A unified conceptual model of coupled reactive transport involving organic and inorganic species in groundwater using a partial disequilibrium approach

The chemical degradation of organic contaminants in subsurface environments is clearly coupled with the local geochemical environment. Abiotic and biologically-mediated transformation pathways of petroleum hydrocarbons, chlorinated solvents, and various pesticides are energetically favorable only under certain conditions (primarily redox-controlled). Furthermore, the viability of microorganisms which participate in biodegradation reactions is strongly dependent on the local redox environment. Conversely, degradation reactions may feed back into local geochemistry by altering redox speciation through coupled oxidation-reduction reactions, changing pH through hydrocarbon mineralization, and by possibly inducing mineral precipitation/dissolution reactions. Transport processes play important roles in such systems, not only in the downgradient migration of contaminant plumes but also in the mixing-in of fresh oxidizing and reducing agents, generating a disequilibrium environment conducive to further reaction.;Mathematical modeling may help provide quantitative or semi-quantitative insights into the chemical evolution of such systems on different spatial and temporal scales, particularly in the presence of heterogeneities. However, reactive transport modeling approaches based upon the commonly-used local equilibrium assumption are not directly applicable due to the disequilibrium state of degrading organic constituents. The empirical approaches used in biodegradation modeling describe reaction kinetics adequately well but do not address interactions with local geochemistry. In this study, a new conceptual approach is developed as a synthesis of these approaches. The proposed model utilizes the two-step approach to reactive transport modeling, with a partial disequilibrium model for calculating equilibrium-based behavior of inorganic species and disequilibrium-based (i.e. kinetic) behavior of the organic compounds. Coupling between the two chemical models is achieved via the thermodynamic driving forces on the degradation reactions, valence electron conservation, and elemental mass balance.;The modeling approach is applied to several contamination problems, with the predicted results indicating good agreement with field and laboratory studies. Other issues beyond contaminant hydrology are also explored, including the generation of anaerobic zones within embedded clay lenses in aerobic aquifers, and the role of biodegradation of Al- and Fe-chelating agents in the formation of spodosols. These examples help to demonstrate the utility of the conceptual model in interpreting the complex interactions between organic transformations, geochemistry, and solute transport in the subsurface.