FLAGSTAFF, Ariz. (AP) — The U.S. Environmental Protection Agency is overseeing an effort to determine the extent of contamination at a former uranium mining site on the Navajo Nation that marked a high priority for assessment.

The EPA announced Monday that it has reached a deal with Chevron USA Inc. to survey the 31-acre Mariano Lake Mine site near Gallup, N.M., and surrounding homes and water wells. Chevron is to report its findings from radiological survey and soil samples to the EPA early next year and is on the hook for EPA’s oversight costs.

The Mariano Lake Mine is one of a handful of sites that the EPA and its Navajo Nation counterpart have targeted for investigation or cleanup so far. They’ve been assessing hundreds of abandoned uranium mines to address what has become a legacy of death and disease across the reservation.

In 2005, the Navajo Nation banned uranium mining on the 27,000 square-mile reservation that stretches into parts of New Mexico, Utah and Arizona. Nearly four million tons of uranium ore were mined from the reservation over more than four decades.

The tribe and the EPA began aggressively targeting the abandoned mines within the last decade. That includes the development of a five-year, multi-agency plan to address contamination that now is in its fourth year.

Chevron, which took over the Mariano Lake Mine from Gulf Mineral, is the fifth party that EPA has required to take action at sites on the Navajo Nation. The mine operated from 1977 to 1982 on a site that includes a 519-foot deep shaft, waste piles and several surface ponds.

Preliminary assessments conducted last year revealed some reasons to be concerned, the EPA said. Long-term exposure to high levels of radium can cause cataracts, fractured teeth and cancer. One mobile home that was in the parking lot of the mine site had to be cleaned and relocated.

One home site has not yet been surveyed, and eight others had no signs of contamination in the homes or within one acre of them, the EPA said.

“We’re confident there’s not an immediate risk, but we are concerned about livestock and the activities of the families,” said Andy Bain, remedial project manager with the EPA.

The Navajo Nation has been pushing for mining waste — tailings or rocks that weren’t rich enough in uranium — to be removed from the reservation. Navajo President Ben Shelly commended the EPA and Chevron for coming to an agreement he said will protect the communities in and around Mariano Lake.

“I look forward to the data that will be generated in this investigation, and I respectfully request U.S. EPA to understand our desires for the most protective clean-up plans that will help restore harmony in our communities and homes,” Shelly said in a statement.

The Skyline Mine near Monument Valley, Utah, is the first mine that EPA itself is cleaning up at a cost of $8 million.

The agency reached an agreement with United Nuclear Corp, and its parent company, General Electric Co., to clean up the Northeast Church Rock Mine. The price tag ranges from $24 million to $300 million, depending on where the waste ends up, the EPA said.

The EPA said it expects a report on the Quivira Mine near Church Rock in northwestern New Mexico next month.

“Those are mines that are highly contaminated, are located nearby people and are important to the Navajo Nation for cleanup,” said Clancy Tenley, associate director for tribal programs at the EPA in San Francisco. “These are their highest priorities, as well as ours.”