Modeling transport and biodegradation of benzene and toluene in sandy aquifer material: Comparisons With experimental measurements

A one-dimensional numerical model is developed for simulating the biodegradation and transport of benzene and toluene in the subsurface environment. Modeled processes include mass exchange between the constituent phases (solid, liquid, gas, and biomass), advective and dispersive transport, and biotransformation, as well as microbial biomass production. Two substrates, two electron acceptors, one trace nutrient, and two microbial populations are modeled. Resulting governing equations include five nonlinear partial differential equations describing component transport in the bulk pore fluids, five nonlinear algebraic equations governing interphase mass exchange, and two ordinary differential equations governing microbial growth. These equations are solved through application of a Galerkin finite element method and a set iterative solution scheme. The utility and validity of the modeling approach is explored through comparisons with laboratory column experiments. Model parameters were estimated independently through laboratory batch experiments, aquifer slurry studies, or from the literature. Simulations are found to provide reasonable agreement with measurements of benzene and toluene biodegradation in saturated continuous-flow columns packed with aquifer material. Sensitivity analyses and comparisons with column data suggest that model predictions are highly dependent upon the microbial parameters, particularly the initial active biomass concentration, the maximum specific substrate utilization rate, and the half-saturation coefficient. The importance of the accurate estimation of these microbial parameters is emphasized.