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 groundwater, combined with solute concentration data, provide important details regarding groundwater geochemical evolution and flow-pathways in the eastern Snake River Plain aquifer. The study was conducted in the vicinity of the Idaho National Engineering and Environmental Laboratory (INEEL), Idaho, which 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 ²³⁴U/²³⁸U atomic ratios range between 0.000085 and 0.000168 (activity ratios of 1.5 to 3.1). All of the samples have natural ²³⁵U/ ²³⁸U ratios, and ²³⁶U was not detected; thus, the trends delineated by the ²³⁴U/²³⁸U ratios reflect natural variations in the aquifer. Groundwater nearest the valleys that provides focused recharge to the Snake River Plain aquifer from the northwest has high ²³³U/²³⁸U ratios when compared to values of regional groundwater flowing southwestward in the aquifer. Mixing of these water masses can account for the intermediate uranium isotope ratios of some of the samples; however, water-rock interaction must also be invoked to account for the observed trends in isotopic data. Uranium and ⁸⁷Sr/ ⁸⁶Sr isotope ratios 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 controlling ²³⁴U/²³⁸U ratios. As a result, ²³⁴U/²³⁸U ratios decrease along flow pathways toward the secular equilibrium values of the aquifer host rock. Uranium and strontium isotopic modification can be explained by incongruent dissolution of the host basalt. Lateral distributions of ²³⁴U/²³⁸U ratios indicate elongate zones of high ²³⁴U/²³⁸U 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 ²³⁴U/²³⁸U ratios than the adjacent aquifer water. Both of these zones are interpreted to contain stagnant waters that are relatively isolated from flow in the regional aquifer due to lower permeability. Physical and chemical evidence strongly suggests that the stagnant zones are dominanted by water from the Big Lost River that infiltrated via flood control ponds (spreading areas), playas, and the riverbed.