Investigation of Transportation of Pollutants in Alluvial Bed of One of Zanjan River Branches Using MODPATH

Groundwater resources,as one of the most important resources of water supply, have been faced with the qualitative decline and increasing the concentration of pollutants in addition to a quantitive problems in recent years. According to the importance of the role of water supply quality in exploitation, recognizing the aquifer and factors affecting it is very important. This study was conducted to investigate the movement of pollutants with the view of assessing aquifer vulnerability in one of the branches of Zanjan River. Accordingly, two types of intrinsic and special vulnerability in the region were investigated by two methods of drastic and HPI. The results of vulnerability index show that the soil of region is effective in decreasing the concentration of heavy metals under study (lead, chromium, nickel, cobalt, zinc) and the concentration reduces in the direction of groundwater flow. The results of vulnerability index showed that the Drastic index is more than 80% in the medium to low and low ranking and the HPI index of the basin with an average of 30.3 is in the low risk, however, this basin has the potential to develop pollution. In order to determine the direction of pollution and the role of alluvial in the area MODPATH numerical model was used to simulate under the influence of the advection process in order to determine the direction of pollutants movement and the role of alluvium of the region. The numerical simulation of the region was carried out for three years after modeling and calibrating the MODFLOW model. Based on the results, the wells located along the groundwater and surface water flow path in the center of the basin have longer length compared to the wells that are around and far from the center of the basin. Based on the results, different patterns can be used such as creating barriers to modify and reduce the amount of pollutants transfer in this basin and other basins leading to Zanjan River.