Evaluating Effective Reaction Rates of Kinetically Driven Solutes in Large-scale, Statistically Anisotropic Media: Implications of Pore Scale Mixing and Preferential Flow Pathways at the Field Scale

The role of high and low hydraulic conductivity (K) regions in heterogeneous flow fields with varying degrees of stratification and the subsequent effect of rate-dependent geochemical reactions is investigated. An example contamination scenario is used to to investigate potential impacts of kinetic sorption and local dispersion on plume migration and channeling. In this study, kinetic sorption is simulated with finely resolved, large-scale domains to identify geo-hydrologic conditions where solute reactions are either rate limited (nonreactive), in equilibrium (linear equilibrium assumption, LEA, is appropriate), or are sensitive to time-dependent kinetic reaction rates. By utilizing stochastic ensembles, local and effective equilibrium conditions are examined, in addition to potential interplay between multiple parameters (i.e. positive or negative feedbacks). In particular, the effect of preferential flow pathways and solute mixing at the field-scale (marcrodispersion) and sub-grid (local dispersion) is examined for varying degrees of stratification and regional groundwater velocities. Results show effective reaction rates of kinetic ensembles with the inclusion of local dispersion yield disequilibrium transport, even for averaged (or global) Damköholer numbers associated with equilibrium transport. Resulting solute behavior includes an additive tailing effect of the plume, and a retarded peak time. This discrepancy between kinetic and LEA ensembles is augmented in highly anisotropic media. Comparisons with non-reactive solutes simulated with the inclusion of local dispersion exhibit not only an increase in plume spreading but also effective plume retardation.