Characterization of spatially variable riverbed hydraulic conductivity using electrical resistivity tomography and induced polarization

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.