<p>The hydrochemical assessment of the surface and groundwater resources in the Orontes River (OR) basin in Lebanon and Syria was conducted using the chemical and environmental isotope methods. This study aims to determine the origin and quality of water, and mean residence time of groundwater in the different aquifers. The river-waters upstream were fresh (total dissolved solids (TDS) &lt; 0.4&#xa0;g&#xa0;l^–1), while its quality north the city of Homs was severely polluted due to waste waters being injected into the riverbed. The groundwater quality of most of the karst springs located upstream or distributed at the foot of the Coastal mountain range was also good, with a Ca–Mg and HCO₃ type. The higher TDS and higher water temperature (<i>T</i>) values that measured in the surface and groundwater downstream are mostly due to the presence of deep faults. The groundwater of most of the karst springs upstream were isotopically depleted, reflecting higher recharge zones (1100–1500) m above sea level (a.s.l). The enriched isotopic values, measured in the western and eastern springs, are likely the result of different groundwater-recharge zones (&lt; 900 m a.s.l for western springs and &lt; 600 m a.s.l for eastern springs). The groundwaters in the upstream and western springs are rapidly replenished (mean residence time &lt; 75 years), while those issued from the eastern springs have a significantly longer residence time (&gt; 100 years).</p>