FLUVIAL DYNAMICS OF TH-230 IN THE CHURCH ROCK EVENT, PUERCO RIVER, NEW-MEXICO

The largest accidental surface release of radioactive materials in the United States occurred 16 July 1979, when a uranium tailings pond collapsed near Church Rock, New Mexico, releasing 378,500 m3 of liquids and 1000 mg of solids into the Puerco River. The resulting flood wave distributed radioactive thorium-230 through an 80-km reach of the river. A detailed analysis of 48 km of the entrenched channel shows that radionuclide concentrations in streambed sediments fluctuated irregularly with increasing distance from the source of contamination instead of declining exponentially as might be expected from hydraulic and geographic theory. Hydraulic calculations at 154 cross-sections in the 48-km reach show that concentrations of radionuclides in channel sediments were inversely related to unit stream power generated during the peak of the flood wave. Concentrations were also inversely related to the length of time that shear stress exceeded critical values during the passage of the flood wave. Regional geologic structure determined the arrangement of channel characteristics, hydraulic behavior of the flood, and the resulting transport and storage of contaminants in the channel. The Puerco River case suggests that in dam-burst events, the variable downstream distribution of hydraulic properties of the flood wave is the primary physical control on the geography of potentially hazardous concentrations of heavy metals. © 1990, Taylor & Francis Group, LLC. All rights reserved.