Andarokh-Kardeh region is located northeast of Iran and provides notable part of potable water of Mashhad, the second most populated city of Iran with over 3 million inhabitants. Stable isotopes (18O, 2H and 13C), hydrodynamic, and hydrochemical indicators have been applied to evaluate water resources quality comprehensively and to present a hydrogeological conceptual model for the Andarokh-Kardeh region. Hydrochemical studies indicate that chemical rock weathering and ion exchange are the main factors controlling the quality of water in the region, while atmospheric (precipitation) as well as anthropogenic sources have no notable effect. Stable isotopes (18O and 2H) studies in surface and groundwater samples demonstrate enriched values in Kardeh Dam Reservoir (KDR) due to evaporation and also demonstrate that the interaction between KDR and downstream groundwater resources is negligible. The δ13C isotope and DIC and DOC concentrations indicate role of carbonate mineral dissolution from Mozdooran Formation, atmospheric and soil CO2 and also the domination of C3 vegetation coverage in the study region. The developed conceptual model for Andarokh-Kardeh demonstrates different hydrogeological conditions during wet and dry periods, and highlights the main process which affect hydrogeology of the region. Water quality studies and the developed conceptual model demonstrate the main factors controlling water quality and the main hydrogeological characteristic of Andarokh-Kardeh region which can be used by water resources scientists for the future water management programmes.

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