Application of the theory of micromixing to groundwater reactive transport models

Abstract

This study extends and applies the theory of micromixing, first introduced in the chemical reaction engineering literature, to the topic of reactive transport in porous media. For all but the simplest linear kinetic and sorption models the fate and transport of a reactive solute depends on the residence times and the details of small‐scale mixing. The latter phenomenon, also called micromixing, is important because it brings into close proximity chemical species that react, and it controls the local concentrations in a flowing system. Solutes with reaction rates or sorption isotherms that depend on species concentration will therefore be affected by micromixing. Two models for micromixing are introduced, the minimum and maximum mixedness models, that provide bounds on the extent of reaction or retardation behavior within the constraints imposed by the residence time distribution (RTD) of a conservative solute in the same flow system. These mixing models prescribe the latest or earliest permissible mixing of parcels of fluid of different residence times, which in turn bounds the degree of reaction of a reactive solute for nonlinear rate laws or sorption isotherms. Simulation results using the bounding models show that micromixing effects are most important for nonlinear reaction curves, solute pulses of short duration, and systems with broad RTD curves. Use of these models is a straightforward and practical way to investigate the importance of a phenomenon for which data are seldom available and whose impact on groundwater reactive transport models has heretofore not been studied in a systematic, bounding manner.