Two of the largest companies involved in natural gas drilling have acknowledged pumping hundreds of thousands of gallons of diesel-based fluids into the ground in the process of hydraulic fracturing, raising further concerns that existing state and federal regulations don't adequately protect drinking water from drilling.

Rep. Henry A. Waxman, D-Calif., who released the information in a statement Thursday, announced that the House Committee on Energy and Commerce, which he chairs, is launching an investigation into potential environmental impacts from hydraulic fracturing.

The process, which forces highly pressurized water, sand and chemicals into rock to release the gas and oil locked inside, gives drillers unprecedented access to deeply buried gas deposits and vastly increases the country's known energy reserves. But as ProPublica has detailed in more than 60 articles, the process comes with risks. The fluids used in hydraulic fracturing are laced with chemicals -- some of which are known carcinogens. And because the process is exempt from most federal oversight, it is overseen by state agencies that are spread thin and have widely varying regulations.

In 2004, the U.S. Environmental Protection Agency examined hydraulic fracturing and determined it can be safe as long as diesel fuel isn't added to the drilling fluids. The agency based its decision in part on a non-binding agreement it struck with the three largest drilling service companies -- Halliburton, Schlumberger and B.J. Services -- to stop using diesel. But the agreement applied only to gas drilling in a specific type of geologic formation: shallow coal deposits. The EPA study has since been widely criticized.

The information obtained by Waxman's group shows that B.J. Services violated that agreement and that Halliburton continued to use diesel in other geologic formations not governed by the agreement. All three companies acknowledged using other potentially harmful chemicals, such as benzene, toluene, ethylbenzene and xylene...continued....