| The natural processes that exist within shallow alluvial aquifers affect the fate of anthropogenic organic contaminants and inorganic chemicals such as iron. The research conducted for this thesis focused on developing an improved understanding of these processes and applying them for the natural remediation of polycyclic aromatic hydrocarbons (PAHs) in a shallow alluvial groundwater system near a former manufactured gas plant (MGP) site. The PAHs present in MGP-impacted groundwater are derived from coal tar, a by-product of the MGP operations. Iron is often present in reduced groundwater systems, but is even more prevalent at former MGP sites due to the disposal of iron oxide coated wood chips, which were used in the gas purification process.; The results of this work demonstrated that natural remediation can be engineered to restore impacted groundwater at MGP sites thereby protecting off-site receptors---for example nearby surface water. Controlled re-aeration of the anoxic groundwater system is the fundamental component of natural remediation. The introduction of air provides oxygen to stimulate the aerobic degradation of the highly reduced PAHs. Aeration at a controlled rate minimizes the potential for reduced iron in the groundwater to be rapidly converted to amorphous iron hydroxides. Instead, reduced iron is oxidized to a dense iron-oxyhydroxide crystal structure on existing surfaces, including the surfaces of bacteria. Moreover, the controlled oxidation of reduced iron near the surfaces of PAH-degrading bacteria appears to provide additional chemical energy that can be used to support the efflux of potentially toxic aromatic compounds or metabolites. This protective mechanism maintains the integrity of the bacterial cell wall and provides a potential competitive advantage in relation to other bacterial strains in this environment. This function appears to help PAH-degrading bacteria survive within this transitional (anoxic to aerobic) natural remediation environment and support the degradation of PAH compounds. The cell functions responsible for capturing this additional energy source have not been identified at this time and present an opportunity for ongoing research to understand the interaction between redox sensitive inorganic species, such as iron, and the natural remediation of organic pollutants in the environment. |
