Quantifying the coupled hydrogeological, microbiological, and geochemical processes that control redox potential is a fundamental issue in understanding the fate and transport of nutrients and contaminants in subsurface systems and thus in protecting drinking water and ecosystem health. In subsurface systems, changes in redox state are often controlled by shifts in the terminal electron accepting processes (TEAPs) of microorganisms, initi-ated by the delivery of limiting terminal electron acceptors such as oxygen, nitrate, or sul-fate. Thus, mixing interfaces between reduced aqueous systems and more oxic "re-charge" water, are zones of increased TEAP dynamics. Despite the well-recognized im-portance of mixing interfaces, few field investigations have targeted these small-scale, transient zones due to difficulties in obtaining hydrologic, geochemical and microbial measurements at relevant spatial and temporal scales. This interdisciplinary study seeks to quantify the solute transport, geochemical, kinetic, and microbiological controls on TEAPs at mixing interfaces within a contami-nated aquifer-wetland system. High-resolution numerical models will be developed to integrate observations, test hypotheses regarding the role of interfaces on the overall re-ducing capabilities of the system, understand processes and guide field/laboratory ex-periments, and evaluate the potential effects of changing hydrologic conditions on the fate and transport of nutrients and organic contaminants. To accomplish this and test the central hypothesis that maximum TEAP dynamics, including microbial activity and transformation rates, are observed at interfaces due to the delivery of limiting electron acceptors or donors we will perform the following tasks: 1) map and quantify the distri-bution of TEAPs across significant mixing interfaces during various hydrologic condi-tions; 2) conduct in-situ kinetic studies of electron acceptor utilization rates at induced mixing interfaces; 3) identify in-situ changes in microbial community directly related to changes in water chemistry; and 4) integrate measured controls using numerical models and test hypotheses regarding the impact of mixing-interface zones on biogeochemical cycling during variable hydrologic conditions. Our novel application of existing and re-cently developed tools will make it possible to quantify the complex linkages between small-scale changes in microbial community structure and activity and the corresponding geochemistry. New and fundamental knowledge of the controls on TEAPs at mixing in-terfaces is expected to improve our understanding of the fate and transport of redox-sensitive species including nutrients and anthropogenic contaminants and thereby im-prove our ability to assess risk and protect drinking water and ecosystem function. The findings of this research are expected to be of great value not only to scien-tists searching for improved ways to measure and interpret complex, coupled earth sys-tem processes but also to industry, regulatory agencies, and the general public. To ensure the broader impacts of this research are widely disseminated we have developed strate-gies to 1) involve students (K-12, undergraduate and graduate) through direct employ-ment, course development, and educator training; 2) attract researchers from other fields through workshops, meetings, and peer reviewed publications; 3) educate the public through widely distributed fact sheets, research site tours and web page design; and 4) increase diversity by encouraging the participation of underrepresented groups.
