FRESNO (AP) — A dozen wells used to pump oil-and-gas in California’s Central Valley have been ordered to stop production to protect underground drinking-water from contamination, officials said Tuesday.

The operators of 10 oil wells in Kern County voluntarily stopped production, while two were issued cease-and-desist orders, said Steven Bohlen, head of oil, gas and geothermal resources for the California Department of Conservation.

Groundwater surrounding the wells will be tested for traces of contamination.

The action came after a review found more than 2,500 instances when the state authorized the injection of oilfield waste into protected water aquifers that could be used for drinking water supplies or crop irrigation.

In addition, California — the nation’s leading agricultural state — enters a fourth drought year with farmers relying heavily on scarce underground water supplies.

“It doesn’t necessarily mean that the aquifers have been spoiled,” Bohlen said in a telephone call with reporters.

He said another 11 wells were shut down last year, and so far testing has found no evidence of contamination.

The state’s Environmental Protection Agency says in a separate report that groundwater was injected with oilfield waste over the years due to confusion, lax oversight and miscommunication among government regulators.