Hydrogeochemical, Isotopic, and Modeling Insights Into Groundwater Salinization in a Coastal Semiarid Aquifer System

Groundwater salinization poses a critical challenge to water security in arid and semiarid regions worldwide. To elucidate the processes governing salinity evolution and support sustainable management, this study integrates hydrogeochemical, isotopic, and solute transport modeling analyses based on 50 groundwater samples collected from 34 wells in the Wadi Aday Basin, Oman. Total dissolved solids (TDS) range from 585 to 1926 mg/L, with chloride concentrations reaching up to 886 mg/L. The primary source of salinity is attributed to the dissolution of evaporitic sedimentary units, with secondary contributions from evaporation, soil salt leaching, and limited natural recharge. Stable isotope data differentiate deeper, isotopically depleted groundwater formed under past humid climatic conditions from shallower wells influenced by episodic surface recharge, leakage from water-supply networks, and evaporative enrichment. Most wells exceed Omani drinking water standards for TDS, Mg, and SO₄, although minor element ratios and vertical EC profiles indicate no seawater intrusion. Groundwater flow and solute transport models developed using MODFLOW-USG and MT3DMS, respectively, were calibrated successfully. Simulations identify an area downstream of a proposed dam as an optimal site for future development owing to higher hydraulic conductivity and enhanced recharge potential, which would help dilute salinity and stabilize water quality. Modeling shows that under "no change" (Sc1), chloride concentrations increased by approximately 11 mg/L, whereas under "construction of a new dam with increased groundwater recharge" (Sc2) concentrations remained relatively stable and were approximately 10-11 mg/L lower than Sc1 by the end of the simulation. Recommended management options include implementing managed aquifer recharge via dam construction, optimizing well design to isolate high-salinity zones, adopting blending strategies, and deepening wells to access fresher groundwater.