With surface water supplies decimated from the ongoing drought, more Californians are forced to use groundwater.
But groundwater is unsafe for nearly 800,000 residents, according to the state's water resources control board.
This problem is the longtime contamination from nitrates and arsenic.
That's meant less drinkable water in California's struggle to survive more than three years of severely dry weather.
"Most areas affected by contamination don't have surface water supplies so they have to find new groundwater sources," said Kurt Souza, a branch chief of the division of drinking water at the California State Water Resources Control Board.
"But that's not always easy to do," Souza added. "Sometimes you can find new ground locations for water and sometimes you can't."
The lack of rain and subsequent heavy demand on ground wells—which are also facing supply problems—is making a bad situation worse, said Sara Aminzadeh, executive director of the California Coastkeeper Alliance, a statewide advocacy group for safe water.
"Contamination is a major concern," Aminzadeh said. "The drought just exacerbates water issues, and the groundwater problem is one of those."