. Aquifer conditions can thus constrain drinking water supply reliability. 

The national study was published in the journal Environmental Science & Technology – Water. 

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