Uranium isotopic evidence for groundwater chemical evolution and flow patterns in the eastern Snake River Plain aquifer, Idaho

The isotopic composition and concentration of uranium and strontium in grou ndwater, combined with solute concentration data, provide important details regarding groundwater geochemical evolution and flow-pathways in the easte rn Snake River Plain aquifer. The study was conducted in the vicinity of th e Idaho National Engineering and Environmental Laboratory (INEEL), Idaho, w hich has a long history of storing and disposing of radioactive waste, some of which has entered the aquifer. Uranium concentrations in INEEL groundwater range from 0.3 to 3.6 ppb, and U-234/U-238 atomic ratios range between 0.000085 and 0.000168 (activity rat ios of 1.5 to 3.1). All of the samples have natural U-235/U-238 ratios, and U-236 was not detected; thus, the trends delineated by the U-234/U-238 rat ios reflect natural variations in the aquifer. Groundwater nearest the vall eys that provides focused recharge to the Snake River Plain aquifer from th e northwest has high U-234/U-238 ratios when compared to values of regional groundwater flowing southwestward in the aquifer. Mixing of these water ma sses can account for the intermediate uranium isotope ratios of some of the samples; however, water-rock interaction must also be invoked to account f or the observed trends in isotopic data. Uranium and Sr-87/Sr-86 isotope ra tios are positively correlated and define a trend toward isotope ratios of the aquifer host rock. These relations indicate that dissolution and/or ion exchange are more important than alpha recoil or selective leaching in con trolling U-234/U-238 ratios. As a result, U-234/U-238 ratios decrease along flow pathways toward the secular equilibrium values of the aquifer host ro ck. Uranium and strontium isotopic modification can be explained by incongr uent dissolution of the host basalt. Lateral distributions of U-234/U-238 ratios indicate elongate zones of high U-234/U-238 ratios extending southward from the mouths of Birch Creek and the Little Lost River. These elongate zones are interpreted as preferential flow paths. Two isolated pockets of groundwater located in the central and western parts of the study area have lower U-234/U-238 ratios than the adj acent aquifer water. Both of these zones are interpreted to contain stagnan t waters that are relatively isolated from flow in the regional aquifer due to lower permeability. Physical and chemical evidence strongly suggests th at the stagnant zones are dominated by water from the Big Lost River that i nfiltrated via flood control ponds (spreading areas), playas, and the river bed.