UBC Theses and Dissertations
Using dissolved gas analysis to investigate the performance of permeable reactive barriers Williams, Randi Lee
The strongly reducing nature of permeable reactive barrier (PRB) treatment materials can lead to gas production, potentially resulting in the formation of gas bubbles and ebullition. For this work degassing in the saturated zone of PRB systems due to the production of gases (primarily CO₂ and CH₄) is investigated using the depletion of Aland N₂, naturally present non-reactive gases, in order to identify, confirm, and possibly quantify chemical and physical processes occurring. Dissolved gas sampling and analysis were conducted at three PRB sites designed for the treatment of groundwater contaminated by mining and industrial activities: the Nickel Rim Mine Organic Carbon PRB Site (Site I), the Campbell Mine Zero-Valent Iron (Fe⁰)/ Organic Carbon Test Cell PRB (Site II), and the Columbia Nitrogen Fe⁰/ Organic Carbon Mixed PRB Site (Site III). At Site I, residence times within the PRB are sufficiently long to allow gas production and degassing. A simple four-gas degassing model was used to analyze the data set, and the results indicate that sulfate reduction is by far the main process of organic carbon consumption within the barrier. The data provided additional information to delineate rates of microbially mediated sulfate reduction and to determine slow and fast flow zones within the barrier. Degassing was incorporated into reactive transport simulations for Site I in order to model 8 years of barrier operation. The simulations adequately reproduce observed dissolved gas trends, although no information on the volume change due to bubble formation or the fate of the trapped gas could be obtained. At Site II, residence times were short and the dissolved gas data could be used primarily as a transport tracer. Zones of preferential and of low flow could be identified within the PRB. At Site III, the strong resemblance of water composition upgradient and downgradient of the PRB suggested that residence times are long and that there is little flow through the PRB. The dissolved gas data could primarily be used as a reaction tracer. The data suggested that gas production and reaction rates are relatively insignificant in the barrier system. The success and failures at Sites I-III could be used to create a set of criteria under which dissolved gas analysis is useful for PRB systems. Treatment material composition, dissolved gas composition in the groundwater influent to the PRB, and residence times through the PRB are important factors to consider.
Item Citations and Data