Competition Between Chemical and Physical Processes in Bioremediation

The performance of an in-situ groundwater remediation project is a complicated function of many factors, including subsurface flow rates and patterns, diffusion/dispersion, rate of contamination from the source, retardation and rate of degradation of the contaminant(s), availability of co-reactants for degradation, etc. The complexity of the interactions between these phenomena makes it difficult to interpret macroscopic behavior, e.g. variation in levels of contamination in an extraction well, or to predict field-scale performance from small-scale data. This work illustrates the use of ParSSim, a parallel simulator of subsurface flow, transport and reaction, to elucidate the mechanisms that control performance of a intrinisic bioremediation project. The coupling of the chemical and physical processes leads to rich and sometimes counterintuitive behavior. For example, an increased dispersivity can lead to higher contaminant concentrations. Different chemical/physical factors may have the same qualitative effects on the concentration of contaminant reaching an observation well, and we illustrate how simulation can be used to discriminate between these factors. Finally we illustrate the use of the simulator to quantify biogeochemical "domains of influence", the regions of parameter space in which a particular reaction or mechanism dominates the process. Such knowledge is valuable in evaluating alternative remediation strategies.