Modeling unsaturated flow and transport processes at the Busted Butte Field Test Site, Nevada

A numerical model was used to simulate the flow and transport processes at the Busted Butte Field Test Site for the purpose of quantifying the effects of hydrogeologic conditions beneath the potential Yucca Mountain repository horizon. In situ experiments were conducted on a 10×10×7 m block comprising a layered Topopah Springs/Calico Hills formation with two imbedded faults. Tracer solution was continuously injected in eight parallel boreholes arranged on two horizontal planes. Twelve collection boreholes were emplaced perpendicular to the injection holes and were both horizontal and inclined. Solution samples were collected regularly using a sampling assembly consisting of an inverted membrane and sorbing-paper sampling pads. Comparisons between measurements and predictions show that, except for the occasional drops of concentrations observed in the field, the current model is able to capture the general characteristics of the system with varying levels of agreement using laboratory-measured mean hydraulic properties. Simulation results and field observations revealed a capillary-driven flow in the system. Good quantitative agreement is generally observed for near-field boreholes, however, this agreement deteriorates and the simulated solute concentration is underestimated at boreholes farther away from the injection points. Increasing the spatial resolution of the simulation improves the model predictions only to a limited extent. Scaling issues may need to be considered to describe flow and transport events, as the travel distance becomes large.