CHEMICAL AND ISOTOPIC SIGNATURE OF A URANIUM-RICH CONTAMINANT PLUME, FRY CANYON, SOUTHEASTERN UTAH: IMPLICATIONS FOR SITE MONITORING AND REMEDIATION

Uranium ore upgrading (1957-1960) and sulfuric acid heap leaching of Cu-U ores (1963-1968) at a site in Fry Canyon, southeastern Utah produced a plume of contaminated ground water. Previous studies by the U.S. Geological Survey and Bureau of Land Management indicated high concentrations of U (1,000-16,000 µg/L), Cu (<4-20 µg/L) and SO₄ (1,200-1,900 mg/L) in the plume, and locally elevated concentrations of U (>100 µg/L) in well water 0.5-km downgradient from the site. In April 2007, we sampled seven surface waters from adjacent Fry Creek, eight previously installed monitoring wells, four local seeps, and one spring. The purpose was to determine if the ²³⁴U/²³⁸U alpha activity ratio (AR) of plume water could be distinguished from that of local uncontaminated (baseline) waters and therefore used as an additional indication of contamination and mixing of waters. Three monitoring wells located in the plume area had a mean AR of 0.934±0.005, whereas the natural seeps and spring, one upgradient monitoring well, and two surface waters upstream from the site had a mean AR of 1.241±0.066 and U of 34-70 µg/L. Four downgradient monitoring wells had AR values of 0.954-1.012 and corresponding dissolved U of 770-380 µg/L. A plot of AR versus 1/U showed that the four downgradient wells clearly fell on a linear mixing trend connecting the contaminant plume and baseline end members. Surface waters collected 0.5-3.5 km downstream from the site also fell on the mixing line and contained 150-256 µg/L U. Downstream movement of contaminated ground water is in the modern alluvium of Fry Creek and in a broader paleochannel that underlies and approximately follows the modern creek channel. Uranium isotopes (1) confirm and track movement of U contamination from the site, (2) permit calculation of proportions of isotopically distinct U in mixtures, and (3) provide an additional tool for monitoring the effects of planned site remediation.