For redox active contaminants, such as selenium, reductive transformations are particularly critical in natural and accelerated attenuation owing to the higher mobility and toxicity of their oxidized states. Reduction of Se is largely controlled by microbial processes. Reducing conditions are therefore important for immobilizing selenium at contaminated sites throughout the San Joaquin Valley (CA). However, the subsurface is physically complex and the resulting redox (micro)environments, and associated metabolic processes, are heterogeneously distributed in both space and time. A qualitative and quantitative understanding of variations in the redox processes operating at the microscale will therefore be critical in developing comprehensive and predictive models describing the dynamics of biogeochemical systems.
The present research project proposes to examine spatial heterogeneity of selenium reduction within physically complex systems representative of natural environments, with structural complexity that integrate, rather than segregate, biological, geochemical, and physical processes. The overall objective is to determine what key physical and biogeochemical factors determine the extent and location of selenium reduction and the transport between compartments at the soil aggregate scale. The focus will be on (1) understanding how localized biogeochemical environments are controlled by the intrinsic microbial activity and dynamics versus mass transfer limitations, (2) identifying the key environmental determinants that control Se-reducers diversity, and (3) establishing how Se-reducers community structure and biogeochemical (micro-)heterogeneity affect the transformation rates of selenium in the subsurface.