Hydrochemical-geophysical study of saline paleo-water contamination in alluvial aquifers

An integrated hydrochemical and geophysical 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 hydrochemical analysis of groundwater, assessment of well logs, and one-, two- and three-dimensional electrical geophysical surveys. The studied area was selected for its characteristic hydrogeological setting. 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. 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 freshwaters. The saline waters show a dilution during upward migration, which is due to mixing with the shallow fresh groundwater. Highly mineralized groundwater is identified even at very shallow depth in correspondence with each plume. 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 this lesser contamination generally only involves the deeper parts of the aquifer.