Investigating Controls on Temporal and Spatial Variation of Dissolved Uranium at Springs in Horn Creek Drainage, Grand Canyon National Park, Arizona, USA

Groundwater is a critical resource in the Grand Canyon, providing water to local communities, millions of annual visitors, and ecosystems. One major concern for this critical resource is the potential for uranium contamination in conjunction with uranium mining efforts. Upper Horn Bedrock Spring (UHB) in the Horn Creek drainage of the Grand Canyon sits below the oldest uranium mine in the region, Orphan Lode mine. UHB spring contains the highest concentration of dissolved uranium in the region, eclipsing the Maximum Contaminant Level (MCL) set by the USEPA by an order of magnitude on average. This research and recent collaboration with the U. S. Geological Survey (USGS) provide further insight into the hydrogeochemical evolution of the groundwater within the drainage, with an emphasis on trying to understand the temporal and spatial variation of dissolved uranium within groundwater sampled in Horn Creek drainage. This thesis specifically investigates two environmental phenomena surrounding dissolved uranium in the drainage using stable isotopes, multivariate statistical analysis, inverse modeling in PHREEQC, and bivariate analysis. The first phenomenon is the relatively rapid change in dissolved uranium concentration as flow moves downgradient from Upper Horn Bedrock spring to Upper Horn Alluvium spring. Approximately 200 m apart, there is roughly an order of magnitude decrease in dissolved uranium between the two springs. Inverse modeling suggests that water-rock interactions could explain the observed change in uranium concentration if the mineral pyrite is present in trace concentrations along the flow path. Inverse modeling and multivariate analysis also suggest that mixing between UHB and a second spring, East Horn Creek – Battleship, could explain the observed change in chemistry. However, stable isotopes collected in 2013 do not support EHC-BS as a mixing partner. The second phenomenon investigated is the variable concentration of dissolved uranium at UHB over time. Twenty-two samples have measured the concentration of dissolved uranium at Upper Horn Bedrock spring across nearly two decades, with concentrations ranging from 151 ppb to 509 ppb. Results indicate that although there is no trend in dissolved uranium over time, there is a relationship between recent precipitation and dissolved uranium. Bivariate analysis comparing recent precipitation (up to 120 days prior to sample collection) and dissolved uranium indicates that wetter periods 9 to 10 weeks prior to sampling correspond to the lowest concentrations of dissolved uranium recorded since 2003. Drier conditions 9 to 10 weeks prior to sampling correspond to higher concentrations of dissolved uranium, however significant variation is present among these higher concentrations.