The Church Rock uranium mill spill occurred in New Mexico on July 16 , 1979 when United Nuclear Corporation 's Church Rock uranium mill tailings disposal pond breached its dam . Over 1,000 tons of solid radioactive mill waste and 93 millions of gallons of acidic , radioactive tailings solution flowed into the Puerco River , and contaminants traveled downstream to Navajo County , Arizona and onto the Navajo Nation . The accident released more radiation , although diluted by the 93 million gallons of mostly water and sulfuric acid , than the Three Mile Island accident that occurred four months earlier and was the largest release of radioactive material in U.S. history . Groundwater near the spill was contaminated and the Puerco rendered unusable by local residents , who were not immediately aware of the toxic danger
Three Mile Island accident
Three Mile Island accident
Environmental impact of nuclear power