The effect of river bend morphology on flow and timescales of surface water-groundwater exchange across pointbars

Head gradients along meandering rivers induce surface water-groundwater exchange across pointbars. We analyze this exchange process by finite-element modeling of groundwater flow and transport through model-generated river planforms taken from previous studies. The river is considered as a constant-head boundary that decreases linearly along the direction of the channel. This leads to an irregular groundwater flowfield in the pointbar that is controlled by channel sinuosity. Simulations under homogeneous aquifer conditions shows that the variable flowpath lengths coupled with the heterogeneous flow velocities lead to non-Fickian transport characterized by power-law residence time distributions. This anomalous transport behavior is enhanced by increased aquifer dispersivity. The broad transit time distribution implies that time-dependent chemical transformation processes in near-stream zones may be equally variable as the flowfield, even in the absence of heterogeneity in aquifer hydraulic properties. (C) 2008 Elsevier B.V. All rights reserved.