Abstract
The importance of the exchange between surface water and groundwater is well recognized because of its effects on multiple fate and transport-related processes, including contaminant migration, biogeochemical reactions, and remediation efficacy. Thorough understanding of hydrodynamic exchange can be challenging due to the nested physical processes that control exchange dynamics, especially considering typical pore- to bedform-scale sediment heterogeneity in streambed porous media. In the presence of sediment heterogeneity, pores with different levels of hydrologic connectivity are expected to be exposed to flow, leading to a spectrum of local transport times. Sediment pores that are well connected create "mobile" zones, whereas poorly connected pores create "less-mobile" zones. Conventional fluid sampling techniques are primarily sensitive to the mobile zones, given the pumping mechanism (induced advection) of sampling. On the other hand, geoelectrical measurements are sensitive to both mobile and less-mobile zones. As such, combining fluid sampling with geoelectrical measurements during an ionic tracer injection results in improved assessment of both mobile and less-mobile zones, and the exchange between these two porosity domains. We have incorporated this combined approach in numerical simulations and field studies of sediment/water interfaces in naturally occurring porous media, such as an urban stream. Our results illustrate that this approach can provide a quantitative assessment of less-mobile porosity exchange dynamics, including their flow-dependent behavior.