High Quality Content by WIKIPEDIA articles! The Church Rock Uranium Mill Spill occurred in New Mexico, USA, in 1979 when United Nuclear Corporation`s Church Rock uranium mill tailings disposal pond breached its dam. Over 1,000 tons of radioactive mill waste and millions of gallons of mine effluent flowed into the Puerco River, and contaminants traveled 80 miles (130 km) downstream to Navajo County, Arizona. Local residents used river water for irrigation and livestock and were not immediately aware of the toxic danger.