Mathematical modeling of in situ bioremediation of volatile organics in variably saturated aquifers.

Abstract

The objectives of this research are to develop a comprehensive mathematical model describing in-situ bioremediation of organic contaminants in a variably-saturated aquifer and to explore the importance and the interplay of various processes involved in natural in-situ attenuation. The mathematical model developed in this research incorporates advective and dispersive transport, mass transfer between constituent phases (water, soil, gas, non-aqueous phase liquid (NAPL), and organisms), and Monod-type microbial transformations and growth. Resulting governing equations are solved in one- and two-dimensions through application of Galerkin and hybrid finite element methods, respectively. The applicability and validity of the modeling approach are explored through comparisons with 1D laboratory column experiments. Model simulations are found to provide reasonable agreement with measurements of benzene and toluene biodegradation in continuous-flow columns packed with aquifer material. Examination of parameter sensitivity and comparison with column data suggest that model predictions are highly sensitive to the microbial parameters. Two-dimensional simulations of the dissolution, transport, and transformation of petroleum hydrocarbons in an unconfined aquifer illustrate the sensitivity of model predictions to microbial and hydrodynamic parameters. Particular attention is directed towards examination of the processes limiting bioattenuation and controlling NAPL dissolution. Simulation results indicate that natural bioattenuation under low groundwater flow conditions is controlled not only by oxygen but also by substrate availability. Oxygen availability is largely determined by advective-dispersive transport and oxygen diffusion in the vadose zone. Substrate availability is directly controlled by NAPL dissolution and indirectly controlled by volatilization and adsorption. Hydrodynamic parameters and water table fluctuations have a direct influence on NAPL dissolution and the mixing of substrate and oxygen, and therefore, have significant impact on the bioattenuation rate. It is demonstrated that NAPL dissolution can be enhanced by biodegradation, flushing, volatilization and adsorption. However, the dissolved mass is permanently removed only through biodegradation. Otherwise it is merely transferred to other phases, potentially creating additional environmental threats. The bioenhancement effect is shown to be limited by microbial growth conditions. Simulations are also presented which examine model sensitivity to temperature, soil moisture content, microbial growth patterns, soil properties, contaminant properties, and alternative electron acceptor respiration.