Geogenic Arsenic Contamination in Northwest of Iran; Role of Water Basin Hydrochemistry

Introduction: Arsenic contamination of  surface and groundwater has been categorizd among high profile  environmental  problems around the world. The matter is of most concern where unsafe water is used for drinking. Sahand region reservoir in northwest of Iran supplies water for drinking, industrial and agricultural purposes throught  irrigation  of 11000 hectares of lands. Materials and Methods: The hydrochemistry status of Sahand dam basin and arsenic plume distribution in water recourse was evaluated by analyzing 308 water samples from surface and ground water according to the standard methods. in addition, geological features information of basin were obtained based on recent site studies and field investigations. Results:Elevated concentrations of arsenic (range = 0-1440 ppb, mean = 171.68 ppb) were detected in water basin and dam. According to the delineating evidences, regional geological background and volcanic activities can be considered as the main sources of the natural genesis of arsenic in the study area. Conclusion: Notable amounts of arsenic sulfide were recorded in seams, gaps , fractures of limestone, marl, sandstone and an overlying ferruginous conglomerate. Concentrations of arsenic varied seasonally highlithing the maximum concentration observed in autumn and early winter (December). Seasonal fluctuations can be probably attributed to changes in geochemical conditions in sediments at the bottom of reservoir.