In situ mobilization of colloids and transport of cesium in Hanford sediments.

Radioactive waste, accumulated during Pu production, has leaked into the subsurface from underground storage tanks at the U.S. Department of Energy's Hanford site. The leaking solutions contained 137Cs and were of high ionic strength. Such a tank leak was simulated experimentally in steady-state flow experiments with packed Hanford sediments. The initial leak was simulated by a 1 M NaNO3 solution, followed by a decrease of ionic strength to 1 mM NaNO3. Cesium breakthrough curves were determined in both 1 M and 1 mM NaNO3 background. Colloidal particles were mobilized during the change of ionic strength. Mobilized colloids consisted mainly of quartz, mica, illite, kaolinite, and chlorite. Electrophoretic mobilities of colloids in the eluent solution were -3(microm/s)(V/cm) and increased to less negative values during later stages of mobilization. Mobilized colloids carried a fraction of the cesium along. While transport of cesium in 1 M NaNO3 background was much faster than in 1 mM NaNO3, cesium attached to colloids moved almost unretarded through the sediments. Cesium attached to mobilized colloids was likely associated with high affinity sorption sites on micas and illites.