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The intensely layered character of the 200-m thick unsaturated zone near the Radioactive Waste Management Complex (RWMC) Subsurface Disposal Area (SDA) at the Idaho National Engineering and Environmental Laboratory (INEEL) critically affects both vertical and horizontal water fluxes. Because of the potential for radionuclide migration from the SDA to the Snake River Plain aquifer, it is important to investigate the role of the unsaturated zone in contaminant transport processes. The unsaturated zone consists of thick layers of fractured basalts interbedded with thinner layers of sediment. These interbeds .and basalts were deposited approximately 50,000 to 450,000 years ago during the late Pleistocene. As a part of a drilling program to develop a standard methodology for subsurface characterization and risk assessment at INEEL, hydraulic properties of the 34-m deep sedimentary interbed (known as the B-C interbed (Anderson and Lewis, 1989)) have been measured at one location in the vicinity of the SDA, including particle size distributions, water retention functions, saturated and unsaturated hydraulic conductivity, and related properties. In porous media, water flux is usually modeled in terms of Darcy's law for steady flow and Richards' equation for transient flow. Both of these formulations require knowledge of the unsaturated hydraulic conductivity (K) of the media, a property that is difficult to measure and highly sensitive to variations in water content. The transient case additionally requires knowledge of the water retention relation, which similarly varies to a high degree within the medium. The interbeds may play several critical roles in long-range transport processes: (a) retardation of downward-moving water as it encounters layer boundaries, (b) generation of perched water, (c) homogenization of preferential flow that has been focused by basalt fractures, and (d) the formation of long-range, highly conductive horizontal flow paths for contaminants. Within these sedimentary layers, there may be little or no impediment to lateral flow. Drastic differences in hydraulic properties between the basalt and interbeds, and within the interbeds themselves, are likely to promote such flow.

water-resources investigations report

Measurement of hydraulic properties of the B-C interbed and their influence on contaminant transport in the unsaturated zone at the Idaho National Engineering and Environmental Laboratory, Idaho