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

Giorgio Pilla,Patrizio Torrese

doi:10.1007/s10040-021-02446-5

Abstract

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.

Highlights

Saline paleo-water uprising into aquifers is a hydrogeological process controlled by the geological and structural setting of the area, and the mechanisms are often not fully understood (Barberio et al 2021; Conti et al 2000; Grobe and Machel 2002; Petitta et al 2011; Re and Zuppi 2011; Schwartz and Muehlenbachs 1979; Yechieli and Sivan 2010)
The alluvial aquifer is strongly conditioned by the presence of the Vogherese Fault, a buried tectonic discontinuity along which the saline waters are mainly distributed (Pilla et al 2007), which is responsible for the uprising of deep, saline paleo-waters
Two main hydrochemical facies can be identified within the Oltrepò Pavese groundwater: a Ca–HCO3 hydrofacies, which characterises most of the groundwater of the Oltrepò Pavese alluvial aquifer; a Na–Cl hydrofacies, which locally characterises the groundwater in some sectors of the Oltrepò Pavese area along the Vogherese Fault (Fig. 5)

Summary

Introduction

Saline paleo-water uprising into aquifers is a hydrogeological process controlled by the geological and structural setting of the area, and the mechanisms are often not fully understood (Barberio et al 2021; Conti et al 2000; Grobe and Machel 2002; Petitta et al 2011; Re and Zuppi 2011; Schwartz and Muehlenbachs 1979; Yechieli and Sivan 2010). An unfavorable distribution of sampling wells makes it necessary to integrate any hydrogeological and hydrochemical study with geophysical surveys (Di Sipio et al 2006; Fadili et al 2017; Jiraporn et al 2020; Kouzana et al 2010; Pilla et al 2010; Pujari et al 2012; Torrese and Pilla 2021) This allows definition of the general hydrogeological setting of the studied area, as well as detailed characterization of localized and restricted zones of the aquifer where the uprising phenomenon of deep saline waters. The alluvial aquifer is strongly conditioned by the presence of the Vogherese Fault, a buried tectonic discontinuity along which the saline waters are mainly distributed (Pilla et al 2007), which is responsible for the uprising of deep, saline paleo-waters This contamination prevents the exploitation of the aquifer, for drinking-water supply This contamination prevents the exploitation of the aquifer, for drinking-water supply (Italian Legislative Decree, Decreto Legislativo 2 febbraio 2001, n. 31, Italian Legislature 2001), and for agricultural (Wilcox 1948) and industrial uses

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