Reactive transport modeling of redox geochemistry: approaches to chemical disequilibrium and reaction rate estimation at a site in northern Wisconsin
The purpose of this study is to investigate the hydrology and redox geochemistry of shallow groundwater discharging to a stream in northern Wisconsin. In this organic-rich aquifer, we observe both oxygen reducing zones and iron reducing zones whose boundaries are roughly constant over time. To investigate the apparent steady state between solute fluxes and redox reaction rates, we develop a reactive transport model of carbon oxidation. We use a “quasi-kinetic, “partial-equilibrium approach to modeling redox reactions, a hybrid approach between traditional equilibrium approaches and fully kinetic approaches that require large computer resources. Our model suggests that observed trends in redox sensitive elements can only be explained by oxidation rates that are both dependent on the predominant electron acceptor and are spatially variable. Our coupled models provide field-based estimates of redox kinetics, which are otherwise difficult to obtain in hydrologically complex systems.