Contaminant displacement caused by polyurethane grouting in the deep vadose zone

Polyurethane grouting is a type of chemical grouting with the potential to immobilize contaminants in the deep vadose zone (DVZ), which lies above the groundwater table and typically below the depth at which open excavation is practical or cost-effective. One concern with grouting is the risk of mobilizing contaminants and dispersing them over a wider area, potentially worsening the situation. This study presents laboratory grouting tests to investigate the contaminant displacement during polyurethane grouting in the DVZ at the Hanford Site. Additionally, the study evaluates the effectiveness of soil desiccation and soil freezing in reducing contaminant displacement by reducing the amount of movable containing contaminant water. The test results indicate that polyurethane grouting immobilizes 9.1 ± 3.5% of contaminants. Soil desiccation effectively reduces contaminant displacement to 2.3%. In contrast, soil freezing does not significantly reduce contaminant displacement, primarily due to two factors: polyurethane flushing and gas production during curing. Gas generated during polyurethane curing can mobilize soil water, leading to contaminant displacement. Test results with different rates of gas production also indicate higher rates of gas production lead to a larger iodide displacement ratios. The resin with the highest measured gas production rate of 1.56 × 10^-4 mol/(mL·h) resulted in a displacement ratio of 0.351, whereas the resin with the lowest measured gas production rate 7.00 × 10^-5 mol/(mL·h) resulted in a much lower displacement ratio of 0.101 when both tests used soil water content of 5%. These findings offer valuable insights for using polyurethane grouting to immobilize contaminants in DVZs.