On the Equivalence of Two Models for Biodegradation During Contaminant Transport in Groundwater

Biodegradation is one of the most important processes affecting contaminant fate and transport in groundwater. Not surprisingly, then, several models have been developed for describing or modeling biodegradation during contaminant transport. Here we compare two such models: a ``simple'' model that is based upon macroscropic properties only, and a ``biofilm'' model that accounts for contaminant diffusion and reaction in biofilms. We demonstrate that the two models are, in fact, equivalent at the macroscopic scale when steady-state conditions prevail. Under these conditions, the equivalent macroscopic first-order degradation rate k can be related to microscopic rate parameters that describe mass transfer across a boundary layer, diffusion within the biofilm, and reaction within the biofilm. Under transient (non-steady-state) conditions, the two models are not strictly equivalent. However, even under these conditions, the error between the two models is negligible in almost all cases. Therefore, in order to describe or predict concentrations at the macroscopic scale, the more sophisticated biofilm model offers little or no advantage over the simple model. To describe concentrations at the microscopic scale, the biofilm model is, of course, still needed.