HYDROGEOLOGY OF THE HANFORD SITE VADOSE ZONE

The vadose zone at the Department of Energy’s Hanford Site in the Pasco Basin, south-central Washington, consists of mostly Pleistocene-age cataclysmic flood deposits, informally referred to as the Hanford formation. Locally, pre-flood deposits of the Cold Creek unit (formerly the Plio-Pleistocene unit) and the Ringold Formation may lie between the Hanford formation and the water table (DOE 2002). A thin veneer of post-Hanford formation, eolian deposits occurs at the surface.

Based on observations of outcrops and a limited number of drill cores, the Hanford formation can be subdivided into 11 textural-structural lithofacies, which group into three facies associations (gravel-dominated [GD], sand-dominated [SD], and interbedded sand- and silt-dominated [ISSD]). GD flood deposits lie along flood channels through the central portion of the Pasco Basin, while ISSD flood deposits occur around the basin margins. SD is transitional between GD and ISSD, and is the principal unit beneath the 200 Area Waste-Management Units, while GD predominates beneath Hanford’s 100 Areas. Flood deposits are inherently heterogeneous and anisotropic due to the highly complex interplay between channels and bars during multiple flood episodes.

Moisture in the vadose zone is typically concentrated along high-contrast bed interfaces, as well as along finer grained layers within the Hanford formation and Cold Creek unit. Precipitation and waste-water discharges may migrate downward along discordant features, such as clastic dikes, or spread laterally, sometimes in a stair-step fashion, along overlapping series of anisotropic, discontinuous strata. High-moisture retaining facies occur in all strata, but are most frequent in ISSD and least frequent in GD flood deposits. Studies are ongoing to evaluate the physical and chemical characteristics of vadose-zone strata and assign hydrologic properties to each sediment type for modeling purposes.

DOE, 2002, Standardized Stratigraphic Nomenclature for the Post-Ringold-Formation Sediments Within the Central Pasco Basin, DOE/RL-2002-39, Rev. 0, U.S. Department of Energy, Richland, Washington.