More than a fifth of the private domestic wells tested nationally as part of a federal study had at least one contaminant at worrisome levels.

A sampling of water from private wells in 48 states including California by the U.S. Geological Survey found that most of the pollutants "of potential health concern" were naturally occurring -- such as arsenic and radon.

Nitrate -- associated with human activities such as fertilizer use, livestock operations and septic systems -- was the exception. It was found in elevated concentrations most commonly in agricultural regions, such as California's Central Valley and the Midwest Corn Belt.

Nationally, the USGS says about 43 million people draw water from private domestic wells, which are not regulated by the federal Safe Drinking Water Act or by California. However some counties, including Los Angeles, do regulate them.

The State Water Resources Control Board estimates 1.2 million to 1.5 million Californians get their household water from private wells.

--Bettina Boxall