ASSESSING GEOCHEMISTRY OF GROUNDWATER DISCHARGING TO THE HORN CREEK DRAINAGE NEAR THE ORPHAN MINE IN GRAND CANYON NATIONAL PARK, ARIZONA

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. The Orphan Mine was the first mined breccia pipe uranium deposit in the area, which produced uranium from 1953 to 1972 and is currently in the process of reclamation. The Horn Creek drainage is located below the Orphan Mine. The eastern fork of the Horn Creek drainage represents an area with groundwater discharging from bedrock high in the drainage that infiltrates into the alluvium and resurfaces lower in the drainage. Water at the head of the drainage emerges from bedrock with elevated uranium concentration (257 µg/L), low uranium activity ratio (UAR), calcium-magnesium-sulfate type water, and low tritium (0.9 pCi/L), then infiltrates into the alluvium and emerges about a half kilometer lower in the drainage with a decrease in uranium concentration (23 µg/L), similar UAR, calcium-magnesium-bicarbonate type water, and elevated tritium (2.6 pCi/L). Additionally, there is a large decrease in uranium (23 to 7.6 µg/L) concentration over a short distance (approximately 20 m) in the alluvial material. Groundwater chemistry from Horn Creek drainage was compared with groundwater samples collected from springs in drainage basins surrounding Horn Creek. Water chemistry at nearby Salt Creek Spring was most similar to water in the Horn Creek drainage alluvium and the water chemistry at the head of Horn Creek drainage with elevated uranium concentration was anomalous. Investigation of multiple isotopic tracers in groundwater provide a better understanding of the processes affecting uranium concentrations, as well as potential mixing with other sources of groundwater, as groundwater flows downgradient through the alluvial system within the Horn Creek drainage.