The Regional Deep Cretaceous Aquifer (RDCA) is the principal groundwater resource in Syria. Isotope and hydrochemical data have been used to evaluate the geographic zones in terms of renewable and non-renewable groundwater and the inter-relation between current and past recharge. The chemical and isotopic character of groundwater together with radiometric ¹⁴C data reflect the existence of three different groundwater groups: (1) renewable groundwater, in RDCA outcropping areas, in western Syria along the Coastal and Anti-Lebanon mountains. The mean δ¹⁸O value (−7.2 ‰) is similar to modern precipitation with higher ¹⁴C values (up to 60–80 pmc), implying younger groundwater (recent recharge); (2) semi-renewable groundwater, which is located in the unconfined section of the RDCA and parallel to the first zone. The mean δ¹⁸O value (−7.0 ‰) is also similar to modern precipitation with a ¹⁴C range of 15–45 pmc; (3) non-renewable groundwater found in most of the Syrian interior, where the RDCA becomes confined. A considerable depletion in δ¹⁸O (−8.0 ‰) relative to the modern rainfall and low values of ¹⁴C (<15 pmc) suggest that the large masses of deep groundwater are non-renewable and related to an older recharge period. The wide scatter of all data points around the two meteoric lines in the δ¹⁸O-δ²H diagram indicates considerable variation in recharge conditions. There is limited renewable groundwater in the mountain area, and most of the stored deep groundwater in the RDCA is non-renewable, with corrected ¹⁴C ages varying between 10 and 35 Kyr BP.

Al-Mazraa is a heavily populated suburb of Damascus (Syria) with agricultural activity. It is adjacent to the Cretaceous Qassioun Mountain Range, from which it is structurally separated by the Damascus fault. Al-Mazraa waterworks abstracts from a shallow Quaternary aquifer, whose recharge processes are unidentified. The functions of Qassioun Mountain, the Damascus fault, the agricultural activities, the ascending deeper groundwater, and the through-flowing Tora River are not well understood and they are, hence, subject to study. The application of hydrochemical parameters and ratios in combination with signatures of δD and δ¹⁸O revealed that recharge predominantly occurs in the outcropping Cretaceous rocks through subsurface passages rather than through influent conditions of the Tora River or through direct rainfall. Interestingly, high Na/Cl ratios indicate contact with volcanic rocks which exist within the Cretaceous anticline and also in the subsurface of the studied Quaternary aquifer. Evidence for deeper circulating groundwater is given, since replenishing waters are up to 4 °C warmer and have much lower nitrate concentrations than the groundwater in the study area. From these points, it is indicated that the Damascus fault is conductive in respect to groundwater, rather than being impermeable, as it is elsewhere.

A drilling project was carried out in Syria to assess the potential of the deep groundwater resources of the Cretaceous aquifer, composed of Cenomanian-Turonian limestones and dolomites. In this context, isotope (14C, 3H, δ13C, δ18O, δ2H) and hydrochemical analyses were performed on wells in and around the Aleppo and Steppe basins. The interpretation includes complementary results from published and unpublished literature. The results provide evidence that many new wells pump mixed groundwater from the Cretaceous aquifer and the overlying Paleogene aquifer. Radiocarbon measurements confirmed dominating Pleistocene groundwater in the Cretaceous aquifer and mainly Holocene groundwater in the Paleogene aquifer. Most groundwater in the Cretaceous aquifer seems to be recharged in the western limestone ridges, stretching from Jebel az Zawiyah (south of Idlep) via Jebel Samane (south of Afrin and A’zaz) to the region north of Aleppo, and in the Northern Palmyrides mountain belt. Some recharge also occurs around the basalt plateau of the Jebel al Hass, south east of Aleppo. It is concluded that the Taurus Mountains and the Euphrates River do not recharge the Cretaceous aquifer. The sources of recharge seem to be occasionally occurring intensive winter storms that approach from Siberia.