Modeling reactive transport of organic compounds in groundwater using a partial redox disequilibrium approach

Abstract

The chemical transformation of organic contaminants in natural groundwater systems is clearly dependent upon local geochemistry which determines the thermodynamically favorable degradation reactions and the nature of local microbial populations. Conversely, groundwater geochemistry may be impacted significantly in terms of pH and redox couple speciation by the chemical transformation of sufficient quantities of organic compounds. Therefore an understanding of the coupling between degradation reactions, local geochemistry, and chemical transport is essential in predicting the chemical evolution of contaminated aquifers. Equilibrium‐based reactive chemical transport models are usually not utilized for problems involving the transport of degradable organic compounds due to slow reaction kinetics and the persistence of intermediate degradation products. In this study we propose a reactive geochemical transport model which considers these types of degradation reactions. An expert system approach is used to postulate a set of sequential, first‐order degradation reactions for the organic compounds based upon thermodynamic considerations and user‐defined rules. Redox disequilibrium provides the driving force for the abiotic or microbially mediated transformation of the organic compounds as well as the associated response of groundwater geochemistry. Coupling between local inorganic geochemistry and reacting organic compounds is achieved by assuring conservation of operational valence and mass balance. The composite geochemical model is in turn coupled with an integral finite difference transport algorithm using a two‐step sequential solution approach. The transport equation is solved separately for each inorganic aqueous species, complex, and dissolved organic species, allowing a high degree of flexibility in problem definition. We apply the model to an illustrative example problem concerning the introduction of aromatic hydrocarbons and chlorinated ethenes into an initially oxidizing aquifer. Modeling results agree well qualitatively with field and laboratory observations reported in the literature in terms of degradation patterns and the effects on groundwater geochemistry.