Isotopic evidence to characterize the sources of sulfate ions in the carbonate aquifer system in Aleppo basin (North Syria)

Sulfur and oxygen isotopic compositions of sulfate (delta S-34(SO4), delta O-18(SO4)) and delta O-18(H2O), delta C-13(DIC) have been used in conjunction with chemical data to constrain sulfate sources and geochemical processes in carbonate aquifers of the Aleppo basin in north Syria. The Aleppo basin comprises two limestone aquifers: the first one is shallow unconfined of Paleogene age and the second is deep confined of Upper Cretaceous age. The chemical data indicate that dissolution of carbonate and evaporites is the main process controlling groundwater quality. In the mainly unconfined modern groundwater, the wide range of delta S-34(SO4) values (10.39-17.6 parts per thousand) indicates a variety of input sources. The likely sources of sulfur include meteoric precipitation and sewage waters which recharge the aquifer through Qweik River crossing the area. These waters have low SO42- concentration (<50 mg/L), and are associated with heavier value of delta O-18(H2O) and high NO3 concentration (>20 mg/L). Sulfate increases in deep confined groundwater in some of which gypsum saturation is reached. The delta S-34(SO4) and delta O-18(SO4) are relatively constant with mean values of 18.5 and 13.5 parts per thousand, respectively. This indicates that gypsum and anhydrite dissolution are the primary sources of sulfate. This water is associated with higher depleted value of delta O-18(H2O). The heavier delta C-13(DIC) with mean value -1.8 parts per thousand and high Mg2+ concentration (similar to 100 mg/L) in the high sulfate deep groundwater, indicates that the de-dolomitization reactions dominate along flow paths where sulfate concentrations increase.