Stream-Groundwater Interactions and Near-Stream Flow Systems: The Influence of Aquifer Heterogeneity and Stream Meandering on Three-dimensional Hyporheic Exchange Flows

This study examines the effects of aquifer heterogeneity and channel pattern on flow interactions between stream and groundwater systems. Previous efforts to model stream-groundwater interactions have generally used simplified, horizontal, two-dimensional domains and treated only one-dimensional vertical exchange between the river bed and the underlying aquifer layer. We have adopted a three-dimensional approach and used MODFLOW to evaluate the magnitude, direction and spatial distribution of stream-subsurface exchange flows induced by stream meanders. Our model includes vertical heterogeneity in hydraulic conductivity and analyzes exchange across the wetted perimeter of the stream. The numerical model consists of a 256 x 256 x 30 (x, y, z) grid with spatial dimensions determined both by the correlated scales used to generate the hydraulic conductivity random fields and by the scale of the stream meanders. Aquifer interactions with a straight river and a meandering river were simulated using four levels of heterogeneity (lnK variance equal to 0.25, 1.0, 2.5 and 4.0). Results of the straight-river simulations are compared to cases that incorporate a simple river meandering pattern in order to evaluate the interaction of degree of heterogeneity and hydraulic short-circuiting on the flow exchange pattern at the aquifer-riverbed interface. Comparison of model simulations with different stream geometries shows that flow variance of rows parallel to the river (longitudinal cross-section) and columns perpendicular to the river flow (transverse cross-section) increases as the lnK variance increases. For a longitudinal cross-section for the simulated meander pattern, the flow variance is greater for all levels of heterogeneity compared to the straight-river case. At a transverse cross-section the situation is the opposite: the flow variance is reduced for the meander case compared to the straight river case, for a given level of heterogeneity. Vertical penetration of water from the stream increases with increasing heterogeneity for both the straight-river and the meander cases.