URANIUM ASSOCIATED WITH WATER RESOURCES NEAR BRECCIA PIPE URANIUM DEPOSITS OF THE GRAND CANYON REGION

Changes in the geochemistry of groundwater interacting with mined and unmined uranium breccia pipes in the Grand Canyon region of the southwestern United States are not well understood. Several uranium mines exist in the region and are in various stages of development from pre-mining, active operations, and reclamation completed. Water resources in the region are limited and generally occur as groundwater in a perched and deeper regional aquifer. Uranium concentrations in groundwater vary spatially, and some locations show anomalously elevated concentrations. Several groundwater sites are sampled on a frequent basis and sites with elevated uranium concentration show high temporal variability. Groundwater emerging from the base of the bedrock cliff in the upper reaches of Horn Creek, located near a historical uranium mine, has the highest uranium concentrations in the region and has high variability (from 151 to 290 µg/L). Uranium concentrations are lower in groundwater emerging from alluvial material lower in the Horn Creek drainage (from 7.2 to 15.4 µg/L). Geochemical indicators, such as major ions, trace elements, tritium, stable isotopes, and radiocarbon, suggest there may be mixing with younger water between the upper and lower groundwater in Horn Creek. The area north of Grand Canyon includes uranium mines located near groundwater discharge at springs which have variable uranium concentrations, with the highest values occurring at Pigeon Spring (50 to 92 µg/L). Multiple lines of evidence suggest water at Pigeon Spring is derived from groundwater upgradient of the nearby Pigeon Mine and may be related to undeveloped mineralized deposits. The relation of the elevated uranium concentrations to nearby breccia pipe uranium deposits depends on groundwater flow paths. Combining uranium concentration data with other geochemical tracers provide a better understanding of the processes affecting uranium concentrations in groundwater and may help in projecting future water quality effects from uranium mines in the area.