A prerequisite to determining risk from a subsurface contaminant plume is to determine chemical concentrations down gradient from the source zone. For a conservative contaminant, down gradient concentrations depend on the initial source zone concentrations and the extent to which the contaminant mixes (i.e., dilutes) with surrounding groundwater. For a (bio)reactive contaminant, down gradient concentrations also depend on the rate of reaction; this can be governed by the rate that the

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... t mixes with other substrates and the reaction rate constant. In this work we examine the effect of a high conductivity preferential flow zone on contaminant mixing and reaction at the pore scale. Two reactive substrates are introduced into a network of pores via two separate and parallel fluid streams. The substrates mix within the porous media via transverse dispersion and react. The lattice-Boltzmann (LB) method is used to solve for interstitial pore velocities. Reactive transport is simulated by combining pore velocities from the LB model with a reactive transport finite-volume model (FVM). The extent of mixing and reaction is quantified by evaluating product formation rates from the two substrates. Results from the LB-FVM simulations indicate that (i) the amount of product formed can be 40-50% higher due to flow focusing in the preferential flow zone, (ii) the amount of enhancement depends on the length and width of the preferential flow zone, and (iii) the bi-molecular reaction rate can have an effect on the total amount of product formed. Results are compared with an analytical solution developed for flow focusing scenarios at the continuum scale, and with reactive fluorescent micro-model experiments which were conducted utilizing an identical porous media configuration to the base preferential mixing scenario evaluated utilizing the LB-FVM. This study suggests that flow focusing can significantly affect the overall extent of mixing and reaction in groundwater flow, and hence may need to be considered whe [...]