River-aquifer interactions play an important role in a wide range of major ecological and hydrological challenges. An accurate characterization of these interactions is important for assessing river water quality and riparian ecology, evaluating stream depletion due to groundwater extraction close to rivers, and predicting flood peaks and low flows. Most studies focus on the large-scale characterization of river-aquifer interactions. However, aquifer and riverbed properties can exhibit strong heterogeneity on the meter-scale. Few studies have been performed that focus on the effect of meter-scale spatial variability of these properties on resulting river-aquifer exchange fluxes.
In this study, river-aquifer interaction has been characterized on the meter-scale by combining field measurements, heat transport modelling and groundwater modelling for a stretch of the Aa River, a typical gaining lowland river in the Nete Catchment (Belgium). Field measurements have been performed at two 20m long sections of the Aa River. Both horizontal and vertical riverbed hydraulic conductivity are characterized with respectively falling-head slug tests and rising head standpipe tests. Furthermore, vertical riverbed temperature profiles are measured, and vertical river-aquifer exchange fluxes are estimated with heat transport modelling. The resulting fluxes are compared with river-aquifer exchange fluxes derived from a local groundwater flow model (MODFLOW) and with fluxes calculated with Darcy’s law based on riverbed K and vertical hydraulic gradients. Moreover, geophysical measurements (ERT and IP) are performed at one of the sections of the Aa River to qualitatively map the spatial structure of the riverbed. A geostatistical analysis of riverbed K, exchange fluxes and geophysical properties has been performed to characterize the spatial variability and anisotropy on the meter-scale.
Results show that riverbed hydraulic conductivity is strongly spatially variable and that it varies over several orders of magnitude within several meters. Elongated structures of high riverbed K along the river flow are observed, which are related to deposition and erosion due to high discharge events, and to the presence and thickness of an organic matter layer at the top of the riverbed. Estimated vertical river-aquifer exchange fluxes based on heat transport modelling show a clear spatial variability on the meter-scale. In general, higher fluxes are observed near the river banks. This indicates that the spatial distribution of the fluxes cannot only be explained by the spatial distribution of riverbed K. An inverse correlation between riverbed K and IP geophysical parameters has been identified. However, the relationship is only semi-quantitative. Therefore, it is recommended to use ERT/IP to qualitatively map zones of high hydraulic conductivity and to target sampling locations of K, rather than in a direct quantitative way. Different methods to quantify river-aquifer exchange fluxes are compared, and the influence of the different assumptions behind these methods is shown.