Large quantities of radiological and chemical wastes are located in the vadose zone at the Hanford Site in southeastern Washington State. Research at Hanford suggests that large quantities of colloids can be mobilized and transported in the vadose zone under transient flow. Our goal is to quantify in situ colloid mobilization and colloid-facilitated radionuclide transport in Hanford sediments under field conditions. We have carried out experiments of colloid transport in four 7.6-m deep lysimeters. The lysimeters have been irrigated with synthetic Hanford rainwater to represent the natural, wet, very wet and chinook conditions. Fiberglass wicks were installed at multiple depths to collect soil solution for colloid analysis. Additionally, the lysimeters are equipped with neutron access tubes to measure soil water content and tensiometers to measure pressure head. We have applied synthesized 200-nm size Eu2O2CO3 colloids and have characterized their surface and colloid properties. Different-sized polystyrene colloids have also been applied as tracer colloids. It is expected that, under natural conditions at Hanford, colloid-facilitated radionuclide transport can occur but the magnitude strongly depends on the recharge rates.
Key Words: radionuclide; colloid transport; vadose zone