A federal watchdog says it will review the U.S. Environmental Protection Agency’s efforts to clean up abandoned uranium sites on the Navajo Nation.

The EPA received funding in 2015 from a $1 billion settlement to address 50 sites in northeastern Arizona that were previously used as mines. The sites were run by Kerr McGee Corporation, later sold to a petroleum company. The EPA’s Office of Inspector General says it wants to determine whether the agency is prioritizing sites that present the greatest threats. Reviews generally take 18 months.

The EPA and Dine College have found high concentrations of uranium in the soil and groundwater. Uranium has been linked to many health problems, including cancer.

The Associated Press contributed to this report.