Several major sedimentary basins, in which thick sequences of Palaeogene marls and Neogene-Quaternary sediments have accumulated, can be distinguished in the arid zones which extend from the Great Nefud in Saudi Arabia to the Euphrates river in Syria. Structurally, they represent areas of relative subsidence formed between the Rutbah-Ha'il arch, the Jordan uplift and the Halab uplift.
The Syrian steppe and adjoining areas can be subdivided into eastern, western and north-western regional ground-water flow features which follow north-westerly (Palmyrian) and south-south-westerly trends.
The eastern regional flow system in the permeable Palaeozoic and Mesozoic strata is related to the development of the broad domal structure of the Rutbah-Ha'il arch. Transmission of flow parallel to a system of north-north-westerly lineaments occurs from the Rutbah area of Iraq to the Wadi-el-Miah area of Syria. Step faulting and the minor anticlinal structures developed in this latter area seem to influence upward leakage from the deeper aquifers into the Cretaceous and Palaeogene chert aquifers. Groundwater transmitted westwards from the Rutbah-Ha'il arch is retarded by the marly facies of the Upper Cretaceous and Palaeogene in the Wadi Sirhan basin. Minor chert aquifers, however, transmit a portion of the flow. Discharge occurs in a zone along the eastern margin of the Wadi Sirhan.
The western regional flow system differs from that in the east in that hydraulic interconnection between the upper aquifers with the underlying Mesozoic is negligible. The recharge zone for this system coincides with the higher portion of Jebel-el-Arab. Groundwater transmission in the complex sequence of basaltic sheets of the Syria-Jordan plateau region is controlled by two fracture systems, in addition to the usual lava flow conduits.
The north-western regional system is recharged in the Anti-Lebanon and flows south-westwards in fractured limestones and dolomites. Discharge occurs mainly via karstic springs. Upward leakage may occur into the Ghouta gravel aquifer and eventually may be lost by evaporation from Ateibe lake. In the S Palmyrian moutains zone, a flow pattern controlled by linear en-echelon structures transmits groundwater in a north-easterly direction. Discharge from the system takes place at Sabkhet-el-Mouh, south of Palmyra. Hydrogen sulphide water, issuing along the south Palmyrian regional fault system, suggests relatively deep groundwater circulation. Isotopic studies indicate that groundwater age increases along the Palmyrian flow system.