Biodegradation during contaminant transport in porous media: 3. Apparent condition-dependency of growth-related coefficients

The biodegradation of organic contaminants in the subsurface has become a m ajor focus of attention, in part, due to the tremendous interest in applyin g in situ biodegradation and natural attenuation approaches for site remedi ation. The biodegradation and transport of contaminants is influenced by a combination of microbial and physicochemical properties and processes. The purpose of this paper is to investigate the impact of hydrodynamic residenc e time, substrate concentration, and growth-related factors on the simulati on of contaminant biodegradation and transport, with a specific focus on po tentially condition-dependent growth coefficients. Two sets of data from mi scible-displacement experiments, performed with different residence times a nd initial solute concentrations, were simulated using a transport model th at includes biodegradation described by the Monod nonlinear equations and w hich incorporates microbial growth and oxygen limitation. Two variations of the model were used, one wherein metabolic lag and cell transport are expl icitly accounted for, and one wherein they are not. The magnitude of the ma ximum specific growth rates obtained from calibration of the column-experim ent results using the simpler model exhibits dependency on pore-water veloc ity and initial substrate concentration (C-0) for most cases. Specifically, the magnitude of lt., generally increases with increasing pore-water veloc ity for a specific C-0, and increases with decreasing C-0 for a specific po re-water velocity. Conversely, use of the model wherein observed tag and ce ll elution are explicitly accounted for produces growth coefficients that a re similar. both to each other and to the batch-measured value. These resul ts illustrate the potential condition-dependency of calibrated coefficients obtained from the use of models that do not account explicitly for all per tinent processes influencing transport of reactive solutes. (C) 2001 Elsevi er Science B.V. All rights reserved.