MASS BALANCE ASPECTS OF PERSISTENT URANIUM CONTAMINATION IN THE SUBSURFACE AT THE HANFORD SITE, WASHINGTON

A groundwater plume containing uranium has persisted beneath the 300 Area at the Hanford Site far longer than predicted by initial remedial investigation activities. Subsequent groundwater monitoring has revealed that the initial assumptions regarding a) uranium inventories in the subsurface, and b) processes that distribute those inventories via environmental pathways, were inaccurate. For the current plume, estimates suggest a contaminated groundwater volume of approximately 1,000,000 m³ that contains a dissolved mass of uranium typically in the range 60~80 kg. These parameters have remained relatively constant since 2002, despite major surface source removal actions, discharge of contaminated groundwater to the Columbia River, and groundwater withdrawal at a water supply well. Estimates for groundwater discharge to the river, based on a 3-D groundwater flow model, suggest a maximum rate of 315 m³/m_shore/yr under current river conditions. Uranium flux via this discharge can be bracketed by applying a range of typical average concentrations for uranium in the plume, and suggest a likely flux of 20~50 kg/yr, with another 10 kg/yr being removed via the water supply well.

Estimates have also been developed for the inventory of contaminant uranium in various subsurface regions, using sediment and water samples that cover the vadose zone, the zone through which the water table rises and falls, and the uppermost hydrologic unit within the unconfined aquifer. The largest inventory is associated with the vadose zone beneath former disposal facilities and may reach several thousand kilograms, which is a relatively small fraction of the amount historically disposed. Contamination is concentrated in the vadose zone beneath former disposal facilities, but may also be widespread in sediment through which the water table has fluctuated, especially during earlier 300 Area operating conditions. How much of the inventory is available for migration under current conditions to resupply the groundwater plume is a principal uncertainty in the conceptual model. However, as mass balance estimates for the plume become more refined and defensible with field data, limits can be placed on the resupply via downward migration from the vadose zone, thus providing key information for evaluating remediation alternatives.