Considering Factors Affecting High Arsenic Concentration in Groundwater Resources of Tabriz Plain Aquifers

The Tabriz plain with an area of more than 700 km2 is extended from the eastern limit of Tabriz city to the salt flats of Urmia Lake. There are two types of aquifers in this plain with different quality of groundwater. The unconfined aquifer, which is extended all over the plain, in recharge areas near the plain margins has good quality water but in the central part of the plain along the Aji Chay river as well as in the west margin of the Plain is saline. The multi-layer confined aquifers, which lie down in central and western parts of the Plain bearing more or less good quality groundwater. The arsenic concentration of these aquifers is also different; the water samples analyzed from the chemical point of view especially with respect to concentration of arsenic show two different groups of groundwater. The aim of this study is considering ofarsenic spatial distribution and factors controlling high arsenic concentration in theaquifers. For this purpose 16 water samples, with evenly distributed in the Plain, were collected from the aquifers, two water samples for each well, one for analyzing major ions and the other for trace elements. They were analyzed in hydrology lab at the Tabriz University. The arsenic concentration in unconfined aquifer and in recharge areas of the plain boundary is low and in confined and deep wells is high. Arsenic concentration compared on the basis of their dependency on hydrogeological conditions, nitrate and phosphate concentrations and pH and the results interpreted by factor analysis and hydrogeochemicalmethods. Attendance of nitrate and phosphate by positive factor and arsenic by negative factor can show the reduction conditions in groundwater system, which caused the arsenic mobilization. In spite of high arsenic concentrations in the water samples, the saturation index of arsenic minerals is very low and under saturation. The arsenic existing in groundwater resources of the area originate from the geological formations and its concentrations depend highly on the hydrogeological and environmental reduction conditions, residence time of water in underground layers and depth of the sampling wells.