PNNL

Abstract

The deep vadose zone (DVZ) poses significant challenges for contaminant remediation due to its depth and the high costs of accessing it. This study examines the use of chemical grouts to mitigate contaminant leaching from DVZ sands, focusing on iodine-129 contamination resulting from nuclear waste processing and disposal at sites where contamination of the DVZ is an issue, such as the Hanford Site in WA. Current remediation efforts at the site struggle to address contaminants at these depths. This research evaluates the effectiveness of three commercially available chemical grouts—an epoxy resin, a polyurethane (PU) grout, and a PU-foam grout, in forming stable sand-polymer monoliths that can encapsulate contaminants like iodine-129 and prevent further migration. The study involved injecting these grouts into sand samples to assess their potential to reduce permeability and leaching of iodide ions, the model contaminant used in this study. The research measured the rate of gas evolution during grout curing, foam expansion, and the effectiveness of each grout in trapping contaminants within the sand matrix. The foam produced by polyurethane, catalyzed by tertiary amines, expands 3 to 12 times its original volume depending on the amount of catalyst used. These foams displaced less than 1% of iodide due to moisture in the samples being consumed by reactions with methylene diphenyl diisocyanate (MDI), leaving little excess moisture and associated mobile iodide as the resins cured. Nearly half (45%) of the iodide was displaced during the injection of epoxy grout when moisture was present. This was attributed to the immiscibility of water containing iodide with epoxy, which was displaced from the monoliths during curing. All MDI grouts had porosity values ranging from 16.59% to 19.45% and diffusivities between 1.84x10-8 to 2.30x10-7 cm²/sec. Catalyzed grouts generally showed lower porosity and diffusivity with higher ratios of added catalyst. Results indicate that the chemical properties and application techniques of these grouts significantly influence their performance, with PU-based grouts showing promise in creating impermeable barriers that reduce contaminant mobility in DVZ sands. This study provides insights into the potential application of chemical grouting as a viable method for the long-term stabilization of contaminants in challenging subsurface environments like the DVZ.