Isotope-hydrochemistry of Arnave karstic spring and Shirin dareh dam reservoir, North Khorasan

Surface and groundwater resources such as Arnaveh karstic spring and Shirin Dareh reservoir are the main sources of drinking water and agricultural activity in the North Khorasan province, northeast of Iran. The main agents of this study are to evaluate the origin, hydrochemical and isotope characteristics of water resources and also suitability of the Shirin dareh reservoir water for drinking and agriculture usages. Water samples were collected in October 2017 from Arnaveh Spring, Shirin dareh River and dam reservoir to measure the EC, TDS, major ions concentrations and stable isotopes (δ18O, δ2H) values. Furthermore, the two rain stations were selected for isotope sampling from rain waters and determining the Shirin dareh isotope Meteoric Water Line. According to hydrochemical characteristics, Ca-Mg-SO4 and Ca-Mg-HCO3 are the dominance waters type in the area. The average electrical conductivity is about 900 µS/cm. The δ2H = 7.14 δ18O + 5.67 (R= 0.96) equation is proposed as LMWL of the study area based on the precipitation samples. Its slope and intercept are lower than the GMWL due to secondary evaporation from rain drops in low humidity condition. The isotopic results suggested that the Arnaveh spring and surface waters (river) are originated from modern atmosphere precipitation. Due to evaporation, isotope composition is more enriched in dam reservoir than in river samples and has a slight deviation from the local meteoric water line. The quality indexes confirm that these water resources is suitable for Irrigation and drinking usages.   1-Introduction: Shirin Dareh Basin is located in northeast of Iran. Shirin Dareh dam is the main source of drinking water and agricultural activity in the Northern Khorasan region. The geochemical investigation provides more information about ground and surface waters mineralization and the processes controlling the deterioration of water quality. Water quality in dams depends on the geological characteristics of the catchment area, the quantity and quality of water entering the reservoirs of dams, the amount of precipitation and the amount of agricultural and industrial activities. Sustainable management of the water resources needs a comprehensive understanding of the sources of water and spatial variability of hydrochemical and isotope characteristics. Water isotopes are commonly used for determining the sources of surface and groundwaters. The aim of this study was to investigate the spatial variations of water quality in the river and Lake of Shirin Dareh dam, as well as the origin and chemical evolution of water resources with different methods of hydrogeochemical and isotopic methods. This type of study was carried out for the first time in this area, and in addition, the local precipitation line of the isotopic rainfall in the Shirin Dareh area was also measured and plotted. 2-Materials and methods: Samples were collected for one period (October 2017) from Arnaveh spring, Shirin Dare River and dam reservoir. Electrical conductivity, total dissolved solids, major ions (Ca, Mg, Na, K, HCO3, SO4, Cl, NO3) and stable isotope (18O,2H) were analyzed. Furthermore, isotope values of the precipitation samples in two rain gauge stations in the study area were also measured to determine the isotope meteoric water line. Hydrochemical diagrams, such Piper, Schoeller, Chadeha, different ions ratio, Gibbs and Saturation Index (SI) were used to evaluate the hydrogeochemical characteristics. Six factors such as residual sodium carbonate (RSC), magnesium adsorption ratio (MAR), sodium percentage (%Na), sodium adsorption ratio (SAR), Permeability Index (PI), and water quality index (WQI) were calculated to evaluate the water quality within the research area for irrigation and drinking activities. 3-Results and discussion: According to the Piper and Chadeha diagram, Ca-Mg-SO4 and Ca-Mg-HCO3 are dominance type of water. Gibbs plot showed that all of the samples fall in the rock dominance. The hydrochemistry results and different ions diagrams indicated that the most important factors controlling the chemistry of water included carbonate, anhydrite and gypsum dissolution and to a lesser ion exchange process. The most of water samples are saturated or oversaturated with calcite, dolomite, gypsum and anhydrite.  Dissolution of halite is negligible; therefore, SI of halite is under saturated in the study area. Arnaveh karstic spring with lower EC value has the effective role in improvement of water quality of the Shirin Dare River. The average of electrical conductivity is about 900 µS/cm. All river and dam samples are plotted between the mixing line of the two samples of upstream and Arnaveh spring; indicate that the water quality of the river after mixing with the spring has improved and the concentration of ions has decreased too.   Many factors such as evaporation and water-rock interaction can control the relative abundance of an isotope in surface water compared to the precipitation. According to the isotope composition of the precipitation samples, The LMWL equation is computed as δ2H= 7.14 δ18O + 5.67 (R= 0.96). Its slope and intercept due to secondary evaporation from rain drops in low humidity condition are lower than the GMWL. The isotopic results suggested that the Arnaveh spring and surface waters (river) are originated from modern atmosphere precipitation. Due to evaporation, isotope composition is enricher in dam reservoir than in the river samples. The isotopes stability signatures show that river samples are non-evaporated and controlled by the dissolution, whilst reservoir samples are more 18O and 2H-enriched due to the evaporation from reservoir surface. The quality indexes confirm that these water resources is suitable for Irrigation and drinking.   4- Conclusion To summarize the geochemical and isotopic techniques and water quality indexes were used to evaluate the origin and chemical evolution of surface and groundwaters in the study area. The groundwater samples from Arnaveh spring is mainly affected by natural factors and processes such as lithology, evaporation and water-rock interaction. The isotope composition (18O and deuterium) indicates the importance of evaporation in dam reservoir, and recharge with modern rainfall. The water quality in the study area seems to be good for different usages.