July 1979: United Nuclear Corporation’s tailings dam breach at Church Rock, NM released more radioactivity than Three Mile Island, the largest single radioactive release in U.S. history.
Key Takeaways
94 million gallons of pH 1.2 acidic tailings solution and 1,100 tons of solid radioactive waste flooded the Puerco River, traveling 80 miles into Arizona.
Cracks in the dam wall were identified in December 1977; UNC sealed them with bentonite/kerosene slurry but ignored consultant calls for regular inspections.
The unlined holding pond violated the Uranium Mill Tailings Radiation Control Act of 1978, allowing seepage that weakened the dam foundation.
Navajo residents were warned days later via English-only signs; many spoke only Diné Bizaad. NM governor refused federal disaster declaration.
UNC settled with the Navajo Nation for $525,000, resumed operations after 4 months, and the site landed on EPA’s National Priorities List in 1983. As of 2016, groundwater migration remains uncontrolled.
