Influence of Heterogeneity Patterns on Active and Passive Spreading and Reaction during In Situ Groundwater Remediation

During in situ groundwater remediation, a chemical or biological amendment is introduced into the aquifer to react with and degrade the contaminated groundwater. Degradation reactions only occur where the amendment and contaminant molecules are sufficiently close that they can react. Thus, maximizing the degree to which the amendment can be spread throughout the contaminant plume is critical for effective in situ remediation. Spreading is driven by spatial variations in velocity, which reconfigure the contaminant and amendment plumes, elongate the interface between them, increase the contact area on which reaction can take place, and sharpen concentration gradients near the plume boundaries, bringing the reactants together to react. Spatially varying velocity induced by injecting or extracting wells (active spreading) or by natural heterogeneity (passive spreading). The spatial patterns of heterogeneity and the range of values of hydraulic conductivity affect the degree of passive spreading and therefore significantly affect the amount of degradation that can occur during in situ remediation. We use numerical simulations of solute transport to evaluate the combined effect of active and passive spreading on contaminant degradation in aquifers with zonal heterogeneity that is typical of fluvial depositional environments. We evaluate different ranges of heterogeneity values between zones, and we demonstrate how the local plume geometry and the local velocity field affect the overall solute degradation. These results have implications for effective design of in situ groundwater remediation in aquifers that have well-characterized spatial distributions of hydraulic conductivity.