In this Nov. 14, 2006 photo, a farm worker adjusts the irrigation system that borders the Sonny Bono Salton Sea National Wildlife Refuge in Calipatria, Calif. A U.S. Geological Survey released gives public-policy-makers the first sweeping look at the extent to which agricultural irrigation, industrial pollutants and other uses of groundwater are adding to problems for underground water reserves, now under heavy demand in California's drought.  Credit: Chris Carlson/ AP

In this Nov. 14, 2006 photo, a farm worker adjusts the irrigation system that borders the Sonny Bono Salton Sea National Wildlife Refuge in Calipatria, Calif. A U.S. Geological Survey released gives public-policy-makers the first sweeping look at the extent to which agricultural irrigation, industrial pollutants and other uses of groundwater are adding to problems for underground water reserves, now under heavy demand in California’s drought.
Credit: Chris Carlson/ AP

SAN FRANCISCO — Nearly one-fifth of the raw groundwater used for public drinking water systems in California contains excessive levels of potentially toxic contaminants, according to a decade-long U.S. Geological Survey study […]

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