NATURE AND EXTENT OF VADOSE ZONE CONTAMINATION AND REMEDIAL APPROACHES AT THE HANFORD FEDERAL FACILITY, WASHINGTON

The U.S. Department of Energy (DOE) Hanford Site in south central Washington State lies along the Columbia River and is one of DOE’s largest legacy waste management and remediation sites. Radionuclide and chemical inventories exist below-ground and are estimated at 550,000 curies of radioactivity and 150 million kg of metals and hazardous chemicals. Contaminants spread through the thick vadose zone (exceeding 60 meters in the central plateau) and created plumes, some of which have migrated into the underlying aquifer. The migration of these contaminants is controlled by the chemical characteristics of the contaminants and the heterogeneities associated with the Mio-Pliocene Ringold Formation and Pleistocene Hanford Formation.

The vadose zone contamination categories can be described as:

1. laterally extensive with intermediate depth (ground surface to about 45 meters) and mostly related to high volume effluent discharge into cribs, ponds and ditches from Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) facilities,
2. dominated by laterally less extensive areas mostly related to leaks from Resource Conservation and Recovery Act tank farms and often commingled at depth with wastes from adjacent CERCLA facilities,
3. created from the high discharge volume from CERCLA facilities and extend from the surface to more than 60 meters below ground and where contamination crossed the entire thickness of the vadose zone and reached groundwater, and
4. miscellaneous lower volume waste sites.

While contaminants in shallow sediments can be removed by excavation or hydraulically controlled by surface engineered barriers, contaminants in the deeper vadose zone rest beneath the influence of these technologies. The purpose of the deep vadose zone remediation is to protect the underlying aquifer by reducing contaminant flux through the use of natural system and /or engineered actions.

The major deep vadose zone remediation technologies considered are soil desiccation, in-situ gaseous reduction, grout injection, soil flushing, and surface barriers. Results of the pilot scale test of soil desiccation are very promising and more test results are forthcoming. Other technologies are being tested at the laboratory and await field demonstration.