Energy companies are using hydraulic fracturing, known also as fracking, to get oil from the state's Monterey shale reserve. The drilling technique involves the use of pressurized water mixed with sand and small amounts of chemicals to get at natural reserves trapped in shale.
The Natural Resources Defense Council said it had growing concerns about the practice.
"Fracking is happening here in California as we speak, presenting an immediate threat that requires urgent attention," NRDC California Legislative Director Victoria Rome said in a statement Tuesday.
Environmental groups say the drilling practice could contaminate drinking water supplies. They're pressing Gov. Jerry Brown to ban the practice. State Sen. Fran Pavley, a Democratic, proposed measures last week to impose tighter restrictions on drillers, the San Fransisco Chronicle said.