Kinetic modeling of microbially-driven redox chemistry of subsurface environments: coupling transport, microbial metabolism and geochemistry

This paper deals with the treatment of subsurface environments as reactive biogeochemical transport systems. We begin with an overview of the effects of microbial activity on the chemical dynamics in these environments. Then, after a review of earlier modeling efforts, we introduce a one-dimensional, multi-component reactive transport model that accounts for the reaction couplings among the major redox and acid-base elements, O, C, H, N, S, Mn, Fe and Ca. The model incorporates kinetic descriptions for the microbial degradation pathways of organic matter, as well as for the secondary redox reactions and mineral precipitation-dissolution reactions. Local equilibrium only applies to fast homogeneous speciation reactions and sorption processes. The model is used to simulate the distributions of chemical species and reaction rates along flow paths in two subsurface environments. In the first case, waters containing moderate levels of natural soil-derived organics supply a regional groundwater system. In the second case, a pristine aquifer is contaminated by an organic-rich leachate from a landfill. In both environments, the microbial oxidation of organic matter causes the disappearance of dissolved and solid oxidants and the appearance of reduced species, albeit over very different spatial scales. In the second case, a pronounced reaction front develops at the downstream edge of the contaminant plume. The reactivity, or biodegradability, of the organic matter is shown to be a major factor governing the biogeochemical dynamics in the plume. The simulations predict different distributions of the biodegradation pathways, depending on whether the organics of the leachate have uniform or variable reactivity. The secondary reactions also have a significant impact on the concentration profiles of inorganic species and the spatial distributions of the biodegradation pathways. Within the downstream reaction front, large fractions of O-2, Mn(IV), Fe(III) and SO42- are reduced by secondary reactions, rather than being utilized in the oxidative degradation of leachate organics, overall, the model simulations emphasize the strong coupling between subsurface heterotrophic activity and an extensive network of secondary reactions. (C) 1998 Elsevier Science B.V. All rights reserved.