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

Understanding the effect of the heterogeneous subsurface environment upon f ate and transport of biologically reactive solutes is necessary for effecti ve held application of in situ bioremediation. We consider a scenario in wh ich an electron acceptor is continuously input into an aquifer having initi al uniform concentrations of an adsorbed pollutant and immobile microorgani sms. Biodegradation is governed by dual Monod kinetics, and the aquifer is assumed to have vertical stratification of pore water velocity and retardat ion factor. We characterize solute transport behavior with a theoretical ap proach and derive a simple expression for the long-term pollutant biodegrad ation rate. Our theoretical approach is based upon the observation that the balanced interactions between the biodegradation and mixing processes caus e the solute and biomass concentration profiles to form traveling waves aft er 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 indep endent of longitudinal and transverse dispersion, the vertical variations o f flow velocity and retardation factor, microbial growth kinetics, and init ial biomass concentration. Numerical simulations verify the appearance of s uch asymptotic transport and biodegradation properties at large times but s how that there is an early transient period which depends upon all these-pa rameters and conditions. Our results demonstrate that transverse dispersion is the, important process controlling the establishment of the asymptotic transport and biodegradation properties.