ANTHROPOGENIC INFLUENCES ON GROUNDWATER IN THE VICINITY OF THE WASTE ISOLATION PILOT PLANT, SOUTHEASTERN NEW MEXICO, USA

The groundwater flow system in the Culebra Dolomite Member (Culebra) of the Rustler Formation is a potential radionuclide release pathway from the Waste Isolation Pilot Plant (WIPP), the only deep geological repository for transuranic waste in the United States. Groundwater levels in the Culebra were not expected to fluctuate, except in response to WIPP construction activities and long-term climatic changes, with response times on the order of hundreds to thousands of years. Groundwater pressure data collected at the WIPP site have recorded perturbations to the Culebra flow system caused by pumping activities associated with the demands of the local (i.e., Permian Basin) hydrocarbon industry for water. The unprecedented magnitude of recent Culebra-based pumping provides an opportunity to assess current understanding of the flow system and investigate the influence of transient phenomena. An ensemble-averaged flow model with a sink term was developed to simulate drawdown associated with the recent pumping activities. The results were evaluated against field observations and used to estimate pumping rates. Variability in subsurface hydrologic response was studied by imposing the best-fit pumping rate on the realizations that comprise the ensemble-averaged model. The ensemble-averaged flow model results show reasonable agreement with observations (model efficiency [EF] coefficient = 0.6). Steepened hydraulic gradients reduce estimates of conservative particle travel times across the simulation domain by approximately one half when compared to unperturbed conditions. This work highlights the need to more rigorously evaluate the impact of transient hydrologic forcings on the Culebra, raises questions about the future of land use in the vicinity of the WIPP, and demonstrates the challenge in reconciling human-timescale observation with geologic-timescale prediction.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. This research is funded by WIPP programs administered by the Office of Environmental Management (EM) of the U.S. Department of Energy. SAND2016-6381A