Transboundary hydrogeology and controlling factors of euphrates water chemistry: Insights from hydrochemistry, stable isotopes, and residence times in Syria and Western Iraq

Arid and semi-arid climates are critical for water security in sustainable river systems. The Euphrates River in western Iraq has been increasingly stressed by both geogenic and anthropogenic factors. This study analyzed hydro-geochemical processes, recharge dynamics, and groundwater flow modeling in western Iraq. Additionally, stable isotope tracing (δ¹⁸O, Cl⁻) integrated with hydro-chemical parameters was used to assess water quality and river-aquifer connectivity in both Syria and western Iraq. A total of 144 groundwater samples were collected annually across 12 stations in western Iraq. Surface water and groundwater data were collected from previous studies at 13 stations in Syria. Results revealed slightly alkaline freshwater conditions (pH 7.6–7.9; TDS 627–888 mg/l), evolution attributed to the dissolution of carbonate and gypsum, agricultural return flows, and industrial effluents. MODFLOW simulations and lumped-parameter modeling indicate bidirectional river–aquifer exchange, with a net river-to-aquifer flux under typical/average hydraulic conditions, although flow may locally reverse toward the river during low-flow periods, with mean residence times of 3–6 years, indicating delayed solute accumulation. Stable isotope enrichment supports evaporative concentration towards the downstream. Overall, the integrated approach reveals spatio-temporal degradation in the Euphrates River water quality controlled by the combined effect of water age, river-aquifer interaction, and anthropogenic inputs. These findings highlighted the need to synchronize monitoring with groundwater age structures and to develop integrated transboundary management strategies for sustainable water use in arid basins.