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There is a need for groundwater modelling approaches that can incorporate both the complexity of riverbeds and the influence of lateral flows through the river banks in the simulation of river–aquifer interaction. A simple and straight-forward approach is proposed for modelling river–aquifer interaction in gaining rivers based on existing building blocks available in MODFLOW. The river water itself is modelled as constant-head cells, while the riverbed is modelled as one or more separate layers. The Horizontal Flow Barrier (HFB) Package is used to simulate the effect of a river bank on lateral exchange fluxes between river and aquifer. This approach has several advantages over conventional MODFLOW streamflow packages: (1) riverbeds can be modelled as several layers with different hydraulic properties; (2) both vertical and horizontal flow through the riverbed can be modelled; and (3) lateral flow through the river banks can be simulated. The capabilities of the new approach are demonstrated in two applications based on a MODFLOW model of the Aa River, Belgium. Application I demonstrates that the approach can be used to model heterogeneous multi-layered riverbeds. Results show that neglecting this heterogeneity can result in an overestimation of river–aquifer exchange fluxes. Application II shows that the approach can be used to simulate the effect of lateral fluxes through river banks. It demonstrates that lateral fluxes can be a major contributor to total river–aquifer exchange fluxes. The proposed approach provides the necessary tools to accurately model river–aquifer interaction for gaining rivers in MODFLOW. [ABSTRACT FROM AUTHOR]

The spatial distribution of hydraulic conductivity (K) in riverbeds is essential to understand and model river-groundwater interactions. However, K in riverbeds varies over several orders of magnitude and its spatial distribution is closely linked to complex geological and fluvial processes. Investigating the local distribution and spatial heterogeneity of K is therefore a challenging task. The use of direct current (DC) and time-domain-induced polarization (IP) geoelectrical methods to map qualitatively the spatial distribution of K within riverbeds is described. The approach is demonstrated for a test site situated in a typical lowland river in Belgium. Inverted geophysical parameters (resistivity, chargeability and normalized chargeability) are compared with estimates of K obtained through slug tests. In general, high values of K are observed in the middle of the river and lower values towards the banks, while the opposite is true for chargeability and normalized chargeability. Therefore, there exists an inverse correlation between K and IP geophysical parameters. Furthermore, geostatistical analyses using variograms show that all parameters have ranges of similar magnitudes. The strong correlation between K and chargeability or normalized chargeability can be explained by the fact that all three parameters are mainly controlled by clay and organic matter content. [ABSTRACT FROM AUTHOR]