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

Groundwater is an important resource used for multiple purposes and should be protected, especially, in 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, combined hydrogeochemical and isotopic fingerprints data approaches were used 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: 25 from shallow aquifer and 11 from deep aquifer. The obtained results from both stretches indicated three dominant hydrochemical facies: Chloride-sodic, Sulphated-sodic, and mixed water type. The determination of groundwater salinization origin, and the understanding of its hydrological and geochemical behaviors, were assessed by a combined statistical and hydrogeochemical approach. The study of correlations 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 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 82H and 818O values for groundwater samples located close to Global Meteoric Water Line 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.