ENVIRONMENTAL GEOLOGY OF THE HANFORD SITE

As a consequence of production and waste storage activities associated with Pu production, the Hanford Site (1,450 km²) has radionuclide and chemical contamination in saturated and unsaturated sediments. The geology of the site strongly influences groundwater flow and contaminant transport. Broadly, the contaminants in the central part of the site move through a 70m thick vadose zone to the groundwater which then has the potential to transport them to discharge zones along the Columbia River. Each transport process, 1) unsaturated transport, 2) saturated transport, and 3) discharge to the Columbia River involves heterogeneous and anisotropic geologic units dominated by variably coarse to fine continental clastic sediments. Specific units include the Neogene Ringold Formation, the late Pliocene to Pleistocene Cold Creek Unit, the Pleistocene Hanford formation, and Holocene alluvial sediments all of which are underlain by Columbia River Basalt. The Ringold Formation forms much of the unconfined aquifer beneath the site and consists of sediments deposited by the proto-Columbia River system over about a 5 Ma aggradational period. Facies include fluvial channel and overbank deposits, lacustrine deposits, alluvial fans, and paleosols. The Cold Creek Unit disconformably overlies the Ringold Formation and consists of eolian, alluvial, and colluvial deposits with paleosols. The Hanford formation occupies much of the vadose zone on the site and consists of boulder to silt size sediments deposited by a series of cataclysmic Ice-Age floods, mostly from glacial Lake Missoula. Mapping of the sedimentary facies in the subsurface using excavations, boreholes and geophysical data, has identified heterogeneity and anisotropy that control unsaturated and saturated flow in the supra-basalt sediments (e.g., polygonal networks of clastic dikes). Understanding the flow and transport properties of sediment facies at Hanford is crucial to development of reactive transport models at a range of scales. Only when we can relate pore- and mm-scale transport phenomena (including a full range of geochemical and microbial reactions) to detailed facies models extensible to the decimeter and kilometer scale will we be able to fully exploit Hanford geology for improving predictions of future risk posed by contaminants at the site.