Here is the best summary of UMTRA I have come across:

The Uranium Mill Tailings Radiation Control Act (UMTRCA) of 1978 gave the U.S. Department of Energy (DOE) the responsibility of stabilizing, disposing, and controlling uranium mill tailings and other contaminated material at twenty-four uranium mill processing sites located across ten states and at approximately 5,200 associated properties.

In the 1950s and 1960s, private firms processed most uranium ore mined in the United States. After uranium mining came under federal control, companies abandoned their mill operations, leaving behind materials with potential long-term health hazards. These mills contained low-level radioactive wastes and other hazardous substances that eventually migrated to surrounding soil, groundwater, surface water, and emitted radon gas.

Under the Act, the DOE established the Uranium Mill Tailings Remedial Action (UMTRA) Project to monitor the cleanup of uranium mill tailings. The UMTRCA gave the Nuclear Regulatory Commission (NRC) regulatory authority over the cleanup and licensing of mill tailing facilities at sites under NRC license. The U.S. Environmental Protection Agency (EPA) had the task of developing cleanup strategies and recording standards for mills. The UMTRA used on-site disposal methods for eleven of the mills, while excavating and disposing of the wastes found at the remaining thirteen sites to remote off-site disposal locations owned by the DOE.

Further Reading
Introduction to Uranium Mill Tailings Remedial Action Program Office (U.S. Department of Energy, Office of Environmental Management)
Uranium Mill Tailings Radiation Control Act of 1978, as Amended: Full Text (Nuclear Regulatory Commission)

This useful posting is referenced in another posting that is almost alarming. There appears to still be concern over the use of uranium mill tailings in houses in the reservation.

It appears that neither UMTRA nor the corresponding Title II program for privately owned uranium mine remediation dealt with everything on the Navajo Nation land. A recent report states:

From 1944 to 1986, nearly 4 million tons of uranium ore were mined from the land from over 500 mines, according to U.S. EPA. When the Cold War ended, the demand for uranium disappeared and the mines on the Navajo land were mostly abandoned. The mines were also left exposed with no warning signs or protective fencing around hundreds of 50-foot high piles of mining tailings, Navajo witnesses said at the hearing.

Leaders of the Navajo are demanding action to clean up these post war abandoned uranium mines. The report notes:

The biggest step the federal government has taken to clean up the Cold War legacy was taken by EPA when the agency removed 6,500 cubic yards of radium contaminated soils around residences near the Northeast Church Rock Mine in New Mexico. It is the only abandoned mine site thoroughly assessed in accordance with EPA’s Superfund program, said Wayne Nastri, the administrator for the agency’s Region 9, at the hearing. The Church Rock site was identified by the Navajo and EPA as the highest priority of about 40 sites at dangerous levels of contamination. The Navajo Nation EPA has identified more than 600 former uranium mines and 1,200 mining-related hazards on their lands. When Nastri replied that his agency needed more time to complete studies, the committee members were not satisfied. Nor were they content with any of the answers from the Energy Department, the Nuclear Regulatory Commission, the Indian Health Service or the Bureau of Indian Affairs.

Waxman told the agencies to coordinate action on testing the health and environmental effects from the uranium mines and report back Dec. 12. “Tell us where you are … [and] what you need to get the job done,” he said. Waxman was not alone. Reps. Betty McCollum (D-Minn.) and Udall said they would be watching for the line items on this issue in the agencies’ budgets in the House Appropriations Committee.