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 reac tive biogeochemical transport systems. We begin with an overview of th e effects of microbial activity on the chemical dynamics in these envi ronments. Then, after a review of earlier modeling efforts, we introdu ce a one-dimensional, multi-component reactive transport model that ac counts 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 precipitati on-dissolution reactions. Local equilibrium only applies to fast homog eneous speciation reactions and sorption processes. The model is used to simulate the distributions of chemical species and reaction rates a long flow paths in two subsurface environments. In the first case, wat ers 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 dis appearance of dissolved and solid oxidants and the appearance of reduc ed species, albeit over very different spatial scales. In the second c ase, a pronounced reaction front develops at the downstream edge of th e contaminant plume. The reactivity, or biodegradability, of the organ ic matter is shown to be a major factor governing the biogeochemical d ynamics in the plume. The simulations predict different distributions of the biodegradation pathways, depending on whether the organics of t he leachate have uniform or variable reactivity. The secondary reactio ns also have a significant impact on the concentration profiles of ino rganic species and the spatial distributions of the biodegradation pat hways. Within the downstream reaction front, large fractions of O-2, M n(IV), Fe(III) and SO42- are reduced by secondary reactions, rather th an being utilized in the oxidative degradation of leachate organics, o verall, the model simulations emphasize the strong coupling between su bsurface heterotrophic activity and an extensive network of secondary reactions. (C) 1998 Elsevier Science B.V. All rights reserved.