Hydrogeological Characterization of Fine-Grained Floodplain Aquifers Using Integrated Geophysical and Rock-Physics Approaches

Floodplains in low-energy depositional environments often feature fine-grained facies with relatively low hydraulic conductivities (< 1 × 10⁻⁵ m s⁻¹). In such systems, the most hydraulically conductive zones are often associated with calcareous-rich deposits, which consist of freshwater unlithified tufa, but also include organic-rich layers. This study examines the spatial distribution and hydrogeological significance of calcareous-rich units under fully-saturated conditions at two contrasting floodplain sites in Southwestern Germany. Site 1 is located within a relatively wide floodplain (~800 m wide), where these calcareous units can reach a thickness of up to 7 m. In contrast, site 2 is located within a narrower floodplain (~100 m wide), where these units reach a thickness of up to 3 m.At site 1, we acquired 2-D geoelectrical and borehole nuclear magnetic resonance data, which were used within a rock-physics framework combining Archie’s law and the Kozeny-Carman model to estimate the hydraulic properties of calcareous-rich units. Collocated pumping test, which indicate hydraulic conductivities of 1 × 10⁻⁶ to 1 × 10⁻⁵ m s⁻¹, were used as ground truth to calibrate the site-specific parameters of the rock-physics models. Such calibrated models may subsequently be applied at other field sites with similar characteristics where only resistivity data are available.At field site 2, we collected geoelectrical and seismic data (P- and S-waves) to identify the spatial distribution of the calcareous-rich units. The resulting resistivity and S-wave velocity models delineated these units in agreement with collocated borehole data. In contrast, the P-wave velocity model did not clearly resolve them but provided useful constraints on the depth to bedrock. At this site, rock-physics approaches similar to those applied at field site 1 will be used to support planned hydrogeological modeling and enable direct comparison between the two sites.Our preliminary results demonstrate the potential of integrated geophysical and rock-physics approaches to identify and characterize hydraulically relevant units within fine-grained–dominated aquifer systems.