INVESTIGATION OF THE PERFORMANCE OF PERMEABLE REACTIVE BARRIERS USING DISSOLVED GAS ANALYSIS

In permeable reactive barriers (PRB’s) comprised of organic material or zero-valent iron, the strongly reducing nature of the treatment materials may lead to gas production, potentially resulting in the formation of gas bubbles and ebullition. These processes affect dissolved gas concentrations, and dissolved gas analyses can therefore be useful to help delineate reaction and transport processes occurring in PRB systems. Dissolved gas data were collected at the Nickel Rim Mine organic carbon PRB near Sudbury, Ontario during July 2003, and at the Campbell Mine zero-valent iron (Fe0)/organic carbon PRB test cell in Balmertown, Ontario during June 2004. Both barriers are designed to treat groundwater impacted by mine drainage. Concentrations of dissolved Ar, CH4, CO2, H2, H2S, N2, and O2 were determined on-site using gas chromatography and a headspace technique. Within the PRBs, Ar and N2 concentrations are depleted in comparison to atmospheric levels. These results indicate that degassing is occurring within the barriers and variations in the degree of Ar and N2 depletion can be used to delineate zones of preferential flow. At the Nickel Rim site, analysis of the dissolved gas data provides additional information to identify dominant terminal electron accepting processes (TEAP’s), and was used to estimate rates of microbially mediated sulfate reduction. Observed CO2 concentrations could be explained by the occurrence of methanogenesis and sulfate reduction. On average, over 90% of the CO2 is produced by sulfate reduction, while the remaining CO2 is produced by methanogenesis. At the Campbell Mine PRB the unsaturated zone is enriched in N2 and Ar, indicating that ebullition and oxidation of CH4 is occurring.