Transport and biodegradation of solutes in stratified aquifers under enhanced in situ bioremediation conditions

Understanding the effect of the heterogeneous subsurface environment upon fate and transport of biologically reactive solutes is necessary for effective held application of in situ bioremediation. We consider a scenario in which an electron acceptor is continuously input into an aquifer having initial uniform concentrations of an adsorbed pollutant and immobile microorganisms. Biodegradation is governed by dual Monod kinetics, and the aquifer is assumed to have vertical stratification of pore water velocity and retardation factor. We characterize solute transport behavior with a theoretical approach and derive a simple expression for the long-term pollutant biodegradation rate. Our theoretical approach is based upon the observation that the balanced interactions between the biodegradation and mixing processes cause the solute and biomass concentration profiles to form traveling waves after a long elapsed time. The theoretical results indicate that the effective macrodispersion coefficients for the depth-averaged solute fronts decrease to zero at large times and that the long-term biodegradation rate is independent of longitudinal and transverse dispersion, the vertical variations of flow velocity and retardation factor, microbial growth kinetics, and initial biomass concentration. Numerical simulations verify the appearance of such asymptotic transport and biodegradation properties at large times but show that there is an early transient period which depends upon all these-parameters and conditions. Our results demonstrate that transverse dispersion is the, important process controlling the establishment of the asymptotic transport and biodegradation properties.