Oil and gas operators have been ordered to close 12 injection wells in California. Regulators said they were concerned that the wastewater disposal wells could affect the quality of groundwater.

The state Division of Oil, Gas and Geothermal Resources revealed that 10 of the wells have been shut down voluntarily, while cease-and-desist orders were issued for the remaining two. All the wells are located in Kern County, are within a mile of the surface and 500 vertical feet underground of a water supply, the Los Angeles Times reported.

According to Jonathan Bishop, chief deputy director of the State Water Resources Control Board, there is no evidence that drinking water has been contaminated.

State Oil and Gas Supervisor Steven Bohlen said that the wells are being closed "out of an abundance of caution for public health."

The news comes a week after it was revealed that more than 300 unlined wastewater pits were being used in Kern County without proper permits. Some of these are used to dispose of wastewater produced during hydraulic fracturing.

The U.S. Geological Survey (USGS) said that more data and research were needed to best understand the potential risks to water quality associated with unconventional oil and gas development in the United States.

The agency searched through the national water-quality databases from 1970-2010 but was only able to assess long-term trends in 16 percent of the watersheds with unconventional oil and gas resources.

"There are not enough data available to be able to assess potential effects of oil and gas development over large geographic areas," said USGS scientist Zack Bowen.