Analytical approximation of biodegradation rate for in situ bioremediation of groundwater under ideal radial flow conditions

We derive the long-term biodegradation rate of an organic contaminant (substrate) for an in situ bioremediation model with axisymmetric flow conditions. The model presumes that a nonsorbing electron acceptor is injected into a saturated homogeneous porous medium which initially contains a sorbing substrate and attached indigenous microorganisms. The derived analytical removal rate depends upon the injection flow rate, the initial substrate and supplied acceptor concentrations, the stoichiometric coefficient for acceptor utilization, and the sorption characteristics of the substrate; the removal rate does not depend upon the dispersion parameters, microbial kinetic parameters, and initial biomass concentration. Numerical simulations confirm the analytical results. The insensitivity of the long- term removal rate to the microbial kinetic parameters and initial biomass concentration suggests that precise estimation of these data may not be necessary to assess bioremediation effectiveness. In the numerical results, however, there exists an initial transient phase during which the removal rate depends upon microbial growth kinetics. This initial phase is significantly prolonged if the initial substrate and injected acceptor concentrations are at nutrient-limiting levels, or if the microbial kinetic parameters and initial biomass concentration do not yield efficient microbial growth and substrate utilization.