Performance assessment modeling for a disposal facility in a semi-arid environment

Abstract Vadose zone hydrology plays a key role on moisture flow and contaminant migration through the vadose zone to the underlying aquifer. In particular, vadose zone residence times are significantly impacted by the geologic media heterogeneities, physical and hydraulic properties of sediments, and the moisture content of sediments. This is especially true for the relatively low moisture contents (e.g., less than 10% volumetric) that are typical of dry semi-arid conditions such as those existing for Hanford sediments in southeastern Washington, USA. The objectives of this article are to present a detailed assessment and an evaluation of the technical basis for selection of the vadose zone conceptual model for a waste disposal facility at the DOE Hanford Site. Specifically, we provide a detailed description and an assessment of the vadose zone modeling used in Environmental Restoration Disposal Facility (ERDF). The ERDF is a shallow near-surface disposal facility that is situated in the Central Plateau portion of the Hanford Site. The current disposal area is approximately 1000 m long, 400 m wide, with a depth of 22 m. As of middle of year 2015, approximately 14 million metric tons of waste had been disposed at ERDF, which occupies approximately 6.5 million m 3 of volume.