Large-Time Behavior of GW Pollutant Plumes Subject to Biodegradation at the Fringe: Mathematical Analysis and its Application to a Large-Scale (~10 km) Field Problem

Engineered bioremediation and monitored natural attenuation are important options for the cleanup of frequently occurring subsurface contamination by organic compounds. Because the contaminant removal occurs only when the substrates, target contaminants, and degrading bacteria are present simultaneously, the controlling mixing processes of the contaminants and substrates dictate the contaminant removal rate. Due to the complex nature of subsurface environments, in-situ bioremediation often involves many physico-chemical and biological processes concurrently. Thus, mathematical modeling is a useful tool -and probably the only effective tool- to identify the rate controlling processes. As a tool for predicting the environmental impact of a spill and/or for screening the effectiveness of possible remediation technologies, its ability to correctly capture the key processes is important. However, classical modeling involving the discretized form of the governing equations over very large spatial domains and long periods is computationally infeasible at this point. In this research, we investigate the large-time solution behavior of a representative bio-reactive transport model assuming the mixing of two required substrates occurs only in the directions transverse to groundwater flow. The processes are governed by the commonly used advection-dispersion-reaction equations. The microbial growth and decay in the model are described by the double Monod kinetics terms and a linear decay term. The flow field is assumed to be uniform. We have developed a practical approach to estimate the size of the microbial reaction zone and the level of microbial concentration. We have found out that the microbial reaction rates are always limited by the transverse transport of the substrates at steady state, provided that the bulk substrate concentrations are much larger than a characteristic value determined only by the microbial kinetic parameters. Thus the reactions can be considered as instantaneous for the purpose of mathematical modeling. This simplification allows us to efficiently find the steady-state solutions for large scale field problems. We will present a field application which indicates the mixing with the ambient oxygen at the plume fringe may successfully constrain the spread of a high total organic carbon (500mg/L) plume, generated from a passive bio-reactive barrier. But the concentration reduction along the center line of the plume is insignificant.