Mashhad, the second biggest city in Iran with populations of more than 2.5 million, depends on both surface water and groundwater resources. There are two large dams around Mashhad, so called Torogh and Kardeh, which are supplying irrigation and potable water for the city. The recharge area of the Kardeh dam is higher than that of the Torogh dam which resulted in slightly negative isotopic values of18O and 2H (-8.3 ‰, -55.4 ‰ and -7.6 ‰, -52.2 ‰ for Kardeh and Torogh dams, respectively). In both dams, the 18O and 2H isotopic compositions within the lake water are more enriched relative to the recharge spring water which can be attributed to evaporation. For example in Torogh dam, the values of 18O and 2H are -6.8 ‰ and -43.7 ‰, respectively, in lake water, whereas in upstream river they are -7.5 ‰ and -48.2 ‰, respectively. The oxygen and hydrogen isotopic compositions of the latter are similar to those in recharge spring water (18O = -7.6 ‰, 2H = -52.2 ‰). The dissolved inorganic content (DIC) concentration in recharge area of the Kardeh (38.6 to 42.7 mg/l) is slightly higher than that of Torogh (35.5 to 42.4 mg/l). This is due to the existence of karstic outcrop in catchment area of the Kardeh dam which can be confirmed by slightly enriched 13C isotope of DIC (-9.5 ‰ and -10.8 ‰ for Kardeh and Torogh, respectively). Although DIC concentration within the Kardeh lake is high (30.3 and 42.7 mg/l for Torogh and Kardeh, respectively) and 13CDIC values are more enriched, there is not much difference in 13CDIC values for Torogh and Kardeh (-8.0 ‰ and -8.5 ‰, respectively). This could be due to enriched CO2 generation through DOC degradation within the lake water (-29.5 ‰ and -28.0 ‰ for DOC of Torogh and Kardeh, respectively).

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