The main purpose of this project was to improve the fundamental mechanistic understanding and quantification of long-term colloid mobilization and colloid-facilitated transport of radionuclides in the vadose zone, with special emphasis on the semi-arid Hanford site. While we focused some of the experiments on hydrogeological and geochemical conditions of the Hanford site, many of our results apply to colloid and colloid-facilitated transport in general. Specific objectives were (1) to determine the mechanisms of colloid mobilization and colloid-facilitated radionuclide transport in undisturbed Hanford sediments under unsaturated flow, (2) to quantify in situ colloid mobilization and colloid-facilitated radionuclide transport from Hanford sediments under field conditions, and (3) to develop a field-scale conceptual and numerical model for colloid mobilization and transport at the Hanford vadose zone, and use that model to predict long-term colloid and colloid- facilitated radionuclide transport. To achieve these goals and objectives, we have used a combination of experimental, theoretical, and numerical methods at different spatial scales, ranging from microscopic investigations of single particle attachment and detachment to larger-scale field experiments using outdoor lysimeters at the Hanford site. Microscopic and single particle investigations provided fundamental insight into mechanisms of colloid interactions with the air-water interface. We could show that a moving air water interface (such as a moving water front during infiltration and drainage) is very effective in removing and mobilizing particles from a stationary surface. Field experiment using a vadose zone lysimeter facility at the Hanford site showed that surface-applied Eu colloids can be translocated rapidly under natural precipitation as well as artificial irrigation. Small amounts of applied colloids were translocated from the surface to a depth of two meters within two months and only 20 mm of cumulative infiltration. Large water infiltration events, mimicking snow melt, enhanced movement of Eu colloids. Nonetheless the majority of Eu colloids remained in the top 30 cm of the soil after 3.5 years of monitoring. These results suggest that colloid and radionuclide transport can occur in the near-surface vadose zone at Hanford under field conditions, but that the magnitude of the transport is less than what has been reported from laboratory studies. We further studied colloid mobilization from undisturbed sediment cores under a flow rate of 18 mm/year, a typical low flow rate at Hanford. Under this low flow rate, we observed continuous colloid mobilization from the sediments, although the total amounts of colloids mobilized are small, only 0.5% of available colloids were mobilized during 5 years of observations. These results demonstrate that colloidal particles are mobile even under the low recharge rates found in a semi-arid site like Hanford. Under higher flow rates, we would expect colloid transport to be even more pronounced. These results of our study are particularly relevant for colloid mobilization and transport related to three process in the vadose zone at Hanford: (1) water infiltration into sediments during rainfall or snowmelt events, (2) groundwater fluctuations as caused by river stage fluctuations, and (3) steady-state, low-flow recharge in deep vadose zone sediments. Transient water flow, like during infiltration or groundwater level fluctuations, are most conducive for colloid mobilization, but even during steady-state, low-flow recharge, colloids can be mobile, although to a much lesser extent. The results of this project have led to a comprehensive and fundamental understanding of colloid transport and mobilization under unsaturated flow conditions at the Hanford site.
