FRESNO, Calif. — In a trailer park tucked among irrigated orchards that help make California’s San Joaquin Valley the richest farm region in the world, 16-year-old Giselle Alvarez, one of the few English-speakers in the community of farmworkers, puzzles over the notices posted on front doors: There’s a danger in their drinking water.

Uranium, the notices warn, tests at a level considered unsafe by federal and state standards. The law requires the park’s owners to post the warnings. But they are awkwardly worded and in English, a language few of the park’s dozens of Spanish-speaking families can read.

“It says you can drink the water — but if you drink the water over a period of time, you can get cancer,” said Alvarez, whose working-class family has no choice but keep drinking and cooking with the tainted tap water daily, as they have since Alvarez was just learning to walk. “They really don’t explain.”

Uranium, the stuff of nuclear fuel for power plants and atom bombs, increasingly is showing in drinking water systems in major farming regions of the U.S. West – a naturally occurring but unexpected byproduct of irrigation, of drought, and of the overpumping of natural underground water reserves.

An Associated Press investigation in California’s central farm valleys —along with the U.S. Central Plains, among the areas most affected — found authorities are doing little to inform the public at large of the growing risk.