Abstracts Chemistry

Unintentional contaminant transfer from groundwater to the vadose zone via gas exsolution and ebullition during remediation of volatile organic compounds

by Andrea Denise Chong

Historical heavy use of chlorinated solvents in conjunction with improper disposal practices and accidental releases has resulted in widespread contamination of soils and groundwater in North America. As a result, remediation of chlorinated solvents is required at many sites. For treatment of source zone contamination, common remediation strategies include in situ chemical oxidation (ISCO) using potassium or sodium permanganate, and the enhancement of biodegradation by primary substrate addition. It is well known that these remediation methods tend to generate gas (carbon dioxide (CO₂) in the case of ISCO using permanganate, CO₂ and methane (CH₄) in the case of bioremediation). It is hypothesized that the generation of gas in the presence of volatile organic compounds (VOCs), including chlorinated solvents, may lead to stripping of the contaminants from the source zone due to gas exsolution and ebullition. This process may lead to ‘compartment transfer’, whereby contaminants are transported away from the saturated zone into the vadose zone, with possible implications for soil vapour intrusion. Two sites in the U.S. undergoing enhanced bioremediation have exhibited behavior suggestive of contaminant transfer into the vadose zone via gas generated during remediation. These sites provided the impetus for a more in-depth investigation into this process. To this extent, benchtop column experiments were conducted to observe the effect of gas generation during remediation of the common chlorinated solvent trichloroethylene (TCE/C₂Cl₃H). Two common in situ treatment strategies were simulated for source-zone subsurface contamination of TCE, including ISCO and enhanced bioremediation. Results confirm that these aggressive remediation methods can lead to gas production and induce vertical transport of contaminants away from the treatment zone, following the formation of a discontinuous gas phase (bubbles). The generation of gas and the potential for unintentional contaminant stripping and transport should be taken into consideration when treating VOCs to avoid release into the atmosphere or into underground structures via soil vapour intrusion. This study also suggests that the suitability of gas-generating remediation techniques in proximity to buildings and in populated areas should be evaluated with care.