The origin of high sulfate concentrations and hydrochemistry of the Upper Miocene–Pliocene–Quaternary aquifer complex of Jifarah Plain, NW Libya

The high uncontrolled groundwater extraction in Jifarah Plain, NW Libya, causes a modification of natural flow systems, inducing seawater intrusion and causing groundwater quality deterioration. The principal aim of this study is to identify the hydrogeochemical processes in this coastal aquifer in order to verify the main sources of sulfate concentration increase that occurs in the system. In order to achieve this aim, water samples were collected from 134 sampling wells in the study area and analyzed for the major cations and anions; physical and chemical parameters were measured, such as water level, electrical conductivity, pH and temperature. The analytical results obtained in the hydrochemical study were interpreted using Piper diagram, ion correlations with Na⁺/Cl⁻, SO₄ ²⁻, Cl⁻ and TDS, in conjunction with calculation of the ionic deviations of the conservative freshwater/seawater mixture and saturation indices using the PHREEQC 2.16 software. The large SO₄ ²⁻ anomaly observed in groundwater near the coast was explained by the presence of seawater intrusion and upconing of deep saline water in these areas. This conclusion is based on high chloride concentrations, the inverse cation exchange reactions and the lower piezometric level compared to sea level. Inland, in Sabratah, the high SO₄ ²⁻ values are related to gypsum dissolution from the Upper Miocene Formation in the lower part of the upper aquifer. These locally high SO₄ ²⁻ concentrations in the south of the study area show overall increase in the upstream direction, which also suggests the dissolution of evaporites from the mountain aquifers in the south. High SO₄ ²⁻ concentration is also related to the effect of the scattered sebkha deposits in some areas along the coast.