Numerical Modeling of Groundwater Flux through a Low-Level Radioactive Waste Repository for Performance Assessment

Abel Porras¹, Changbing Yang², Kan Tu¹, Brad Broussard¹, and Alex Sun²
¹Texas Commission on Environmental Quality, 12100 Park 35 Circle, Austin, Texas 78753
²Bureau of Economic Geology, 10100 Burnet Rd., Austin, Texas 78758

A low-level radioactive waste (LLRW) repository is being constructed within the low-permeable Triassic Dockum Group mudstones (Cooper Canyon formation) in a semi-arid area of west Texas. Due to the absorption of radionuclides through groundwater pathways, the near-field environments of the repository may be potentially impacted. Thus, a performance assessment is required to determine the dose to a receptor utilizing the contaminated groundwater. A performance assessment is a systematic risk analysis that quantitatively evaluates potential doses to receptors and is typically carried out through numerical modeling. One of the key aspects of a performance assessment is to evaluate the amount of water infiltrating through the bottom of the repository. In this study, the numerical model HYDRUS was used for simulating variably saturated groundwater flow in a vertical 2D domain. The model domain consists of the waste repository, the cap system, and about 130 ft (40 m) of underlying geological units. Groundwater flow was simulated for a time period of 100,000 yr with a synthetic initial condition of hydraulically equilibrated groundwater in the simulated domain. A specific net inflow was imposed upon the top boundary as a proxy for the complex atmospheric boundary condition, which considers precipitation, evapotranspiration and root water uptake. While the specific flux constrained with observed hydrogeologic data and site conceptual models can be varied with current and future dry or wet conditions, this study was focused on groundwater fluxes above and beneath the repository due to different model lower boundary conditions. Ultimately, water flux beneath the repository calculated in this numerical model would be used in the transport model using GoldSim as a platform to estimate potential radiological doses at a sandstone layer beneath the repository at depth of approximately 225 ft (68 m) below the ground surface. Results under this specific repository configuration show similar downward fluxes despite the different boundary conditions applied at the bottom of each model. The findings of this study may be useful conducting groundwater flow simulations for performance assessments of other low-level radioactive waste repositories at semi-arid areas in lowpermeability formations.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012