Title: Geophysical-hydrochemical study of saline paleo-water contamination in alluvial aquifers

Abstract

An integrated geophysical and hydrochemical study of the saline paleo water uprising into the alluvial aquifer of the Oltrepò Pavese plain sector (Po Plain, northern Italy) is presented. This study involved one-, two- and three dimensional electrical geophysical surveys, hydrochemical analysis of groundwater and assessment of well logs. Geophysical surveys specifically involved both electromagnetic surveys undertaken over vast areas for a speedy assessment of sub-vertical conductive bodies connected to the uprising of high salinity waters through structural discontinuities and more detailed 1D to 3D electric resistivity surveys for the accurate investigation of the sectors where the uprising phenomenon of deep saline waters occurs. The studied area was selected for its characteristic hydrogeological setting (Pilla et al., 2010; Torrese and Pilla, 2021). The alluvial aquifer is strongly conditioned by the presence of a buried tectonic discontinuity along which the saline waters are mainly distributed. These waters rise along the discontinuities in the bedrock and flow into the overlying alluvial aquifer. This particular setting conditions the distribution of saline waters into the alluvial aquifer. Contamination from saline waters is not spatially and vertically homogeneous within the aquifer. The spatial distribution of Na–Cl waters suggests the existence of plumes of highly mineralized waters that locally reach the aquifer, diffuse and mix with fresh waters. Detailed 3D imaging revealed irregular-shaped shallow saline water contaminations within the alluvial aquifer. Deep saline paleo-waters show a dilution during upward migration. This is due to the mixing with shallow fresh groundwater. Highly mineralized groundwater is identified even at very shallow depth in correspondence of each plume, which is located above a structural discontinuity. On the other hand, there is a lower degree of contamination in those sectors of the aquifer that are further away from the structural discontinuities and generally only involves the deeper parts of the aquifer. The results from our study are applicable in similar hydrogeological contexts where the aquifer's contamination by saline water is caused by mixing of freshwaters with brines or where the fossil saltwater, located different kilometers far from the coastline, are remainder of ancient marine ingressions.