Church Rock tailings spill
Early in the morning of July 16, 1979, there was a breach in the earthen retaining dam of a tailings pond at the United Nuclear Corporation's (UNC's) Church Rock uranium mill. The acidified liquid and tailings slurry spilled through the damaged portion of the retaining wall into an arroyo that is a tributary to the Rio Puerco river system. An estimated 94 million gallons of acidic waste water were released to the Pipeline Arroyo, and hence to the North Fork of the Puerco, when an earthen tailings dam located at the United Nuclear Corporation Church Rock Uranium Mill failed on the morning of July 16, 1979. The mill process fluids, which had a pH of less than 2 and a gross alpha particle activity of 128,000 picocuries per liter (pCi/1), traveled downstream into Arizona before dissipating into the stream bed near Chambers. The incident, which became known as the "Church Rock Tailings Spill," remains the largest release (by volume) of low-level radioactive waste in U.S. history.
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