Modeling transport of contaminants influenced by complex microbial processes

Abstract

Many current models that describe biodegradation and transport of contaminants in porous media do not include provisions for nonideal (nonlinear, rate-limited) sorption, microbial lag, inhibition, microbial community heterogeneity, and cell transport. Therefore, a more complete model was developed that incorporates bacterial lag, inhibition effects, and cell transport/elution, as well as nonlinear, rate-limited sorption. The performance of the new model was evaluated by using the model to simulate the results of a series of miscible-displacement experiments conducted using a range of porewater velocities, substrate concentrations, and initial cell densities. The results show that the model can simulate the substrate breakthrough curves very well. The model was also able to predict the total biomass growth. The calibrated values obtained for the maximum specific growth-rate coefficient, the mean lag time, and lag-time variance were within the range of values obtained from batch experiments. These results suggest that the model performed well and that it successfully describes the system. The model was used to investigate the coupled interactions among sorption, biodegradation, and transport, and the results show that biodegradation can significantly influence the first, second, and third spatial moment when sorption is nonlinear or rate-limited depending on initial/boundary conditions, residence time, biomass growth dynamics, and time-dependent sorption/desorption processes. The influence of heterogeneous microbial communities on biodegradation and transport of contaminants was investigated in the last part of the study. A one-dimensional mathematical model was developed that incorporates multiple populations, each subject to its own set of growth-related coefficients. Breakthrough curves produced for different combinations of growth rates, half-saturation constants and initial biomass concentrations for multiple species exhibit oscillatory behavior under certain conditions, which is attributed to competition between different species. The results suggest that the existence of heterogeneous microbial communities can have a significant influence on biodegradation and transport. The results presented herein illustrate the significant impact that factors such as microbial lag, microbial community heterogeneity, and nonideal sorption/desorption can have on the transport of biodegradable contaminants in porous media. One product of this dissertation is the development and evaluation of a more comprehensive model that represents many important processes involved in transport of biodegradable contaminants. The use of this type of model should enhance our ability to investigate and hopefully understand the complex systems inherent to the subsurface wherein multiple coupled processes influence contaminant transport and fate.