Hydrochemical and statistical study of groundwaters in Gabes-south deep aquifer (south-eastern Tunisia)

Abstract

Groundwater quality assessment is important to ensure sustainable safe use of water. In the region of Gabes, groundwaters represent the main source of water supply for drinking, agriculture and industry seeing the shortage in surface water. In the present paper, an attempt has been made, for the first time in this region, to study the spatial and temporal evolution of the hydrochemistry and to identify factors and phenomena governing the assessment parameters of Gabes-south groundwaters quality, based on (1) an integrated analysis of physical–chemical parameters (2) use of Geological Information System (GIS) and (3) statistical methods. To this purpose, water samples were collected during the period of 1995–2003 from nine boreholes to represent Gabes-south deep aquifer. The water samples were investigated with respect to salinity, concentrations of hydrogen (pH), calcium (Ca 2+), magnesium (Mg 2+), sodium (Na +), potassium (K +), chloride (Cl −), sulphate, SO 4 2 - , bicarbonates HCO 3 - and fluoride (F −). Spatial distribution maps of these parameters have been created using GIS contouring methods with Arcview 3.2. The analytical results obtained were interpreted using hydrochemical methods reinforced by statistical analysis (trend analysis, principal component analysis and correspondence analysis) which has been widely used to analyse large sets of variables in environmental studies. Chemical analysis show that during the considered period, salinity and major elements concentrations vary little with time, and decrease in the direction of the groundwater flow. The waters are mostly SO 4, Cl, Na and Ca type. The chemical composition of these waters is related to the lithology of the aquifer, to their origin and to the exploitation conditions. Water quality analysis indicates that Gabes-south groundwaters are mainly composed of SO 4, Cl, Na and Ca type. Results showed that fluoride concentrations exceed the World Health Organisation (WHO) maximum permissible limit of 1.5 mg/l. Therefore, the water of these boreholes must be defluorinated before use for drinking.