The U.S. Environmental Protection Agency is initiating a comprehensive study on the potential health and water quality impacts of hydraulic fracturing, a common technique used to extract natural gas by injecting a mixture of water, chemicals and sand into underground coal beds.

Environmentalists have long criticized the lack of oversight of the technique, which they say has the potential to contaminate drinking water. In a 2004 study, the EPA concluded that hydraulic fracturing didn’t pose such a threat, but the research was criticized for not including water tests.

ProPublica reports:

The use of hydraulic fracturing has significantly increased well beyond the scope of the 2004 study,” EPA spokeswoman Enesta Jones wrote in response to questions from ProPublica. The old study, she said, did not address drilling in shale, which is common today. It also didn’t take into account the relatively new practice of drilling and hydraulically fracturing horizontally for up to a mile underground, which requires about five times more chemical-laden fluids than vertical drilling.