Titolo della tesi: Contribution of hydrochemistry and isotope geochemistry to the study of hydrodynamics and mineralization of the Mornag aquifer system, Northeastern Tunisia
Abstract
Groundwater is the main source of drinking and irrigation water in Mornag basin, Northeastern of Tunisia. In recent decades, the overuse of groundwater has resulted in a generalized water table decline and groundwater quality contamination.
The present investigation corresponds to a hydrogeological, hydrochemical and isotopic synthesis based on the results of the analyses of 39 groundwater samples taken from dug- wells, boreholes and 1 spring during dry and wet seasons (September 2022 and May 2023). It is interested in the study of the spatio-temporal evolution of the
physic-chemical parameters, major and trace elements, stable isotope of water molecule and Carbone-13 of the Mornag aquifer system.
The geological study show that the lithological units in the study area range from the Eocene to Quaternary. The permeable units host an important and complex multilayer aquifer system. The monitoring of the piezometry indicates that groundwater flows from the south towards the northeast and from the southwest towards the
northeast, indicating that the recharge areas are located at the foothills of the bordering reliefs and the discharge
area are located in the Gulf of Tunis.
The hydrochemical study reveals the presence of two major water types namely the Ca-Cl-SO4 and the Na-Cl water facies. This may reflect that groundwater chemical composition is mainly influenced by water-rock interaction processes related to the carbonate dissolution, the reverse ion-exchange with clay minerals and the dissolution of evaporitic minerals (gypsum and halite).
The assessment of groundwater suitability for drinking water shows that all groundwater samples are not potable according to WHO standards, particularly in the northern and northeastern parts, characterized by soft brackish water. Moreover, the use of water quality indexes for irrigation such as PS, SAR, Na, SSP, MH, KI and IWQI highlight that the Mornag groundwaters range from hard to unsuitable for irrigation practices.
The oxygen-18 and deuterium data has allowed for the identification of three distinct groups of Mornag groundwater. Waters with depleted δ18O and δ2H contents, suggesting a recent origin related to rapid infiltration of precipitation at the foothills. Waters with enriched stable isotopes contents, indicating an evaporative signature associated with return flow of irrigation water and the recharge by saline evaporated water, in the coastal area. The third group influenced by paleoclimatic conditions.
Keywords: Groundwater, Mornag aquifer, spatio-temporal evolution, major elements, trace elements, isotopes.