Influence of transient flow on contaminant biodegradation

The rate of biodegradation in contaminated aquifers depends to a large exte nt on dispersive mixing processes that are now generally accepted to result from spatial variations in the velocity field. It has been shown, however, that transient flow fields can also contribute to dispersive mixing. The i nfluence of transient flow on biodegrading contaminants is particularly imp ortant since it can enhance mixing with electron acceptors, further promoti ng the reactive process. Using numerical simulations, the effect of transie nt flow on the behavior of a biodegradable contaminant is evaluated here bo th with respect to the development of apparently large horizontal transvers e dispersion and also,vith respect to enhanced mixing between the substrate (electron donor) and electron acceptor. The numerical model BIO3D, which s olves for advective-dispersive transport coupled with Monod-type biodegrada tion of substrates in the presence of an electron acceptor, was used for th e simulations. The model was applied in a two-dimensional plan view mode co nsidering a single substrate. Transient flow fields were found to yield lar ger apparent transverse dispersion because the longitudinal dispersivity al so acts transverse to the mean flow direction. In the reactive case, the tr ansient now field increases substrate-oxygen mixing, which in turn enhances the overall rate of biodegradation. The results suggest that in the case o f moderate changes of flow directions, a steady-state flow field can be jus tified, thereby avoiding the higher computational costs of a fully transien t simulation. The use of a higher transverse horizontal dispersivity in a s teady flow field can, under these conditions, adequately forecast plume dev elopment.