Understanding the effects of permeability heterogeneity and connectivity on surface water-groundwater exchange using multiscale models

Abstract:

Permeability heterogeneity and channel morphology both control surface water-groundwater exchange (hyporheic exchange) and therefore influence stream ecosystem processes and biogeochemical cycles. However, the effects of permeability heterogeneity are less understood than channel morphology due to the challenge of measuring permeability at relevant scales in the field. Permeability fields in streambeds often have sharp, discontinuous contrasts ranging by orders of magnitudes (e.g. sand-clay contacts). To better understand the effects of permeability heterogeneity we modeled hyporheic exchange in bimodal permeability fields consisting of sand and clay in a representative low-gradient stream. Heterogeneity was developed using a Markov Chain approach for representing sand and clay stratasets using commonly observed characteristics of sediments including volumetric proportions and mean lengths. The channel topography was generated and characterized using Fourier series. Hydraulic head was assigned along the streambed as a function of topography and stream velocity. Initial results show that sedimentary structure, specifically the positioning of sand strata, controls hyporheic exchange depth and fluxes. Particle tracing simulations test the hypothesis that uncertainty in solute residence times increases as the connectivity of permeable sand strata increases due to the partitioning of solutes between connected and stagnant flow paths. These results will lead to a better understanding of how heterogeneity and the structure of highly permeable strata influences hyporheic residence times and biogeochemical processes.