Regulators in California ordered 12 oil and gas wells in Central Valley to stop drilling over growing concerns that groundwater near the wells may become contaminated, ThinkProgress reported.

California Resources Corporation, E&B Natural Resources Management, and Chevron USA voluntarily shut down production at ten wells, while Modus, Inc. and Western States International, Inc. were served with cease-and-desist orders to force them to stop drilling.

No evidence of contamination has been found in the groundwater yet, but as the wells “are located within a mile of the surface and within 500 feet of water supply,” the threat of contamination is great.

“As we’ve said before, the protection of California’s groundwater resources — as well as public health — is paramount, especially in this time of extreme drought,” said Steven Bohlen, head of oil, gas, and geothermal resources for the California Department of Conservation (CDOC), in a statement. “Halting injection into these wells is a significant step towards that goal.”

As the CDOC pointed out, “drilling for oil and gas yields far more water than oil.” Last year, 3.3 billion barrels of water were produced during drilling efforts in California compared to 205.3 million barrels of oil.

The water produced during drilling is “unsuitable for human use … [and] must be disposed of.” The brackish water is usually “injected back into the reservoir from which it was produced, either for enhanced oil recovery or to maintain reservoir pressure to prevent subsidence.”

This is not the same thing as the water that comes about from fracking, Bohlen explained.

“To be clear, in standard oil and gas operations, the producers skim off the oil and reinject the water back where it came from. So in some cases, what goes back underground is cleaner than what came out. Still, out of an abundance of caution, we want to be sure that no injection is taking place into zones containing water that could, with treatment, be used for human activities.”

The California Environmental Protection Agency also announced earlier this week that it had found more than 2,500 cases in which “the state authorized oil field injections into protected water aquifers that could be used for drinking or irrigating crops,” according to TP. Of the wells that were making those injections, 2,100 of them are still active, and the state has labeled 176 of those as “high priority concerns.”

Environmental organizations in California have filed a legal petition asking Gov. Jerry Brown to shut down these illegal injection and wastewater sites and to place a “moratorium on fracking and other well stimulation techniques.” Given California’s current record-breaking drought, which isn’t likely to subside any time soon, the effects of water contamination are amplified. Any measures to protect what little drinkable water California does have are paramount.