Field trials of subsurface chaotic advection: Stirred reactive reservoirs

Chaotic advection refers to the mixing of fluid elements which arise from repeated stretching and folding of fluid parcels [7,8]. Chaotic advection can be generated by time-dependent Darcy flows and has the potential to enhance mixing under laminar conditions in subsurface reservoirs or other porous media [3,4,10]. Enhanced mixing has many possible applications in environmental science and engineering. Remediation of contaminated aquifers is particularly relevant where mixing between the injected reagent and contaminant is a critical step. To assess whether chaos can be invoked at scale in a natural porous medium, a field trial is being designed in the sandpit area at the University of Waterloo Groundwater Research Facility at CFB Borden located near Alliston, ON, Canada, where we propose to use a transient reoriented dipole flow for subsurface stirring. This paper describes the design criteria associated with this phase of the field trial and presents preliminary modelling results for the determination of key flow system parameters. The Borden aquifer was modelled using Visual MODFLOW® Flex, a 3-D software for groundwater flow and heat/contaminant transport. Assumptions included aquifer homogeneity and isotropic and confined flow. At this initial screening stage, focus was given to horizontal flow fields generated. Simulation results showed that after at least four periodic reorientations of dipoles spaced 1.50 m apart with a pumping rate and duration of 2.50 m³/d and 3 hours, respectively, there is significant crossing of flow paths in the Borden aquifer that indicates high potential for rapid mixing.