Stable Isotopes for Sustainable Management of Agricultural Water: Case of Mateur Plain (North Tunisia)

Abstract

Water stable isotopes have been used since the 61’s as natural tracers of the hydrological cycle. Water entering a catchment have a characteristic fingerprint of its origin and therefore can help to identify where the water in the system comes from. A multicriteria approach in studying hydrodynamics of a multilayer aquifer system has been used in the Mateur plain (Northern Tunisia). It aims to reconstruct the hydrogeological framework of the shallow and the two-deep aquifers distinguished in this area and then to define their hydrochemical and isotopic features. In order to determine the hydrogeochemical processes controlling the water quality and origin, 70 samples from surface water, springs and groundwater were collected and analyzed for various parameters. Physical and chemical parameters of groundwater such as electrical conductivity, pH, total dissolved solids, Na, K, Ca, Mg, Cl, HCO3, SO4, Sr and water molecule isotopes (δ18O, δ2H and 3H) were determined. This dataset reveals that water chemistry is regulated primarily by the combination of three processes: (1) weathering of minerals such as silicates and calcites; (2) ion reverse exchange with host rocks, and (3) mixing with Cl-rich water from the Lake Ichkeul. Saturation indices calculated by PHREEQC model show that nearly all water samples were saturated to undersaturated with respect to carbonate and evaporite minerals. Mixing between all water masses of the system (shallow, deep groundwater, surface water and springs) is indicated by homogeneous values of δ18O and δ2H contents. Isotopic composition plotted close to the Global Meteoric Water Line confirms that groundwater is mostly recharged by modern precipitation with an evaporation effect revealed by d-excess values mainly around the Lake Ichkeul in relation with the wet/dry fluctuation of the surrounding swamp. Tritium analysis confirmed the presence of modern groundwater and a relatively homogeneous recharge process. On the other hand, groundwater sampled in intensive farming areas have nitrate contents exceeding World Health Organization (WHO) standards indicating that the long-term agriculture activities are affecting groundwater quality.