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.