Publication

Colloid-sized particles are commonly detected in vadose-zone pore waters and are capable of binding chemicals with sorptive affinities for geologic materials. Published research demonstrates that colloids are capable of facilitating the transport of sorptive contaminants under conditions of steady pore water flow, when volumetric moisture content and pore water velocity are constant Less is known about the role of colloids in governing contaminant mobility under transient-flow conditions, which are characteristic of natural vadose-zone environments. The objective of this study is to elucidate the influences of flow transients on the mobilization and transport of in situ colloids and colloid-associated contaminants. We conducted column experiments in which the mobilization of in situ colloids and Cs-137 was induced by transients associated with the drainage and imbibition of Cs-137 contaminated-sediments. Our results demonstrate that substantial quantities of in situ colloids and colloid-associated Cs-137 are mobilized as volumetric moisture content declines during porous-medium drainage and as volumetric moisture content increases during porous-medium imbibition. We also find that the colloid-effect on Cs-137 transport is sensitive to changes in pore water ionic strength. That is, the quantities of colloids mobilized and the capacity of the these colloids to bind Cs-137 decrease with increasing ionic strength, leading to a decrease of the mass of Cs-137 eluted from the columns during porous-medium drainage and imbibition.