Investigation of groundwater residence time (alluvial and karst) in Pol-e-Zahab study area due to the spatiotemporal varations of isotopic composition (δ2H , δ18O) of rainfall and groundwater

In this research, we obtained mean values of -2.9‰ to -6.6 ‰ for δ18O and -14.4 ‰ to -31.6‰ for δ2H for precipitations of Pol-e-Zahab area. The Pol-e-Zahab meteoric water line (δ2H = 6.6*δ18O + 12.1) represents high deuterium excess (12.1‰) and elevated values of d-excess of rainwaters from SarpoleZahab, Qasr-e Shirin and Kerend weather stations (between 8.7‰ and 20.9‰) confirms the impact of Mediterranean atmospheric water vapor. In Pole Zahab study area, the isotopic composition of the ground water resources has less variation than that of rainwaters. The mean values of the isotopic composition for the groundwater (alluvial and karst) ranges between -3.5 ‰ and -8.6 ‰ for δ18O and between -2.29 ‰ and -6.36 ‰ for δ2H. In general, karst water isotopic composition (δ18O and δ2H) in the study area has less seasonal variations, which indicates the existance of sufficient time for mixing of infiltrated water with water in saturated zone and in turn indicate high groundwater residence time in formation. The least isotopic seasonal variations were observed in largest karstic springs of the region, including Srabgrm, Ghaleshahin, Gharebolagh and Ryjab springs. Consequently, the water of these springs have the greatest residence time in formation and in turn high relative age of water. Around Marab karstic spring, development of karst channels and faster flowing of groundwater resulted in less residence time and low relative age of water.