HR: 0800h
AN: H11D-1285    [Abstracts]
TI: Reaction-Transport Modeling of Biological Mediated Isotope Fractionation With Complete Substrate Consumption as Boundary Condition
AU: * Chernyavsky, B M
EM: bchern04@yahoo.com
AF: Geobiology Isotope Laboratory, Dept. of Geology, University of Toronto, 22 Russel street, Toronto, ON M5S 3B1 Canada
AU: Wortmann, U G
EM: uli.wortmann@utoronto.ca
AF: Geobiology Isotope Laboratory, Dept. of Geology, University of Toronto, 22 Russel street, Toronto, ON M5S 3B1 Canada
AB: Reaction-transport modeling of biological mediated processes is a commonly used technique to analyze the subsurface distribution of dissolved species. Coupled with isotope geochemical data, it becomes a powerful tool to investigate metabolic processes. However, numerical instabilities limits its usefulness to cases without complete substrate consumption. This is a consequence of the reaction term dependence on total substrate concentration and isotopic species concentration. Formulating a correct boundary condition is furthermore complicated by the fact that the isotopic ratios for infinitesimal small concentrations are usually not know a priory, and that in many cases, the depth of complete substrate consumption is an unknown as well. Here we present a scheme which allows for complete substrate consumption and computes isotopic ratios for infinitesimal small concentrations if the upper boundary concentrations are known. We first resolve the floating lower boundary condition problem by the introduction of an additional boundary condition requiring that only the total concentration, but also the concentration gradients at the lower boundary are zero. This allows us to solve the reaction-transport equation without introducing any a priory assumptions about the location of the lower boundary. The problem now becomes well posed and can be solved with an iterative method where a sequence of the solutions converges toward the unique point satisfying both lower boundary conditions. We applied this approach to solve the problem of biological isotope fractionation in case of complete substrate consumption. We therefore reformulated the model proposed by Joergensen (1979) into a set of coupled equations which computes the depleted and enriched isotope species at the same time. To simulate a realistic pattern of bacterial activity, and to improve model stability, we further introduced a Monod type concentration-controlled limiter for the bacterial consumption function. The preliminary results are in agreement with the expected trends.
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0419 Biomineralization
DE: 0430 Computational methods and data processing
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1805 Computational hydrology
SC: Hydrology [H]
MN: Fall Meeting 2005