Suitability assessment of deep groundwater for drinking and irrigation use in the Djeffara aquifers (Northern Gabes, south-eastern Tunisia)

The multilayered Djeffara aquifer system, south-eastern Tunisia, has been intensively used as a primary source to meet the growing needs of the various sectors (drinking, agricultural and industrial purposes). The analysis of groundwater chemical characteristics provides much important information useful in water resources management. Detailed knowledge of the geochemical evolution of groundwater and assessing the water quality status for special use are the main objective of any water monitoring study. An attempt has been made for the first time in this region to characterize aquifer behavior and appreciate the quality and/or the suitability of groundwater for drinking and irrigation purposes. In order to attend this objective, a total of 54 groundwater samples were collected and analyzed during January 2008 for the major cations (sodium, calcium, magnesium and potassium), anions (chloride, sulfate, bicarbonate), trace elements (boron, strontium and fluoride), and physicochemical parameters (temperature, pH, total dissolved salts and electrical conductivity). The evolution of chemical composition of groundwater from recharge areas to discharge areas is characterized by increasing sodium, chloride and sulfate contents as a result of leaching of evaporite rock. In this study, three distinct chemical trends in groundwater were identified. The major reactions responsible for the chemical evolution of groundwater in the investigated area fall into three categories: (1) calcite precipitation, (2) gypsum and halite dissolution, and (3) ion exchange. Based on the physicochemical analyses, irrigation quality parameters such as sodium absorption ratio (SAR), percentage of sodium, residual sodium carbonate, residual sodium bicarbonate, and permeability index (PI) were calculated. In addition, groundwater quality maps were elabortaed using the geographic information system to delineate spatial variation in physico-chemical characteristics of the groundwater samples. The integration of various dataset indicates that the groundwater of the Djeffara aquifers of the northern Gabes is generally very hard, brackish and high to very high saline and alkaline in nature. The water suitability for drinking and irrigation purposes was evaluated by comparing the values of different water quality parameters with World Health Organization (WHO) guideline values for drinking water. Piper trilinear diagram was constructed to identify groundwater groups where the relative major anionic and cationic concentrations are expressed in percentage of the milliequivalent per liter (meq/l), and it was demonstrated that the majority of the samples belongs to SO₄–Cl–Ca–Na, Cl–SO₄–Na–Ca and Na–Cl hydrochemical facies. As a whole, all the analyzed waters from this groundwater have revealed that this water is unsuitable for drinking purposes when comparing to the drinking water standards. Salinity, high electric conductivity, sodium adsorption ratio and sodium percentages indicate that most of the groundwater samples are inappropriate for irrigation. The SAR vary from medium (S2) to very high (S4) sodicity. Therefore, the water of the Djeffara aquifers of the northern Gabes is dominantly of the C4–S2 class representing 61.23 % of the total wells followed by C4–S3 and C4–S4 classes at 27.27 and 11.5 % of the wells, respectively. Based on the US Salinity Classification, most of the groundwater is unsuitable for irrigation due to its high salt content, unless certain measures for salinity control are undertaken.