The transport and biodegradation of an organic compound (quinoline) were studied in a system of layered porous media to examine the processes that affect microbial growth near hydraulic layer interfaces. From a set of independent experiments, a mathematical model for the microbial kinetics was developed to describe the time rate of change of the concentrations of two organic compounds (quinoline and its first degradation product), one electron acceptor (oxygen), and microorganisms. This mathematical model was incorporated into a two-dimensional numerical model for flow and transport, so that simulations of the laboratory system could be conducted and results compared with the observed data. The model was formulated from the single-phase perspective (i.e, it did not include mass-transfer limitations between the aqueous and microbial phases). These comparisons suggest that, for some systems, a single-phase model can adequately describe the reactive processes that occur between aqueous components and microorganisms. Microbial lag was explicitly accounted for in the degradation kinetics. For the system described here, the inclusion of microbial lag was important for describing transient concentration pulses observed in the low-conductivity layer.