The GAO, an independent, nonpartisan agency that works for Congress, reviewed existing scientific reports on shale drilling, and spoke to state regulators, industry experts and environmental groups.
Regulators from Arkansas, Colorado, Louisiana, North Dakota, Ohio, Oklahoma, Pennsylvania and Texas said state investigations found that the part of the drilling process called hydraulic fracturing, or fracking, has not been identified as a cause of groundwater contamination, the report notes.
Travis Windle, a spokesman for the industry group the Marcellus Shale Coalition, suggested that the GAO report, "like so many other independent reports, determines that hydraulic fracturing is safe and that this critical, tightly-regulated technology has never impacted groundwater."
But the GAO also noted that, according to studies and publications, "underground migration of gases and chemicals poses a risk of contamination to water quality." For example, the GAO said that the Ohio Department of Natural Resources found in 2008 that a gas well with a faulty seal allowed natural gas to build up and migrate into the local aquifer, infiltrating drinking water wells.
George Jugovic, president of the Pennsylvania environmental group PennFuture, said he doesn't think the public cares which specific part of the drilling process poses a threat to health or the environment.
"I don't think it serves the industry well to shy away from what is a legitimate public concern," Jugovic said, noting that every industrial process has some risk. The question is what risk people are willing to live with, he added.
Hydraulic fracturing has made it possible to tap into deep reserves of oil and gas but has also raised concerns about pollution. Large volumes of water, along with sand and hazardous chemicals, are injected underground to break rock apart and free the oil and gas.
Contaminated wastewater from the drilling process can leak from aquifers via faulty well casings. Also, some studies have shown air quality problems around gas wells, while others have indicated no problems.
The industry and many federal and state officials say the practice is safe when done properly, and regulators are strengthening many rules on air pollution and the disclosure of chemicals used in fracking. But environmental groups and some scientists say there hasn't been enough research.
In a separate but related report, the GAO said both federal and state agencies face challenges in regulating shale oil and gas wells, such as a lack of data and limited legal authority. But they also found that some states — such as Ohio and Pennsylvania — have strengthened regulations in recent years, based on recommendations from independent reviews.
The second report also found that environmental regulators in North Dakota, Pennsylvania and Wyoming mentioned challenges related to hiring or retaining staff. Ohio didn't report that problem.
The GAO also said that federal agencies use different methods to estimate recoverable shale gas reserves, but the overall trend has been sharply upward, as companies successfully drill in more and more locations.
The GAO didn't make any formal recommendations about shale gas regulation in the reports.