ایزوتوپی کارست و منشأ چشمه های آبخوان مرزی خراسان شمالی

Due to the declining of the water resources in the alluvial aquifers, karst resources are used as the main sources of domestic water in the entire world, spatially in the arid and semiarid regions. Existence of large carbonate formations in many parts of Iran and the withdrawal from these sources for drinking and agriculture purposes by springs and wells are the indication of the significant importance of these resources. Proper management of these aquifers requires an understanding of the current situation and its variations in different conditions. Hydrogeological (quantitative) and hydrogeochemical-isotope (qualitative) researches of groundwater are essential as practical and scientific solutions for management actions. Precipitation is an important source of groundwater supply in karst and alluvial aquifers. Therefore, temporal and spatial evaluation of the isotopic behavior of precipitation is important. Because isotopic characteristics of water slightly change along paths and keep its origin characteristics, it can be useful in identification probable flow paths and different sources. In the northeast of Iran, The karstic aquifers are important sources for drinking and agricultural purposes. The first step to better management of these aquifers is to determine the water origin and catchment area, as well as the hydrogeochemical processes that govern them. The studied aquifer located in Northern Khorasan province, which has a wide outcrop of Tirgan limestone. This large and borderline aquifer, despite its high recharging, discharge by 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 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. The isotopic samples of the study area were measured at the IAEA Laboratory in Vienna and Utrecht University, and expressed per mile (‰). In this research, the piper diagram, ions ratios, hydrochemical diagrams, physicochemical parameters and, Isotope characteristics were used for determining springs water origin in the study area.
In the Arnaveh, Rezghaneh and Ayoub, increasing in the sodium and potassium concentrations can be related to existence of epikarst and shale-marl layers in the catchment area of these springs or deep regional flows and passing through shale-marl layers (Shuriyeh Formation) due to the high difference of the elevation between recharge area and springs. Increasing the concentration of sulfate in the Ayuob spring can be due to the dissolution of gypsum and anhydrite minerals or pyrite oxidation and the influence of deep regional flows also. According to the 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 discharged value has reverse relation with both EC and temperature values some springs due to the larger catchment area and the further development of Karst. In Arnaveh, Sarani, Estarkhi, Ghordanlu, electrical conductivity decrease slightly with increasing of the discharge in rainy months. In Estarkhi spring, Electrical conductivity is tangible from other springs because of more karst development and wide catchment area. 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 d-excess 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 enrichment 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, due to the influence of winter rainfall at elevations on Ayoub, Rezghaneh, have lower and lighter isotope values than other springs. 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.