Church Rock uranium mill spill occurred in the U.S. state of New Mexico on July 16 , 1979 , when United Nuclear Corporation 's tailings disposal pond at its uranium mill in Church Rock breached its dam . The accident remains the largest release of radioactive material in U.S. history , having released more radioactivity than the Three Mile Island accident four months earlier . The mill was located on privately owned land about north of Gallup , New Mexico , and bordered to the north and southwest by Navajo Nation Tribal Trust lands . The breach released more than of solid radioactive mill waste and of acidic , radioactive tailings solution into the Puerco River through Pipeline Arroyo . An estimated of uranium and 43 trillion picocuries of alpha contaminants traveled downstream to Navajo County , Arizona , and onto the Navajo Nation . In addition to being radioactive and acidic , the spill contained toxic metals and sulfates . The spill contaminated groundwater and rendered the Puerco unusable to local residents , mostly Navajo peoples who used the river 's water for drinking , irrigation , and livestock . They were not warned for days of the toxic dangers from the spill
Three Mile Island accident
Three Mile Island accident
Church Rock uranium mill spill