ASSESSMENT OF POTENTIAL URANIUM ORE MIGRATION FROM A URANIUM MILL NEAR THE UTE MOUNTAIN UTE RESERVATION, UTAH

The White Mesa uranium mill, located 6 km north of the Ute Mountain Ute Indian Reservation in southeastern Utah, began operating in 1980 to extract uranium (U) and vanadium (V) from U ores mined in the region. To evaluate potential offsite migration of radionuclides and selected trace elements from the mill, samples were collected from water, sediments, and vegetation near the mill from 2007-2009. Water samples from the shallow, unconfined aquifer were analyzed for U and U isotopes, selected trace elements, stable oxygen and sulfur isotopes, and dissolved gases. Samples of sediment from ephemeral streams (fine fraction passing a -200 sieve) and new growth from sagebrush were analyzed for 43 elements, including U and elements associated with U ore. Tree rings in cores from living cottonwood trees were dated, and concentration of U in recent growth rings measured. Total U concentrations in water samples ranged from 1.89 to 114 μg/L, with a median value of 11.6 μg/L. The activity ratio of ²³⁴U to ²³⁸U measured in water ranged from 3.29 to 1.26 and indicated that the mill was likely not the source of U in groundwater with the possible exception of one spring. Uranium concentrations in ephemeral stream sediments ranged from 1.5 to 16.2 mg/kg. The concentration of U in seven ephemeral stream sediment samples from drainages east and south of the mill exceeded the background concentration of 3.6 mg/kg (background was established as the maximum concentration of U measured at three control sites over 5 km north of the mill). Uranium concentrations in sagebrush samples ranged from 1.3 to 171 μg/g, with a median of 15.3 μg/g. Elevated concentrations of U and V were found in sagebrush samples collected north-east, east, and south of the mill, downwind of predominant wind directions at the site. Pattern recognition modeling of the multi-element sediment data matrix identified several sites near the mill with potential U ore fingerprints. Uranium concentrations in recent growth rings from cottonwood tree cores were below the analytical detection limit (0.1 μg/g) and dendrochronology verified the trees were alive prior to mill operations. Results from this study can assist development of monitoring strategies for existing and planned U mine and mill facilities.