3-D imaging reveals how polymers can aid groundwater remediation

Abstract:

Polymer solutions can aid groundwater remediation by improving the dispersion of injected chemicals that degrade contaminants in situ. This improved dispersion is believed to be linked to the generation of spatiotemporal fluctuations induced by the polymers, though this “elastic turbulence” has only been observed in model 2-D systems. To better understand this behavior in geologically-relevant settings, we directly image the unsteady flow of polymer solutions in transparent 3-D model rocks using confocal microscopy. We find that polymers can indeed produce a form of elastic turbulence, but surprisingly, the unstable flow is not spatially homogenous. Instead, we observe discrete pockets of unstable flow, giving rise to spatially patchy elastic turbulence. By simultaneously mapping the geometry of the pore space, we show that the formation of these unstable pockets is determined by variations in the pore-scale geometry, as quantified by a dimensionless parameter that characterizes the persistence of elastic stresses in the flow. Further, by using flow visualization to directly measure the energy dissipated by these fluctuations, we develop a general model that links this pore-scale flow behavior to the macroscopic flow resistance and to enhanced macroscopic dispersion. Thus, our work provides a general framework by which dispersion of chemical additives for groundwater remediation can be predicted and controlled—potentially helping to expand access to fresh water in the U.S. and improving public health for those relying on groundwater.