Geochemical and multi-isotopes (δ18O, δ2H, δ13C, 3H and δ37Cl) evidences to karst development and flow directions in transboundary aquifer, Northeast of Iran

The first systematic research of the geo-hydrogeological and hydrogeochemical characteristics and isotopic signatures was carried out in the transboundary karstic aquifer in northeast of Iran, in order to investigate the groundwater origin, flow directions and karst development. The karstic springs of the area are characterized by discharge rate of 50–500 L/s, EC values of 370–1050 μS/cm and the Ca–Mg–HCO3 water type. Stable isotope values of the precipitations resulted in a LMWL (δ2H = 7.2 × δ18O+8.6, R2 = 0.96, n = 96) with a lower slope and a lower intercept than the global meteoric water line (GMWL) due to isotope kinetic fractionation effects during precipitation. The δ18O precipitation gradient is −0.32‰ per 100 m of altitude. Isotopes data reveals that recent Mediterranean meteoric water (rain and/or snow) is the main origin of groundwater in the study area. The depleted isotope composition of some of the springs can be attributed to their higher catchment area and more recharge by snow. Groundwater budget analysis reveals that a large amount of water migrates toward the neighboring country. Fold and fault zones can be important variables on groundwater local and regional flows in the karstic aquifer. In terms of karst development, the Sarani spring in transboundary karstic aquifer with lower δ18O, δ37Cl and EC values and higher δ13CDIC has conduit flow regime and more karst development in its catchment area. In comparison, the isotopic values as well as physic-chemical characteristics of the springs confirm larger residence times (>30 years) and lower karst development in the other karstic springs. Consequently, the geo-hydrogeological and tectonic settings as well as isotopic approaches enhances knowledge in both groundwater flow direction and karst development and, ultimately, to better evaluate and manage water resources in the study area, and also in other transboundary karstic regions