Studying Exchange with Less-mobile Porosity at the Laboratory Scale: Experimentation and Multiphysics Simulations

Exchange between mobile and less-mobile porosity zones in heterogeneous porous media can impact redox zonation and contaminant transport in the hyporheic zone and near-stream aquifer. Field and laboratory-scale experiments have shown that pairing geoeletrical methods with fluid sampling enables the quantification of paired more- and less-mobile porosity, including less-mobile zones dominated by advection or diffusion. Specifically, geoelectrical methods are sensitive to solute tracer dynamics throughout the total porosity of the porous medium whereas fluid sampling is more sensitive to the more-mobile domain. Simultaneous measurements of the combination of bulk and fluid conductivity improves quantification of less-mobile solute dynamics compared to traditional fluid-only sampling approaches that rely on fluid conductivity alone. Upon injection of a tracer into the porous medium, exchange of solute between more- and less-mobile zones causes a lag (hysteresis) between the change in the bulk conductivity of the sample and fluid conductivity in the more-mobile zone. In this study, we used controlled laboratory column experiments, combined with numerical modeling, to explore the hysteretic relationship between bulk conductivity and fluid conductivity in sandy sediments with embedded cobbles. These column results inform the design and interpretation of field experiments in glacial kettle ponds with similar bed material. A plastic disk placed within the sand column normal to flow direction was used to represent the effect of embedded cobbles. A hydraulic shadow, or dead zone, formed downgradient of the plastic disk and causing previously well-connected sand pores to function as less-mobile porosity within the column. Experiments were run at varied pressure gradients to assess the response of natural fluid flux perturbations on less-mobile dead zones dominated by advective exchange. The experimental configuration was also modeled in COMSOL Multiphysics to simulate coupled electrical and fluid dynamics to compare to the column results and investigate additional less-mobile porosity configurations.

Exchange between mobile and less-mobile porosity zones in heterogeneous porous media can impact redox zonation and contaminant transport in the hyporheic zone and near-stream aquifer. Field and laboratory-scale experiments have shown that pairing geoeletrical methods with fluid sampling enables the quantification of paired more- and less-mobile porosity, including less-mobile zones dominated by advection or diffusion. Specifically, geoelectrical methods are sensitive to solute tracer dynamics throughout the total porosity of the porous medium whereas fluid sampling is more sensitive to the more-mobile domain. Simultaneous measurements of the combination of bulk and fluid conductivity improves quantification of less-mobile solute dynamics compared to traditional fluid-only sampling approaches that rely on fluid conductivity alone. Upon injection of a tracer into the porous medium, exchange of solute between more- and less-mobile zones causes a lag (hysteresis) between the change in the bulk conductivity of the sample and fluid conductivity in the more-mobile zone. In this study, we used controlled laboratory column experiments, combined with numerical modeling, to explore the hysteretic relationship between bulk conductivity and fluid conductivity in sandy sediments with embedded cobbles. These column results inform the design and interpretation of field experiments in glacial kettle ponds with similar bed material. A plastic disk placed within the sand column normal to flow direction was used to represent the effect of embedded cobbles. A hydraulic shadow, or dead zone, formed downgradient of the plastic disk and causing previously well-connected sand pores to function as less-mobile porosity within the column. Experiments were run at varied pressure gradients to assess the response of natural fluid flux perturbations on less-mobile dead zones dominated by advective exchange. The experimental configuration was also modeled in COMSOL Multiphysics to simulate coupled electrical and fluid dynamics to compare to the column results and investigate additional less-mobile porosity configurations.