Oil and gas service companies injected tens of millions of gallons of diesel fuel into onshore wells in more than a dozen states from 2005 to 2009, Congressional investigators have charged. Those injections appear to have violated the Safe Water Drinking Act, the investigators said in a letter to the Environmental Protection Agency on Monday.

The diesel fuel was used by drillers as part of a contentious process known as hydraulic fracturing, or fracking, which involves the high-pressure injection of a mixture of water, sand and chemical additives  including diesel fuel  into rock formations deep underground. The process, which has opened up vast new deposits of natural gas to drilling, creates and props open fissures in the rock to ease the release of oil and gas.

But concerns have been growing over the potential for fracking chemicals  particularly those found in diesel fuel  to contaminate underground sources of drinking water.

“We learned that no oil and gas service companies have sought  and no state and federal regulators have issued  permits for diesel fuel use in hydraulic fracturing,” said Representative Henry A. Waxman of California and two other Democratic members of the House Committee on Energy and Commerce, in the letter. “This appears to be a violation of the Safe Drinking Water Act.”