BERKELEY -- Worried that the state's fresh food supply is in danger, Berkeley chef Alice Waters and at least 100 others have signed a petition urging Gov. Jerry Brown to put a moratorium on hydraulic fracking for oil and gas extraction.

Fracking uses high-pressure water, chemicals and sand to fracture underground rock where oil and gas reside. Opponents say the practice wastes precious fresh water and contaminates existing groundwater with chemicals that then migrate to underground drinking and agricultural water supplies.

Waters, owner of Chez Panisse, is currently in Rome and unavailable for comment. But Naomi Starkman, a food activist who approached Waters on the project, said on Thursday that "chefs care about fracking because they care about where their food comes from. Fracking uses a lot of fresh water and chemicals and we're trying to extract this oil from farmland."

Oil companies for years have used fracking in California in Kern County, the Los Angeles and Long Beach area, said Adam Scow, California director of Food and Water Watch, a consumer advocacy organization which is helping collect the signatures for the petition.

"Now oil companies are buying up leases in the Central Valley from Los Angeles up to Santa Cruz County and in the Salinas Valley, where there is the potential to extract 13 billion barrels of oil," Scow said. "Fracking is allowing oil companies to go to all these old oil fields that were thought to be tapped out."

Tupper Hull, a spokesman for the Western States Petroleum Association, said California chefs are forgetting who to thank when they cook their food.

"There is a certain irony here because these chefs, unless they are cooking with wood, coal or hydrogen, which I'm pretty sure they are not, are using a great deal of natural gas," Hull said. "This country is blessed with an abundance of natural gas, and prices are at record lows because of the technology of hydraulic fracturing."

Hull said that despite what the chefs say, "this technology has never been known to have contaminated groundwater."

The chef's petition to get Brown to put a moratorium on fracking started Wednesday. In addition to Waters, it is signed by John Clark, a chef at Foreign Cinema in San Francisco, and chefs at Oliveto and Pizzaiolo in Oakland, among others.

Contact Doug Oakley at 510-843-1408. Follow him on Twitter.com/douglasoakley.