Geochemical, isotopic and statistical monitoring of groundwater quality: Assessment of the potential environmental impacts of the highly polluted CI water in Southwestern Tunisia

Abstract

Groundwater chemistry depends on a number of natural and anthropogenic processes. Its evolution under natural conditions or in response to human perturbations remains of major concern. Thus, the main purpose of this study is to evaluate the groundwater quality deterioration in Southwestern Tunisia where the increasing water demand is mainly supplied by the deep confined aquifers of SASS (Système Aquifère du Sahara Septentrional) multi-layered system. The permanent exploitation of the deepest Continental Intercalaire (CI) aquifer leaded to increasing water quality deterioration. Recently, the hydrogeological regime of the aquifer system undergoes a tremendous change resulting in hydrocarbon (HC) contamination of the largest aquifer in Southern Tunisia. A mixing of petroleum substances and CI thermal water is currently pumped from a number of CI water boreholes in Kebili region owing to a spectacular on shore hydrocarbon seepage at SW Tunisia. This contamination is attributed to natural migration by continuous upward petroleum flows via permeable faults, plans of weakness and/or leakage through drilling wells (spilled oil).The most noteworthy finding of the current study is that the measured chemical composition of CI aquifer water reveals a significant wider distribution than expected. The impacts of the HC pollution are expressed by scattered geochemical, isotopic and statistical data. The hydrogeochemical results indicate that in the Western field, the CI is characterized by low mineralized water (TDS from 0.6 to 2.7 g/l) while towards the East, the CI water becomes mostly polluted with salinity ranging between 3 and 20 g/l and cadmium (Cd) concentrations between 12 and 90 μg/l these values are widely above the permissible limits for both human consumption and agriculture activities. Similarly, the environmental isotopic fingerprinting indicates that CI thermal waters in the contaminated field are influenced by mixing with deep groundwater. The mixing process seems to be facilitated by regional tectonic features such as deep fractures and major faults in the contaminated area. Correspondingly, the statistical analysis (Principal Component Analysis (ACP) and Hierarchical Cluster Analysis (HCA)) confirm, in turn, the scattered results suggesting the dissimilarity of groundwater composition between the contaminated boreholes and the sampled waters from the western side.