Every day in the United States, at least 2 billion gallons of fluids are injected into over 172,000 wells to enhance oil and gas production, or to dispose of fluids brought to the surface during the extraction of oil and gas resources. These wells are subject to regulation to protect drinking water sources under EPA's UIC class II program and approved state class II programs. Because much of the population relies on underground sources for drinking water, these wells have raised concerns about the safety of the nation's drinking water. GAO was asked to review EPA's oversight of the class II program. This report examines (1) EPA and state roles, responsibilities, and resources for the program, (2) safeguards to protect drinking water, (3) EPA oversight and enforcement of class II programs, and (4) the reliability of program data for reporting. GAO reviewed federal and state laws and regulations. GAO interviewed EPA and state officials and reviewed class II programs from a nongeneralizable sample of eight states selected on the basis of shale oil and gas regions and the highest number of class II wells.