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 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.