Residence time of groundwater issuing from the South Rim Aquifer in the eastern Grand Canyon

In the eastern Grand Canyon, secondary porosity created by north trending faults, folds, and breccia pipes, facilitates groundwater flow through the South Rim carbonate aquifer. Springs associated with the South Rim Aquifer have low H concentrations, [Ca']/[Mg '] ratios close to unity, and variable uranium concentrations. For a geochemical comparison, springs are subcategorized on the basis of geology and discharge. Type I springs are associated with high-angle normal faults and have high discharge rates. These springs discharge Ca+-Mg+, HC03waters, have H concentrations < 2 TR, and UFU activity ratios > 3 AR, which suggest long groundwater residence times. Type II and IV springs are located on canyon mesas and have low discharge rates. These springs are predominantly Ca+Mg+, SO/ waters, have tritium ratios between I and 6 TR, and U/U activity ratios between I and 2 AR Higher H and U concentrations and low U/U activity ratios in the latter waters may be due to shorter groundwater residence time. Based on H concentration, the occurrence of dedolomitization, and the resultant uranium isotope fractionation in groundwater, the minimum residence time of water discharging from the South Rim Aquifer is indicated to be > 40 years