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Reising, Lauren J ¹ ; Neupauer, Roseanna M ² ; Mays, David C ³

¹ Â鶹ÊÓƵ
² Â鶹ÊÓƵ
³ University of Colorado Denver

During in situ remediation of contaminated groundwater, a treatment chemical can be injected into the aquifer to react with and degrade the contaminant. In this type of remediation, degradation reactions only occurs where the treatment chemical and contaminant are close enough that mixing, resulting from molecular diffusion and pore-scale dispersion, will bring them together. Active spreading techniques use pre-defined injections and extractions of water at wells surrounding the contaminant plume to generate flow fields that spread the plumes of groundwater contaminant and treatment chemical in a manner that increases their contact, thus increasing the extent of the region where mixing can occur. Determining the optimal sequence of injection and extraction steps that maximize spreading and contaminant degradation is not obvious. Therefore, we attempt to understand why a specific injection and extraction step in a sequence enhances contaminant degradation more than another, with the goal of providing a mechanistic approach for designing active spreading injection and extraction sequences.

In porous media, mechanical dispersion is the main driving force of mass transfer across the interface between the treatment chemical and contaminant. Given that dispersive mass flux is greater parallel to flow compared to transverse to flow, the maximum possible dispersive mass flux across the interface will occur when the local velocity vector is normal to the interface. Therefore, injection and extraction sequences that generate flow fields with a large component of the local velocity vector normal to the interface increase the potential for more mixing and thus more reaction. Here we compare two injection and extraction steps: a single-well case and a three-well case. We show a relationship between the amount of contaminant degradation and the dispersive mass flow rate across the interface, which we quantify based on the hypothetical interface between the treatment chemical and contaminant plumes and the local velocity vectors. These findings support the use of the dispersive mass flow rate as an effective measure of the reaction potential of a specific injection/extraction step and provide a mechanistic approach for designing active spreading injection/extraction sequences.