Krypton-81 and Groundwater Flow in the Culebra Dolomite Near the Waste Isolation Pilot Plant, New Mexico

The Waste Isolation Pilot Plant (WIPP) in New Mexico is the first geologic repository for disposal of transuranic nuclear waste from defense-related programs of the US Department of Energy. It is constructed within halite beds of the Permian-age Salado Formation. The Culebra Dolomite, confined within Rustler Formation evaporites, is a potential pathway for radionuclide transport from the repository to the accessible environment in the human-disturbed repository scenario. Although extensive subsurface characterization and numerical flow modeling of groundwater has been done in the vicinity of the WIPP, few studies have used natural isotopic tracers to validate the flow models at this site. We performed measurements of the cosmogenic isotopic tracer 81Kr (half-life 229,000 yr) in two Culebra monitoring wells near the WIPP site, and compared 81Kr model ages with the results of an ensemble of flow models. The 81Kr model ages were ~130,000 and ~330,000 yr for high-transmissivity and low-transmissivity portions of the aquifer, respectively. These model ages are in broad agreement with flow model results, when taking into account diffusive exchange of Kr between the aquifer and the stagnant zones of the confining formations.