PNNL

Abstract

Contaminants in the deep vadose zone (DVZ) pose a long-term threat to groundwater, human health, and the environment. It is often a challenge to treat contaminants in the DVZ due to the diversity of pollutants and the presence of fine particles representing inaccessible DVZ regions. As such, many current groundwater treatment methods are ineffective or too costly. Polyurethane grouting is a type of chemical grouting that is potentially advantageous to immobilize contaminants in the DVZ. Polyurethane resin has low viscosity with the ability to penetrate and fill the pore space between fine particles to encapsulate contaminants and decrease soil permeability. In this study a series of laboratory pressure grouting and leaching tests were conducted to simulate and assess the effectiveness of polyurethane grouting for immobilizing contaminants in the DVZ. Soil with fine particles was prepared with 127I that served as a non-radioactive surrogate for radioactive 129I, a contaminant of concern at the Hanford Site, a decommissioned nuclear production complex site. The soil was grouted with polyurethane under overburden soil pressure to simulate the grouting in the DVZ. After curing, leaching tests were used to measure and compare contaminant diffusivity and leachability index values. Through comparison with control soil tests (i.e., ungrouted), the effectiveness of polymer grouting is quantified and contaminant leaching mechanisms are elucidated. In addition, grouted soil was characterized by X-ray Computed Tomography (XCT) to analyze the distribution of cured polyurethane in the grouted soil. Changes in porosity and saturated hydraulic conductivity of grouted soil were also measured. Results showed that the cured polyurethane was distributed nearly homogeneously and approximately half of the voids were filled with cured polyurethane. Grouting reduced the saturated hydraulic conductivity of soil by 37%. The effective diffusivity of 127I in the grouted soil decreased by more than 80% as compared with the control, ungrouted soil. The leachability index of the grouted soil was 6.5; meeting the criteria established by the U.S. Nuclear Regulatory Commission (NRC) standard for materials intended for burial with radioisotopes. Results also show that soil with reduced moisture content can exhibit lower effective diffusivity, higher leachability index, and smaller amount of contaminant displacement caused by pressure grouting. The results obtained in this study provide a valuable assessment of polyurethane grouting for contaminant immobilization in the DVZ and indicate this approach may be a viable method for contaminant remediation in DVZ soils.