During in situ groundwater remediation, reactions occur in a narrow reaction front in which the amendment and contaminant are close enough to mix. Active spreading, in which injection or extraction wells create spatially variable velocity fields, can be used to increase the surface area of the reaction front, thereby enhancing reaction. This study used four active spreading flow fields that are building blocks to more complex remediation hydraulics to evaluate how the flow field and the plume position control contaminant degradation in both homogeneous and heterogeneous aquifers. At the plume scale, reaction depended on mechanical dispersion across the reaction front, which is proportional to both the local velocity and the local contaminant concentration gradient. Mechanical dispersion and, consequently, the amount of degradation, was highest when the reaction front was perpendicular to the local velocity, producing a high local dispersion coefficient. This effect was amplified where flow was diverging due to sharpening of the concentration gradient.
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