Subsurface Carbon Cycling in Bioremediation of Acid Mine Drainage: Experiments and Modeling

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).0854510SilversteinAcid Mine Drainage (AMD) is a serious environmental problem resulting from oxidation reactions in sulfidic rock, which is mediated by iron-oxidizing bacteria (IOB). The principal objective of this proposal is to develop a fundamental quantitative understanding of the coupled biogeochemical processes involved in the pyrite oxidation cycle and the carbon cycle, facilitating the evaluation and design of a sustainable remediation process. The proposed work is a follow-up to previously-funded bench-scale studies of the use of organic carbon additions for mitigation of AMD, which demonstrated that addition of organic carbon to acid generating rock stimulates the growth of heterotrophic microorganisms, consuming oxygen and sequestering ferric iron, thus inhibiting pyrite oxidation. The proposed research, which combines experiments and modeling across a range of scales, consists of the following major tasks: (1) Monitor reactions of AMD generation and inhibition under simulated environmental conditions for sufficient duration to determine mineral and organic carbon storage and transport from the interstitial pore to waste rock pile scale. (2) Develop expressions for microbial and chemical kinetics and diffusion rates in waste rock to use in computational models. (3) Characterize transport and storage of reactive organic and mineral constituents along flow paths in waste rock media over the range of spatial and temporal scales, to evaluate the sustainability of carbon addition for in-situ AMD remediation. (4) Incorporate the understanding gained from (1) - (3) into a rigorous reactive transport model and combine it with models of complex unsaturated flow in waste rock systems to predict the behavior at the waste-rock column and pile scale. (5) Carry out sensitivity analysis to evaluate the robustness of carbon addition in inhibiting AMD formation in waste rock under a variety of environmental and hydrologic conditions. The remediation strategy being explored in this project for in-situ control of AMD generation can potentially result in significant reduction in cost compared with current approaches. Moreover, the study of carbon addition to waste rock is an opportunity to identify the mechanisms that control carbon cycling in the subsurface after perturbation of an existing biogeochemical system. Once developed, the multi-scale model approach could be used to assess sustainability of other bioremediation methods such as constructed wetlands and bioreactive subsurface barriers. The multi-disciplinary nature of the research will provide unique opportunities for graduate student research and training in environmental engineering - with broad exposure to hydrology, geochemistry, microbiology and modeling. Large lab columns will provide educational and outreach opportunities targeted at undergraduate students, stakeholder groups and K-12 students in Colorado, where the environmental impacts of AMD are widespread. A diverse group of undergraduate student researchers will be recruited through the University of Colorado REU site in environmental engineering and the Summer Multicultural Access to Research and Training (SMART) program.