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

James Kent Fitzgerald, University of Nevada, Las Vegas


In the Grand Canyon, porosity is created by north trending faults, folds, and breccia pipes. Type I springs are associated with high-angle normal faults and have high discharge rates. These springs discharge Ca{dollar}\sp{2+}{dollar}-Mg{dollar}\sp{2+},{dollar} HCO{dollar}\sb3\sp-{dollar} waters, have {dollar}\sp3{dollar}H concentrations {dollar}{dollar}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{dollar}\sp{2+}{dollar}-Mg{dollar}\sp{2+},{dollar} SO{dollar}\sb4\sp{2-}{dollar} waters, have tritium ratios between 1 and 6 TR, and {dollar}\sp{234}{dollar}U/{dollar}\sp{238}{dollar}U activity ratios between 1 and 2 AR. Higher {dollar}\sp3{dollar}H and {dollar}\sp{238}{dollar}U concentrations and low {dollar}\sp{234}{dollar}U/{dollar}\sp{238}{dollar}U activity ratios in the latter waters may be due to shorter groundwater residence time. Based on {dollar}\sp3{dollar}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 {dollar}>{dollar}40 years.