Permeability Changes Due to Secondary Mineral Precipitation During Weathering of Low-Level Radioactive Waste Glass in the Vadose Zone

A film depositional model has been incorporated into the Subsurface Transport Over Reactive Multi-phases (STORM) code, which simulates coupled unsaturated flow, solute transport, energy transport, geochemical reactions, and porosity/permeability changes due to mineral precipitation and dissolution. STORM has been applied to simulations of the weathering of low-level radioactive glass disposed in the shallow subsurface, conducted over geologic time scales. As the glass dissolves, secondary minerals precipitate, which consist mostly of clays. The film depositional model is based on the assumption that the clays are deposited on the pore walls as a continuous film, which may cause a reduction in permeability. The film depositional model is developed for a discrete pore-size distribution, which is determined using the unsaturated hydraulic properties of the porous medium. This facilitates the process of dynamically updating the unsaturated hydraulic parameters used to describe fluid flow through the media. Results are presented from two-dimensional, field scale simulations of the simultaneous dissolution of low-level radioactive waste glass and secondary mineral precipitation using the film depositional model. These results are used to assess the impact of permeability changes on glass weathering rates over a 10,000-year period. This research was performed in part using the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory (PNNL). PNNL is operated for the Department of Energy by Battelle.