Quantifying the means by which hydrogeologic, microbiologic, and geochemical processes combine to drive spatial and temporal variability in elemental cycles is a fundamental issue in addressing environmental concerns related to human health and long-term ecological sustainability. The biogeochemical cycling (fate and transport) of numerous chemical species, including nutrients and anthropogenic contaminants, are controlled by changes in the reduction-oxidation potential or redox state of a system. Many redox reactions including the degradation of organic contaminants and nutrient cycles are mediated by the metabolic activities of microorganisms. Microbial metabolism, through a series of terminal electron accepting processes (TEAPs), impacts the form, mobility, toxicity and persistence of many chemical constituents in aqueous systems. My research focuses on understanding the controls on the spatial and temporal variability of TEAPs which is necessary to evaluate health and safety concerns such as: chemical routes of exposure (risk assessment), natural attenuation and bioremediation capabilities, and the management of redox sensitive environments such as wetlands and estuaries.