Use of environmental isotopes and GIS techniques for the estimation of recharge rate in Damascus Basin, Syria

Environmental isotopes and GIS techniques have been used in order to evaluate and quantify recharge rate in Damascus basin. The Damascus basin is comprised of two main aquifers: the first one is located in the mountain karst area (NW) of Upper Cretaceous age and the second which is located in Damascus depression (SE) of Neogene-Quaternary age. The δ '1'8O and δ '2H composition of the groundwater and surface water samples are distributed around the local meteoric water line (LMWL), and indicate that all the groundwater in both aquifers is related to direct recharge from rainfall infiltration to the Cretaceous aquifer or indirect infiltration from the surface water to the Quaternary - Neogene aquifers. Using chloride masse balance (CMB), the mean recharge rate is estimated to be 60% of the mean annual rainfall in the Upper Cretaceous aquifer. The isotope pattern of the Quaternary-Neogene aquifers reflects the different sources of recharge, but the main recharge occurs by the indirect infiltration from Barada river during the flood period. The high values of '1'4C activities (50 to 70% pmc), and the high value of tritium content support this suggestion. In addition, the direct mean recharge of rainfall infiltration is estimated about only 2% through the unsaturated zone in Damascus depression. Isotopic signatures in the northeastern part of this aquifer indicate a considerable recharge contribution by subsurface flow discharged from the adjacent carbonate aquifer of Upper Cretaceous. The use of GIS technique show that the recharge potential (RP) is very high in the mountain karstic area in northwestern part of the basin. However, the RP is low in the Damascus depressing but show high RP in the center which is mainly attributed to high density drainage recharge. Theses results are in agreement with the isotope and chemical results (author).