Gas and oil companies have deflected congressional inquiries about whether they are drilling near underground drinking water sources and how they are disposing of the chemical-laden wastewater their operations produce, according to a news release issued by Rep. Henry Waxman, D-Calif., and Rep. Edward Markey, D-Mass.

The congressmen, both members of the House Energy and Commerce Committee, tried to get answers to these questions by sending letters to 14 oil and gas service companies that use a controversial drilling technique called hydraulic fracturing. The process involves pumping millions of gallons of water mixed with sand and chemicals deep underground to break rock and release the gas beneath. When the water resurfaces, it contains natural toxins like benzene, which can carry cancer risks. It can also contain small amounts of chemicals added to enhance drilling.

But the 14 companies -- which include Halliburton and Universal Well Services -- said that because they are "well servicers" and not "well operators," they don't maintain the information the congressmen are asking for. Markey and Waxman are members of the Energy and Commerce Committee, which in February began investigating the potential environmental and human health impacts of natural gas drilling.

Now the congressmen are turning to 10 industry giants for the information, including BP America, Encana Corporation and Chesapeake Energy Corporation. In letters (PDF) sent on Monday, they gave the companies until July 26 to notify them whether they'll comply with the request and until Aug. 6 to actually deliver the information.

The lawmakers' quest for information shows how responsibility for drilling operations can be diffused among a variety of contractors, each doing a different job.

For instance, BJ Services, a company that designs wells and pumps the water underground, told the congressmen that it "does not track or maintain such data because it is the responsibility of the well operator to drill in compliance with the applicable statutes and regulations concerning subsurface aquifers."

When it began its investigation earlier this year, the committee cited (PDF) several stories by ProPublica about water pollution linked to gas drilling. In one of those stories, ProPublica found that drilling wastewater in Pennsylvania was being sent to municipal sewage treatment plants, which didn't have the equipment to properly remove the chemicals from it. As a result, the wastewater contaminated a river that provides drinking water to 350,000 people in the Pittsburgh area. In another story, ProPublica found that many sewage plant operators in New York state don't want to take wastewater because they fear their facilities can't properly treat it.

ProPublica has also uncovered several instances in which underground sources of drinking water have been affected by drilling.