Simulation of less‐mobile porosity dynamics in contrasting sediment water interface porous media

Abstract

Considering heterogeneity in porous media pore size and connectivity is essential to predicting reactive solute transport across interfaces. However, exchange with less‐mobile porosity is rarely considered in surface water/groundwater recharge studies. Previous research indicates that a combination of pore‐fluid sampling and geoelectrical measurements can be used to quantify less‐mobile porosity exchange dynamics using the time‐varying relation between fluid and bulk electrical conductivity. For this study, we use macro‐scale (10 s of cm) advection–dispersion solute transport models linked with electrical conduction in COMSOL Multiphysics to explore less‐mobile porosity dynamics in two different types of observed sediment water interface porous media. Modeled sediment textures contrast from strongly layered streambed deposits to poorly sorted lakebed sands and cobbles. During simulated ionic tracer perturbations, a lag between fluid and bulk electrical conductivity, and the resultant hysteresis, is observed for all simulations indicating differential loading of pore spaces with tracer. Less‐mobile exchange parameters are determined graphically from these tracer time series data without the need for inverse numerical model simulation. In both sediment types, effective less‐mobile porosity exchange parameters are variable in response to changes in flow direction and fluid flux. These observed flow‐dependent effects directly impact local less‐mobile residence times and associated contact time for biogeochemical reaction. The simulations indicate that for the sediment textures explored here, less‐mobile porosity exchange is dominated by variable rates of advection through the domain, rather than diffusion of solute, for typical low‐to‐moderate rate (approximately 3–40 cm/day) hyporheic fluid fluxes. Overall, our model‐based results show that less‐mobile porosity may be expected in a range of natural hyporheic sediments and that changes in flowpath orientation and magnitude will impact less‐mobile exchange parameters. These temporal dynamics can be assessed with the geoelectrical experimental tracer method applied at laboratory and field scales.

Formation Criteria for Hyporheic Anoxic Microzones: Assessing Interactions of Hydraulics, Nutrients, and Biofilms

“…Microzone formation and function in stream sediments is an emerging topic; modeling efforts on microzone formations in the hyporheic zone have been limited until recently (Briggs et al, 2015;MahmoodPoor Dehkordy et al, 2018;Sawyer, 2015). Most microzone research comes from observational and experimental studies, but those have not yet yielded conclusive controls on microzone formation and function (e.g., Briggs et al, 2018;Hampton et al, 2019;Harvey et al, 2013;Zarnetske et al, 2011).…”

Evaluation of Dual Domain Mass Transfer in Porous Media at the Pore Scale

“…The mass‐transfer rate is commonly determined by adjusting model variables until model results match the data (e.g., Rao et al 1980; Knox et al 2016). Despite notable previous work that employs DDMT models, one open question is what exactly “immobile” porosity means in many aquifer systems (e.g., Wheaton and Singha 2010; MahmoodPoor Dehkordy et al 2018; Foster et al 2021).…”