Karst isotope and origin of springs in transboundary aquifer of North Khorasan

Karstic water resources, especially in dry areas, are considered to be the most important source of drinking water in the world. The first step to better management of these aquifers is to determine the water origin and catchment area and the hydrogeochemical processes that govern them. The studied karstic aquifer located in North Khorasan province, which has a wide outcrop of Tirgan limestone. This large and borderline aquifer, despite its high recharging, is discharge by a small number of karstic springs (Arnave, Rezghane, Asiazoo, Ghordanlou, Ayoub and Sarani) with a discharge between 15 and 500 L/s. Springs and precipitation water samples were collected for a one-year period and the main ions and stable isotopes (δ18O and δD) were analyzed to determine the meteoric water line, water origin and and geochemical processes governing the karstic resources of the area. The EC value varies between 250 and 800 in cold water springs and about 1020 µS/cm in the Ayoub hot Spring. According to the Hydrochemical results and ionic ratios, the dominant facies in this karstic aquifer are calcium-magnesic bicarbonate facies due to the dissolution of carbonate rocks and sulfate-calcium resulting from the dissolution of gypsum and pyrite. The physicochemical parameters display that the discharged value has reverse relation with both EC and temperature valuessome springs due to the larger catchment area and the further development of Karst. For the first time, the isotopic meteoric water line of the area is calculated based on the data taken as δ2H = 7 δ18O + 6.32, so that the slope and dexcess are less than GMWL due to the effect of secondary evaporation from rain during precipitation. All springs are located on the LMWL and close to the Mediterranean line, which suggests that the rainfall of the area is mainly originates from the Mediterranean air masses. The enrichtment of the isotope composition of the Arnave spring is due to less evaporation during water flowing through the marl limestone and the high thickness of epikarst in its catchment basin. The isotope results confirm hydrological and hydrochemical results also. The chatchment area of these springs is in its adjacent anticlines. Using data from various rainfall stations around the springs, the value of δ18O varies between 0.32 to 1.6 ‰ per 100 meters, and the elevation of the catchment area of the springs varies from 2200 to 2700 meters that confirms the chatchment areas.