The 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, which operated from June 1977 to May 1982, was located on privately owned land about north of Gallup, New Mexico, and was bordered to the north and southwest by Navajo Nation Tribal Trust lands. The milling of uranium ore produced an acidic slurry of ground waste rock and fluid (tailings) that was pumped to the tailings disposal area. 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 46 curies of a