On July 16, 1979, the earthen dam at the southern holding pond, or cell, of the United Nuclear Corporation’s uranium mill in New Mexico failed, sending tons of contaminated mine tailings and millions of gallons of acidic liquid into an adjoining arroyo and down the Puerco River.

“At the time of the spill, we were not aware of the danger from the tailing,” wrote one woman whose family tended cattle near the site.

Large cracks were observed in the dam in 1977 and plugged with a bentonite clay slurry. The failure was blamed on geological structures under the dam and settling of the material, which allowed cracks to develop.

After the release, Navajo mill employees were sent to notify residents living downstream, and within weeks signs were posted to keep people in New Mexico and Arizona from using the river water.

United Nuclear excavated 3,500 tons of sediment from the Puerco River, which was estimated to be 1 percent of what had been released by the spill.

Two weeks after a United Nuclear vice president testified before a House Interior and Insular Affairs subcommittee that the ongoing shutdown would hurt his business, the NRC is said to have allowed the mill to reopen under the condition that the company eventually find a new repository.

Decades later, the site is being considered for an additional 1 million cubic yards of contaminated mining material.