IN-SITU COLLOID MOBILIZATION IN HANFORD SEDIMENTS UNDER UNSATURATED TRANSIENT FLOW CONDITIONS: EFFECT OF IRRIGATION PATTERN

Colloid transport is a potential means for facilitated off-site relocation of radioactive wastes at the Hanford site of Washington State. In this study, a series of column experiments were conducted to examine the effect of irrigation pattern on releases of in-situ colloids in two Hanford sediments under saturated and unsaturated transient flow conditions and its dependence on solution ionic strength, irrigation rate and sediment texture. The results show that during transient flow more in-situ colloids were released than during steady state flow. The number of short-term hydrological pulses proved more vital than total irrigation volume or persistence time length for increasing the amount of mobilized in-situ colloids. However, increasing ionic strength diminished this effect. At an irrigation rate equal to 5% of the saturated hydraulic conductivity, transient multi-pulse flow in 2 mM NaNO3 was equivalent to a fifty-fold reduction of solution ionic strength (from 100 mM to 2 mM) in a single pulse flow in terms of their positive effects on in-situ colloid mobilization. It was observed that colloid concentrations in the effluent from coarse Hanford sand in the first two pulses of elution were insensitive to the change of experimental irrigation rate, due probably to dominant dispersion of the loosely attached colloids into the streamlines of macropore flow that was induced by non-uniform water distribution between micropores and macropores. However, in subsequent irrigation pulses, effluent colloid concentrations were much lower at the low than the high irrigation rates, due likely to rate-limited detachment of the tightly (more chemically) attached colloids, which is subject to the hydrodynamic forces overcoming the DLVO forces. The experiments also indicate that mechanical straining of colloids in the fine sand greatly decreased the amount of mobilized colloids, although the fine sand contained about one order of magnitude greater concentration of the in-situ colloids than the coarse sand. This study highlights that transient flow is critical for in-situ colloid mobilization, and implies that long-term assessment of colloid release should consider the rainfall pattern and geometrical structure of flow pathways.