Multi-region Transport and Competitive Ion Exchange in Partially Saturated Porous Media

In most natural subsurface settings cesium sorbs very strongly to sediments, effectively limiting its transport. At the Hanford Site in Washington State (USA), vadose zone migration of 137 Cs from subsurface high-level radioactive waste tanks has been detected over 40 meters below the ground surface. In response, a comprehensive investigation of geochemical and hydrologic processes controlling cesium mobility in Hanford sediments was initiated. Geochemical laboratory studies investigated the ability of electrolytes in the hypersaline tank fluid to outcompete 137 Cs for exchange sites in the Hanford sediments. Depending on tank temperatures, sodium concentrations in the tank waste could be as high as 19 M. Batch and saturated laboratory column studies provided the basis for a quantitative multisite, multicomponent ion exchange model of Cs + competition with Na + , K + , Ca ++ , and Mg ++ in a composite Hanford soil. To test the validity of this model under unsaturated conditions, a series of reactive transport column experiments were performed in an ultracentrifuge at different liquid saturations. For each experiment, a constant, uniform saturation was maintained using a steady influx of a 5 M sodium nitrate solution with 5.4E-5 M cesium iodide. Our interest was the potential for enhanced cesium transport due to the presence of immobile liquid and/or bypassed regions. We were also interested in the variability of cesium reactivity that may be associated with specific regions, especially the case where the mobile liquids were selectively exposed to less reactive mineral surfaces. To this end, mobile and immobile fluid fractions, dispersion, and the rate of mass transfer between mobile and immobile regions were determined from the tracer breakthrough. At higher saturations (≈65%), the tracer and cesium behavior could be predicted to a large degree using a single mobile region with the previously developed multicomponent ion exchange model. At lower saturation (≈23%), however, the tracer breakthrough indicated a relatively large immobile fluid fraction, which could be described only with a multi-region approach. In this experiment, however, cesium broke through earlier and at higher concentrations than predicted by multi-region theory combined with the existing cesium ion exchange model. This behavior is consistent with a lower density of exchange sites in the immobile fluid region.