Deep vadose zone contaminant immobilization with polyurethanes and epoxy chemical grouts

The deep vadose zone (DVZ) lies below the depth where practical and cost-effective removal of contaminants through open excavation is feasible. In situ grouting is a potential approach to immobilize contaminants and prevent their spread into groundwater aquifers. Polyurethane and epoxy-based grouting resins have low viscosities, enabling them to infiltrate low-hydraulic conductivity soils commonly found in the DVZ. When these resins cure, they form a solid polymer grout that effectively fills voids within the soil to decrease soil hydraulic conductivity and diffusivity, which limits contaminant transport. In this study, the performance of three different resin-based materials, polyurethane, polyurethane foam, and epoxy, was evaluated on the basis of decreasing soil porosity, soil hydraulic conductivity, and iodide diffusivity in simulated contaminated soil representative of a DVZ environment. The test results indicate that the iodide leachability index of specimens grouted with these different resins all exceed the value of 6, which fulfills the criteria set by the U.S. Nuclear Regulatory Commission (NRC) standard for solid waste intended for shallow burial. All grouted samples demonstrate an exponential relationship between porosity and diffusivity that aligns with Archie's rule, where samples with lower porosity tend to exhibit lower diffusivity. The minimum porosity of grouted soil obtained is 5.1%, and the minimum iodide diffusivity recorded is 2.11 × 10⁻⁸ cm²/s. The results also show that lower soil water content leads to lower porosity, diffusivity, and hydraulic conductivity in polyurethane grouted samples. The findings from this study provide valuable guidance for the use of polyurethane and epoxy grouting in limiting contaminant transport in DVZ environments.