Simulating infiltration in unsaturated basalt for the Large-Scale Aquifer Pumping and Infiltration Test at INEEL

The Large-Scale Aquifer Pumping and Infiltration Test (LPIT) conducted at the Idaho National Engineering and Environmental Laboratory was modeled using TOUGH2 to simulate the highly transient water infiltration and perched-water conditions in the fractured basalt and sedimentary interbeds existing at the site. The fracture and matrix continua of the basalt were represented using a dual-permeability approach. Six perched-water hydrographs, measured during the infiltration test, were used for calibration with iTOUGH2 to estimate six parameters that controlled unsaturated flow in the fractured basalt and the dense clay sedimentary interbed underlying the basalt flow. These parameters included the interfacial area between the basalt fracture and matrix continua, the basalt fracture continuum permeability, the basalt matrix continuum permeability, the interbed matrix continuum permeability, and the interbed van Genuchten capillary pressure parameters. The intent of the calibration was to obtain large-scale properties of the lithological units affecting the field-scale LPIT test. Finally, we tested the applicability of the dual-permeability conceptual model for representing transient variably saturated flow in fractured basalt. Parameters obtained from the calibration were within the range of hydrogeological parameters measured from cores obtained in the field, substantiating the physical relevance of the calibration effort.