Reactive transport modeling: an application to redox geochemistry of groundwater discharging to a stream in northern Wisconsin
Elizabeth H. Keating
Abstract: Reactive-transport models are useful tools in interpreting water chemistry data when both transport and geochemical reactions are thought to be important processes. Most existing models suffer from two limitations: 1) assumption of chemical equilibrium, a rarely-achieved state for many mineral weathering reactions and for most redox reactions, and 2) a paucity of published examples of calibration to field data. The goal of this dissertation has been to address both of these limitations, with particular emphasis on modeling approaches to redox reactions. An existing reactive-transport model has been modified to approximate a quasi- kinetic, pailial-equilibrium approach to redox reactions. A model calibration is presented using hydrologic and groundwater chemistry data from each of three sites along Allequash Creek in northern Wisconsin.
Water chemistry data collected at these sites exhibit both small-scale (on the order of centimeters to meters) and intermediate-scale (on the order of kilometers) variability. Five geochemically distinct waters were identified in groundwater discharge zones near and beneath Allequash Creek. Equilibrium geochemical modeling suggests that silicate hydrolysis and oxygen and iron oxide reduction associated with the oxidation of naturally-occurring organic carbon are important reactions in this aquifer. These reactions do not appear to be at equilibrium. Equilibrium geochemical models and a coupled reactive-transport model (HYDROGEOCHEM) were used to investigate groundwater flow paths and reaction kinetics at these sites.
Acid base, redox, and precipitation/dissolution reactions were incorporated into the reactive-transport model. Redox reactions were modeled using a quasi-kinetic, partial-equilibrium approach developed as part of this dissertation. Calibration results suggest that silicate hydrolysis kinetics are pH-dependent, as others have suggested. The rate of organic carbon oxidation in this aquifer strongly depends upon the predominant electron acceptor. each of these three sites, a uniform approach to site calibration significantly reduced Although uncertainties were present in the hydrologic characterization of each of these three sites, a uniform approach to site calibration significantly reduced the degrees of freedom in the model calibration process. The modeling method presented here provides a useful construct for testing and eliminating hypotheses concerning important hydrologic and geochemical processes in a groundwater discharge zone.
PhD Thesis, University of Wisconsin-Madison, 1995, 216 p.
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