Hydrogeophysical imaging of alluvial aquifers: electrostratigraphic units in the quaternary Po alluvial plain (Italy)

The integration of surface geological and geomorphological information with borehole point-data and geophysical (e.g., geoelectrical) images of the subsurface yields spatially consistent representations of alluvial aquifers heterogeneity at different scales, from depositional systems to basin fills. Such an approach requires a conceptual framework to match the stratigraphic units with their evidence from ground-based DC resistivity methods to effectively fill the gaps between sparse borehole data and to obtain valid representations of sedimentary heterogeneities. Such an approach is applied to characterize two sites of the Quaternary aquifers of the central Po Plain (Italy), which represent (1) the middle-upper Pleistocene braided to meandering river depositional systems sitting on Southalpine crust and (2) their down-current counterparts, where they are involved by the latest uplift and deformation due to the tectonic activity of the Apennine frontal thrusts. Electrical resistivity was considered as a proxy of the litho-textural properties of hydrofacies and their major hierarchical association at depth and was interpreted in accordance with the depth-decreasing resolution of ground-based resistivity methods. Thus, it was possible to identify the geophysical signature of hydrostratigraphic units through "Electrostratigraphic Units", i.e., sedimentary volumes identified by resistivity contrasts that spatially preserve the vertical polarity. Hydrostratigraphy and electrostratigraphy were then joined together through a site-specific relationship between electrical resistivity and hydraulic conductivity, which takes into account the prevailing process of current conduction, the litho-textural properties of hydrofacies and the groundwater electrical conductivity. At the scales of aquifer systems and complexes, this approach permitted to establish the conceptual framework to match hydrostratigraphy, electrostratigraphy, average hydrodynamic properties and distribution of heterogeneities.