Toward enhanced remediation methods using chaotic advection

Many remediation activities in the terrestrial subsurface involve the need to recover/emplace distributions of scalar quantities (e.g. dissolved phase concentration and heat) from/in volumes of saturated porous media. These scalars can be targeted by pump-and-treat technologies, where target fluids are abstracted from the porous medium, or by amendment technologies, were specific chemicals or substrates are injected into the porous medium in order to promote beneficial transformations of water quality or mineralogy. Application examples include in situ leaching for precious metals, recovery of dissolved contaminant plumes, or harvesting of heat energy from geothermal reservoirs. While conventional pumping methods work reasonably well, costs associated with maintaining pumping schedules are high and improvements in efficiency would be welcome. In this paper we discuss how transient switching of the pressure at different wells can intimately control subsurface flow, generating a range of "programmed" flows with various beneficial characteristics. Some programs produce chaotic mixing flows which deliver rapid transport, whilst others create encapsulating flows which can confine pollutants for in situ treatment. In a simplified model of an aquifer subject to balanced injection and extraction pumping, chaotic flow topologies have been predicted theoretically and verified experimentally using Hele-Shaw cells. Mixing enhancement due to chaotic advection and kinematic confinement of aquifer volumes are key features of the chaotic dynamics. Understanding these phenomena may form the basis for new efficient technologies for groundwater remediation or amendment.