Origins and processes of groundwater salinisation in Barka coastal aquifer, Sultanate of Oman

The quality of groundwater resources in many coastal aquifers in the world is threatened by salinity due to several contamination sources and processes. The aim of this work is to identify the sources of salinity and to improve understanding of the major processes controlling groundwater chemistry in Barka coastal region, Sultanate of Oman. Data of pH, EC and concentration of major ions available for 99 groundwater samples in spring 2010 were analysed using statistical and multivariate tools, PHREEQC code and molar ratios. The EC of the groundwater varied from 1,252 to 25,800 μS/cm with an average value of 8,409 μS/cm. Three water types were found in the groundwater: Ca–Mg–Cl, Na–Cl and Ca–Cl. They suggest the mixing of freshwater infiltrated from rock aquifers of Jabal Al Akhdar Mountain with the salt water caused from seawater intrusion followed by reverse cations exchange reactions. Piezometric map, and geochemical criteria and indices confirmed the occurrence of seawater intrusion into the coastal aquifer. The mixing rate of freshwater-seawater ranged from 0.04% to 57.72% through the study area. PHREEQC modelling shows that the groundwater was under-saturated with halite, anhydrite and silica, and saturated with calcite and dolomite. Pearson's correlations, principle component analysis, ionic changes (Nachange, Cachange, Mgchange and Kchange), scatter diagram and diagrams of some cations versus Cl− justified that dissolution of halite, anhydrite and silica minerals, and reverse cations exchange reactions controlled the groundwater chemistry in the study area.