Factors influencing the major ion chemistry in the Tihama coastal plain of southern Saudi Arabia: evidences from hydrochemical facies analyses and ionic relationships

Coastal aquifers across the globe have been a subject of extensive research mainly because of problems related to saline water intrusion as a consequence of global climate changes and overabstraction of groundwater. The present work deals with the assessment of factors which affects the groundwater chemistry in parts of the Tihama coastal plains in the Jazan Province of southern Saudi Arabia. Hydrochemical parameters (pH, EC, TDS, major ions, NO₃ and SiO₂) were obtained from 263 wells and were interpreted. Hydrochemical facies analyses revealed two main types of groundwater facies, i.e., the mixed groundwater facies predominantly confined to the eastern and central part of the study area (away from the coast) and groundwater facies dominated by SO₄ + Cl + NO₃ ions found mostly in the coastal zones in the western part of the study area. The average TDS value of the mixed groundwater facies is 945.7 mg/l, whereas the average TDS value is 3149.7 mg/l for the SO₄ + Cl + NO₃ anionic species. Greater influence of rainfall recharge can be observed in the mixed-water species, while saline water mixing or rock–water interaction can be seen in the SO₄–Cl–NO₃ type of anionic species. However, ionic ratios point more toward rock–water interaction rather than saline water mixing in the groundwater facies dominated by SO₄–Cl–NO₃ ions. This interpretation is also supported by the presence of sabkhas (salt flats) in the coastal zones where its interaction with the groundwater has resulted in high Cl and SO₄ concentration. Cl/Br ratios which have often been used to detect saline water intrusion in coastal zones also do not support saline water intrusion in the study area. Ionic ratios point toward dissolution and base ion exchange as the main factors influencing the groundwater chemistry. Silicate weathering and silica dissolution are also found in the area. Saturation indices point toward gypsum and halite dissolution. The average NO₃ concentration is 157.80 mg/l and is mostly attributed to the agricultural practices in the region. The presence of shallow unconfined aquifers facilitate the easy percolation of irrigation return flows enriched in NO₃ concentration from the application of inorganic fertilizers on the agricultural farms.