Integrated approaches to support the hydrogeological coastal plain conceptualization (Metaponto, Southern Italy)

The integration of geophysical, geognostic and hydrogeological methods was implemented to support the conceptualization of a complex coastal aquifer to obtain a better understanding of the aquifer hydrodynamic. This approach involved the use of electrical resistivity survey, in terms of characterization and monitoring acquisitions, seismic survey and hydrogeological loggings. The integration of these techniques provided a more comprehensive understanding of the coastal aquifer system, helping to identify potential risks and opportunities for sustainable management. The integrated geophysical and hydrogeological approach was tested for understanding the coastal aquifer of the Metaponto plain with the final purpose of developing sustainable management strategies. Four geological formations and hydrogeological units can be schematically identified. They are, from the bottom to the top: the Argille Subappennine Formation (Subapennine Clays); the marine terraced deposits; the alluvial, transitional, and marine deposits, hereinafter the plain deposits); and the coastal deposits. The marine terraced deposits constitute the outcropping aquifer far from the coast; the plain deposits and secondly the coastal deposits constitute the aquifer along a wide coastal strip in the Metaponto plain, the Argille Subappennine is the deep aquifer bottom in both cases. This exemplification does not take into account the considerable variability in the granulometric characteristics of the deposits of the plain, which are determined by the succession of phases of erosion and deposition, variations in sea level, the position of the coastline as well as the extensive meandering of the main rivers of the plain, including the nearby Basento River. Inside the Metaponto coastal plain, which extends about 40 km along the Ionian coast, a test site was selected close to the Metaponto municipality, between the Cavone and Bradano Rivers (Southern Italy). In order to elucidate the effects of these complex geological events on the main hydrogeological attributes and thus improving knowledge of the plain’s aquifer, an extensive survey campaign was conducted using a multimethodological approach. Three electrical resistivity tomography (ERT) were realised, from inland to the coast, crossing test-site, of lengths ranging from a minimum of 940 m to a maximum of 1,820 m, integrated by thirty-four HVSR (Horizontal to Vertical Spectral Ratio) measurements. In order to make a comparison between geophysical and stratigraphical data, improving geophysical results, tree boreholes were drilled. Every borehole was instrumented with an open pipe piezometer equipped with a permanent multipolar electric cable and electrodes along the vertical (every 2 meters), in order to investigate groundwater table and soil resistivity variations. Periodically (every three months), multi-parameter logs were carried out along the water column of piezometers by means of probes equipped with sensors for measuring temperature, electrical conductivity, pH, oxygen demand, and oxidation reduction potential. At the same time, surface-to-borehole ERT were acquired with electrodes spacing of 3 m. The ERT investigations, integrated with HVSR data, show excellent correlation with the geological information of the area and borehole coring data. The results highlighted the irregular geometries related to the morphological evolution of the coastal area as well as the geological and stratigraphic complexity of an area where the evolution of major rivers in their morphological dynamism, related to past geological events, have continuously influenced the current geometrical stratigraphical patterns and the grain size variability.The systematic and periodical use of multi-parameter logs integrated with surfaceto-borehole ERT investigations, provided preliminary assessments of the spatial variability of chemicalphysical characteristics of groundwater related to seawater intrusion phenomenon. Focusing on the risk of salinization for seawater intrusion, this approach can support conceptualization and numerical modelling, i.e., permitting the bounding of areas where saltwater intrusion is a potential risk and where some measures, such as groundwater recharge, may be needed to maintain the water supply