The origin of Bazangan Lake, the largest lake is located 120 km northeast of Mashhad city-Iran, and the groundwater/surface water interaction in this lake is investigated using geochemistry and stable isotopes. Bazangan Lake is surrounded by red siliciclastic sediments of Pestehleigh formation (Paleogene) and the carbonate cliffs of Kalat formation (Maastrichtian). Lake water mainly supplied from the Mozdouran and the Kalat karstic formations. The groundwater of these formations discharges to the lake water through several springs and underground seepage face. The fairly enriched 13C value of dissolved inorganic carbon in lake water (average of -4.5 ‰) confirms the contribution of calcite dissolution from these karstic formations. The high electrical conductivity (15500 mohs/cm) of the lake water is attributed to the dissolution of underlying gypsum layers and to the high evaporation rate at this area (average of 2700 mm/year). This can be confirmed by high concentration of SO4 (4600 mg/l) and the enriched isotopic values of 18O and 2H (average of 1.7 ‰ and -6.2 ‰, respectively) for the lake water. DOC of lake water (8.5 mg/l, 27.7 ‰) is higher than that of recharge spring water (0.6 mg/l, 26.2 ‰), which is due to the presence of phytoplankton groups within the lake water.

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