Application of the Lagrangian particle-tracking method to simulating mixing-limited, field-scale biodegradation

Abstract

Measured (or fitted) reaction rates at field-scale sites are commonly observed significantly lower than batch-scale rates. The reduced rates are usually attributed to poor mixing of reactants. In this study, the Lagrangian particle tracking and reaction (PTR) method is used to characterize the effect of mixing for different types of reactions at a range of scales (from laboratory columns to field-scale tests). In the PTR method, the reactants are represented by particles. The particle/particle reactions are determined by a combination of two probabilities: 1) the physics of transport and 2) the energetics of reaction. The first is a direct physical representation of the degree of mixing in an advancing interface between dissimilar waters, and as such lacks empirical parameters except for the user-defined number of particles, which can be determined from concentration autocovariance. First, the PTR method is used to simulate two column experiments of bimolecular reaction and transport. When compared to the solution of the advection-dispersion-reaction equation (ADRE), the experiments and the PTR simulations showed on the order of 20\% to 40\% less overall product, which is attributed to poor mixing. The poor mixing also leads to higher product concentrations on the edges of the mixing zones. Second, the PTR method is extended to biodegradation, which is commonly characterized by Michaelis-Menten (Monod) (M-M) chemical kinetics. The PTR method not only matches the M-M equation under ideal conditions, but also captures the characteristics of non-ideal conditions such as imperfect mixing, disequilibrium, and limited availability of biologically active sites. These features are shown using hypothetical systems and are also successfully applied to a column study of carbon tetrachloride (CT) biodegradation. Finally, the extended PTR method is used to simulate a field bioremediation project at the Schoolcraft, Michigan site. The remediation was conducted by injecting a denitrifying bacterium, along with sufficient substrate, into the aquifer to degrade a plume of CT. Comparisons between simulated results and field measurements indicate that, unlike previous applications of the ADRE, the PTR method is able to match the field-scale experiment using the rate coefficients from batch experiments.