ASSESSING TEMPORAL CHANGES IN GEOCHEMISTRY AT SPRING SITES LOCATED IN AN AREA OF BRECCIA PIPE URANIUM DEPOSITS

The chemical changes of groundwater over time in the area of breccia pipe uranium mining withdrawal around the Grand Canyon are not well understood. Four springs, including Pigeon, Willow in Hack Canyon, Horn Creek, and Salt Creek Spring, have been monitored over time for geochemical constituents. Pigeon and Willow Spring are located north of the Grand Canyon and discharge from perched groundwater units. Pigeon Spring, had elevated uranium (U) (44 µg/L), compared to other perched springs (1.5 to 15 µg/L), prior to the initiation of mining operations at the nearby Pigeon Mine in 1982. Subsequent sampling at Pigeon Spring between 2012 and 2017 has shown a large variability of U concentrations over time with a maximum of 92 µg/L and a minimum of 56 µg/L and tritium values have also fluctuated over the same time period ranging from 0.5 to 5.4 pCi/L. Willow Spring in Hack Canyon ranged from 19 to 28 µg/L U. These springs had elevated selenium (Se) concentration relative to the USEPA drinking water standard of 50 and Se values ranged from 51 to 72 µg/L and 38.2 to 75.4 µg/L respectively. Two springs south of the Grand Canyon which discharge from a deeper regional groundwater source, Horn Creek and Salt Creek Springs, showed generally less variability of U concentration over time, Horn 7.2 to 9.3 µg/L (with one outlier of 29 µg/L) and Salt 29 to 34.9 µg/L. Selenium concentrations at Horn and Salt were lower than the springs north of the Grand Canyon (5 to 6.9 µg/L and 9 to 12.1 µg/L respectively). Time series results from the two springs discharging from perched springs north of the Grand Canyon showed more variability for U and Se concentration compared with the two springs south of Grand Canyon discharging from a more regional groundwater source. Further time series sampling at these and other springs is needed to understand geochemical variability in the perched and regional groundwater systems surrounding the Grand Canyon.