The Orontes basin contains significant karstic water resources, which largely fed the Orontes River before the extensive development of irrigation.
Three important lithological units constitute the huge water reservoir of the Orontes River basin: two thick limestone formations from the Jurassic era and Mid-Cretaceous era supply the springs located in the upstream part of the basin and a more recent formation from Eocene-Miocene supplies the springs located in the downstream part of the basin.
The Orontes basin also includes several secondary aquifers from the Upper Cretaceous era and Paleogene era that outcrop over large areas. Because of their low porosity and relatively high permeability, these formations with modest water resources are often significantly over-exploited. However, their initial piezometric level is sometimes completely or partially recovered after particularly wet years.
THE JURASSIC AND MID-CRETACEOUS AQUIFER COMPLEX
As shown on the different cross-sections, the thicknesses of the fractured and generally karstified aquifers formations from the Jurassic and Mid-Cretaceous eras, may each approach or exceed one thousand meters thick. Even if these formations are certainly not equally fractured and karstified, they contain groundwater flowing through them and from one to the other, using tectonics faulting or fractures, even if they are separated by lower Cretaceous quite impermeable formations. At the scale of basin, we can simplify and consider the Jurassic and Cretaceous formations as a unique complex aquifer forming a very large reservoir in hydraulic continuity.
Furthermore, in areas where this Lower Cretaceous layer forms a thin barrier hydraulic heads differences can form between the two aquifers, and large transfers of water can be expected, because the of the considerable extension of these two formations.
In the southern and central part of the basin, this vast karstified reservoir supplies many springs, which have an average annual flow up to 13 m3/s. Their regime is more or less stable throughout the year because of the very large water reserves, the high hydraulic conductivity, the well-developed internal drainage and the extended confinement of this complex reservoir.
The recharge of the Jurassic-Cretaceous reservoir is particularly important in the highest areas of the basin, specifically in the Mount Lebanon and the Anti-Lebanon Where limestone formations outcrop; the recharge rate could reach 60%. Taking place mainly during winter season, the recharge can locally extend to the spring during the melting of the snow cover.
THE EOCENE-MIOCENE AQUIFER COMPLEX
In the north of the basin, significant groundwater resources are contained in the limestone Eocene–Miocene formations, which form a regional shallow aquifer but with high extension. Where they outcrop, these highly cracked, fractured and karstified formations offer particularly favorable infiltration conditions, similar to those mentioned above for the aquifer complex Jurassic and Mid-Cretaceous. Water resources of these formations are particularly high despite precipitation, which are so much lower than those observed on the Mount Lebanon. These limestone Eocene-Miocene formations are the source of many springs: those of Ruj depression and those located along the Orontes upstream from the border, including Ayn Alzarqa, a remarkable spring with a stable and particularly high flow.
Inventory of groundwater sources and resurgences of major interest of the Orontes basin, contains around 30 springs, including several major located close to the Lebanon-Syrian border, as Ayn ez Zarqa and Ayn at Tannur
The evolution of their annual average flow between the 1960s and the 2000s is difficult to access precisely. We generally observe a significant decrease because of the intensive use of water more specifically groundwater pumping from deep wells, due to recent and rapid development of new irrigated lands, mainly in Syria.
Supplied by groundwater coming from Jurassic & Mid-Cretaceous aquifer, the main sources in Lebanese territory are Ayn ez Zarqa (Orontes spring, 1.01) and Ain el Laboue (1.02), in Syrian territory close to the border are Ayn at Tannur (2.01), Uyun as Samak (2.02) and Ayn al Damamel (2.03).
Remote groundwater discharge of the same Jurassic and Mid-Cretaceous aquifer has been much more affected downstream, in particularly in the East of the Ghab areas. Some springs have even been dried.