In normal years, California residents get about 30 percent of their drinking water from underground aquifers. And in droughts like the current one—with sources like snowmelt from the Sierra Nevada mountains virtually non-existent—groundwater supplies two-thirds of our most populous state's water needs. So it's sobering news that about 20 percent of the groundwater that Californians rely on to keep their taps flowing carries high concentrations of contaminants like arsenic, uranium, and nitrate.
That's the conclusion of a ten-year US Geological Survey study of 11,000 public-water wells across the state. The researchers tested the wells for a variety of contaminants, looking for levels above thresholds set by the Environmental Protection Agency and/or the California State Water Resources Board.