Cal. PRC § 3220

The owner or operator of any well on lands producing or reasonably presumed to contain oil or gas shall properly case it with water-tight and adequate casing, in accordance with methods approved by the supervisor or the district deputy, and shall, under his direction, shut off all water overlying and underlying oil-bearing or gas-bearing strata and prevent any water from penetrating such strata. The owner or operator shall also use every effort and endeavor to prevent damage to life, health, property, and natural resources; to shut out detrimental substances from strata containing water suitable for irrigation or domestic purposes and from surface water suitable for such purposes; and to prevent the infiltration of detrimental substances into such strata and into such surface water.