Engineering Groundwater Remediation by Active Plume Spreading

Abstract:

In situ groundwater remediation conceives the subsurface as an engineered reactor in which an injected amendment degrades a contaminant. This presentation presents theory, experiments, and modeling to illustrate how in situ remediation depends on the art and science of plume spreading. Degradation reactions depend on mixing, which depends on plume spreading, because the injected plume must be blended with the surrounding groundwater. Plume spreading results from nonuniform velocity, which always results from heterogeneous permeability, which we call passive spreading. But plume spreading can also be engineered through injection and extraction of water through a manifold of wells surrounding the injected plume, which we call active spreading. Recent work on active spreading theory has demonstrated a key point: Active spreading is most effective when the local velocity is perpendicular to the local interface between the injected plume and the contaminated groundwater, which results from longitudinal dispersivity being an order of magnitude larger than transverse dispersivity. Here we illustrate this key point with experimental observations of an injected dye plume, measured by laser-induced fluorescence in refractive index-matched porous media, with active spreading imposed by an oscillating sequence of injections and extractions designed to make the local velocity perpendicular to the local plume interface. Reactive transport is simulated numerically. These experiments and observations demonstrate how active spreading provides a framework to design flows for in situ remediation, which opens up the possibility of improved control of the subsurface as an engineered reactor.