On July 16, 1979, a break in the tailings pond dam of the United Nuclear Corporation's Church Rock, New Mexico* mill occurred. Approximately 94 million gallons of the tailings liquid which had been impounded behind this dam were released into an adjacent arroyo along with an estimated 1,100 tons of tailings solids. The spilled solution traveled down the so-called "pipeline arroyo" and into the north branch of the Rio Puerco arroyo. Beyond this point it continued past the location where the north and south branches of the Rio Puerco join immediately northeast of Gallup New Mexico, continued across the remainder of the state of New Mexico, and extended into the state of Arizona for ~20 to 25 miles, where the flow of the Rio Puerco terminates.
In September, 1979, Pacific Northwest Laboratory (PNL) responded to a request from the New Mexico Environmental Improvement Division and the U.S. Nuclear Regulatory Commission (NRC) to provide immediate on-site sampling and radionuclide analysis capability. The principal objective of PNL's work on-site at the United Nuclear Corporation mill was to provide capabilities for immediate analyses of samples which had been collected from the Rio Puerco environment. The conditions for sample preparation and sample analysis were optimized so that the total amount of time required from the time the sample was collected until the data regarding radionuclide concentrations for that sample were available could be less than 48 hours. The concentrations of 210Pb, 226Ra, 230Th, and 23SU were determined in ~2400 samples from a survey of the Rio Puerco contamination and from subsequent cleanup operations initiated by the United Nuclear Corporation. The purpose of these later analyses was to determine the effectiveness of the cleanup operations that were ongoing and to evaluate what additional cleanup would be required. This on-site analysis of radioactive contamination constituted the principal task of this project, with the identification of those portions of the arroyo exceeding the NMEID proposed cleanup criteria being the major output.
The following conclusions have been drawn as a result of the investigation of the Church Rock uranium mill tailings pond dam failure:
