CAMERON, Ariz. — In the summer of 2010, a Navajo cattle rancher named Larry Gordy stumbled upon an abandoned uranium mine in the middle of his grazing land and figured he had better call in the feds. Engineers from the Environmental Protection Agency arrived a few months later, Geiger counters in hand, and found radioactivity levels that buried the needles on their equipment.

The abandoned mine here, about 60 miles east of the Grand Canyon, joins the list of hundreds of such sites identified across the 27,000 square miles of Navajo territory in Arizona, Utah and New Mexico that are the legacy of shoddy mining practices and federal neglect. From the 1940s through the 1980s, the mines supplied critical materials to the nation’s nuclear weapons program.

For years, unsuspecting Navajos inhaled radioactive dust and drank contaminated well water. Many of them became sick with cancer and other diseases.

The radioactivity at the former mine is said to measure one million counts per minute, translating to a human dose that scientists say can lead directly to malignant tumors and other serious health damage, according to Lee Greer, a biologist at La Sierra University in Riverside, Calif. Two days of exposure at the Cameron site would expose a person to more external radiation than the Nuclear Regulatory Commission considers safe for an entire year.