Colloid Formation and Transport during Leakage of REDOX Tank Waste Solution into Hanford Vadose Zone

To improve the understanding of radionuclide migration in the DOE Hanford Site, the processes of the REDOX tank waste solution leakage into the vadose zone sediments, and subsequent rainfall displacement were simulated in laboratory columns at ambient and elevated temperatures. We define the solution-sediment interactions as three stages: the leading edge of the waste plume, waste plume stage, and dilution stage. Secondary colloids formed at different stages and temperatures were characterized for their chemical composition, mineralogy, and size distributions. Maximum colloid formation occurred at the leading edge of the waste plume, due to maximized geochemical disequilibrium. Maximum transport of the secondary colloids also occurred at the leading edge, where pores have not yet been plugged by particle production, and new colloids have not yet fully attached to primary grain surfaces. Because of their abundance, small size, high surface reactivity and mobility, the secondary colloids can be important in enhancing contaminant transport under Hanford type geochemical conditions.