Hydro-geochemical modeling of Hajeb Layoun-Jelma basin (Central Tunisia)

Groundwater is the main water source in all the world and especially in the arid and semi-arid regions. The Hajeb Layoun Jelma basin is the principal source of water supply for Sidi Bouzid and Sfax region. In the last decades, pollution is considered a common groundwaters problem, representing a severe and harmful threat to the water resources. In this context, this work has been taken place. The Hajeb Layoun-Jelma basin is the selected site. The main objectifs is to provide its actual water quality and quantity situation. This research aims to perform a geochemical view of the two principal aquifers, implement a numerical mode to control the basin, and assess the vulnerability to the shallow aquifer’s pollution using different models. The groundwater hydrochemistry study’s main objectives are to determine the water chemistry origins and assess the groundwater suitability for drinking and irrigation purposes. Twenty-eight water samples were collected in 2017 (wet period) from shallow and deep aquifers and analyzed for different physicochemical parameters (temperature, pH, EC, salinity, Na+, Ca2+, K+, Mg2+, Cl-, HCO3 -, and SO4 2-). The shallow aquifer shows high salinity in most water samples (93% > 1 g.l-1). The deep aquifer has moderate salinity (21% of samples exceeding 1 g.l-1). The results show that both aquifers’ water mineralization is controlled by the dissolution of carbonates/gypsum and water evaporation. The drinking water quality assessment shows that 100% and 57% extremely poor water for the shallow and the deep samples, respectively, coincide with the Na-Cl water type. The water quality evaluation for irrigation uses indicates that the shallow samples show quality less than the deep one and revealed that most samples in the Hajeb Layoun-Jelma basin are not appropriate for irrigation uses. The recharge rate estimation was made using the multi-criteria method. The results show that the shallow and the deep aquifer receive an average recharge rate, from rainfall, about 31.5 mm/year (infiltration: 15%) and 34 mm/year (16.2%), respectively. The numerical model was developed using Modflow code under GMS software. The hydrodynamic model system permitted to estimate the hydraulic conductivity distribution. It also allowed estimating abstraction’s effect on the water table evolution by two pumping scenarios (2019-2050) (Sc1: constant pumping rates, Sc2: doubled pumping rates). The hydrodynamic models show the continuous water table decrease after 30 years. -5- The groundwater vulnerability assessment of the shallow aquifer of Hajeb Layoun Jelma basin was made using both intrinsic and simulation models. The DRASTIC model was used as intrinsic tool for identifying the susceptible zones to contamination for the shallow aquifer. The vulnerability maps indicated that the dominant vulnerability classes are a low class (55%) followed by the moderate class (43 %) in the pesticide model and the “low” classes (86 %) in the standard model. A high vulnerability characterizes only 1 % of the study area to pesticide contamination. The superposition of the standard and the pesticide DRASTIC maps with the land use map shows that many agricultural zones are located in the area characterized by “high” to “moderate “vulnerability. The study suggests that these “DRASTIC” maps can be a valuable tool for local authorities for groundwater and land use management. MTDMS is used to evaluate the transport of salts in the shallow aquifer. The salt transport model results show that the salinization process affects the areas close to the north part’s mountains. The high salinity concentration is related to the irrigated area. These investigations could constitute a basis for decision-makers for water resources management and prevent pollution risks.