Hydrogeochemical processes on inland aquifer systems: A combined multivariate statistical technique and isotopic approach

Abstract

Groundwater is an important resource used for multiple purposes and should be protected, especially, in semi-arid and arid regions. The aim of this study is to assess the main processes on groundwater salinization from Menzel Habib shallow and deep aquifers, southeastern Tunisia, which could be applied to further groundwater quality assessment. Indeed, there were used combined hydrogeochemical (major elements) and isotopic fingerprints (18O, 2H) data approaches using multivariate statistical methods, highlighting, Hierarchical Clustering Analysis and Principal Component Analysis. A total of 36 groundwater samples were collected from the Menzel Habib aquifer system: twenty-five from shallow aquifer and eleven from deep aquifer. The obtained results from both aquifer layers indicated three dominant hydrochemical facies: Chloride-sodic, Sulphated-sodic, and mixed water type, according to Piper classification. The determination of salinization origin of groundwater, and the understanding of its hydrological and geochemical behaviors, were assessed by a combined statistical and hydrogeochemical approach. The study of correlations established between major elements and Total Dissolved Solids is, thus, an important tool where dissolution of evaporites that can originate from the Triassic materials of Hadifa mountain, the precipitation and/or dissolution of carbonates, cationic exchange and inverse cationic exchange processes are the main processes associated to groundwater salinity increase. Moreover, the HCA has allowed to classify the groundwater samples into two clusters: the first one with low to moderate salinity and the second one with high salinity. In addition, the application of stable isotopes allowed a better understanding of the hydrodynamic functioning of this aquifer system. The isotopic data suggests a relatively high δ2H and δ18O values for groundwater samples located close to Global Meteoric Water Line (GMWL) that reflects recent recharge for the aquifer system by direct rainwater infiltration, respectively from bordered reliefs in the study area. It also showed a depletion in isotopic composition for almost groundwater samples that indicates the importance of evaporation on the hydrochemistry of the area.