Isotopic and hydrogeochemical evaluation of springs discharging from high-elevation karst aquifers in Lar National Park, northern Iran

Stable isotopes oxygen-18 (δ¹⁸O) and deuterium (δD) and the hydrochemistry of the main springs of well-developed karst aquifers in Lar watershed in the Haraz basin, northern Iran, were investigated. Water samples were collected in the recession period for analysis of major ions and the stable isotopes. Predominant hydrochemical types of main karst springs samples were Ca²⁺ – Mg²⁺ – \( {\mathrm{HCO}}_3^{-} \) type. The hydrochemical composition of the karst springs is dominated by Ca²⁺, Mg²⁺, \( {\mathrm{HCO}}_3^{-} \)and \( {\mathrm{SO}}_4^{2-} \)ions. The hydrochemical results indicate that dissolution of carbonate rocks (limestone and dolomite) governs the major ion concentrations, with minor effects of silicate weathering, ion exchange and precipitation effects on concentrations of Na⁺, Cl⁻, K⁺, and \( {\mathrm{SO}}_4^{2-} \). In the northern parts of the study area, hydrothermal fluid affects the chemistry of the draining springs. The isotopic content ranges from −43.8 to −52.9‰ and from −7.09 to −8.97‰ for δD and δ¹⁸O, respectively. The karst spring samples lie above the local meteoric water line, similar to the δ¹⁸O and δD signature of the snowpack samples, and have small spatial and temporal variability. The similarity of the isotopic composition of the springs to the snowpack suggests that the dominant recharge is by snowmelt water.