Sahand Dam with 135 Mm3 storage located in southwest of Hashtroud city is one of the important dams of East Azarbijan. Water storage of this dam supply drinking, industrial water demand of Hashtroud city. Furthermore, it used for irrigation of 11 hectares of the sector land. According to importance of this water resource, quality of dam reservoir water and related surface water and ground water resources were monitored in recent eight years (1388-1381). The results of analyzed 308 water samples from study area, which include physical parameter (EC, pH, COD, BOD)major(Ca2+, Mg2+, Na+, K+, , , , ), minor (PO4-3, NO3-, NH4-)and trace (Fe, Mn, Pb, Zn, Cd, Cu, As)ions show arsenic anomalies in the sector. However, arsenic concentration of some of the water resources which related to Dam storage is tenfold of WHO standard(10ppb). The aim of this research is the investigation of hydro chemical anomalies in stored water of Sahand Dam and related surface and ground water resources and determination of their origin. Therefore, graphical methods and factor analysis were used as well as field evidences and geological studies and the results indicate 3 water types and 5 origin for water resources and 5 main effective factors on hydrochemistry of the area. Epithermal concentration of arsenic in the Miocene formations can be the main origin of arsenic anomalies in the water resources that based on obtained hydrochemical results, separation and releasing from its origins in the water resources generally occur via complex formation with bicarbonate ion.