Water quality for drinking and irrigation usage was examined in Kardeh dam basin in NE Iran. Thirty-two surface and groundwater samples were collected and analyzed for major ions of Ca2?, Mg2?, Na?, K?, HCO3 -, CO3 2-, SO4 2- and Cl- by using standard analytical methods of titration and atomic absorption spectrophotometry at geochemistry laboratory of Ferdowsi University of Mashhad. Dominant cation in most of the water samples are Ca2? and Mg2?, and dominant anion is HCO3 -. Water quality index (WQI) was calculated based on physicochemical parameters such as pH, EC and major ions. The WQI values were less than 100 (maximum permissible value) for all samples and suitable for drinking usage; nevertheless, water quality decreased from northwest toward the southeast of studied area. Also, based on modified NSFWQI, the water resources were classified into average and good categories, which are suitable for irrigation uses. More than 40% of the samples are not suitable for irrigation uses based on magnesium hazard values. Carbonate rocks have the main effect on hydrogeochemical facies and the water quality in studied area. According to drinking and irrigation indices, water quality is reducing from upstream toward downstream to the southeast of the basin.

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