Groundwaters in many parts of the world, particularly in dry regions such as Iran are as one of the most important sources of water supply for drinking and irrigation. Also in recent years as a result of over explotation and the increasing use of chemicals dissolved in water, the groundwater quality has been decreased. Therefore, due to contamination of these valuable resources, it is important to identify the sources of groundwater pollution. In Najafabad plains, located in Zayanderud river basins, despite the existing water supply schemes, increasing demand for water and reducing the amount of surface water, demand for groundwater has been increased in recent year. Undoubtedly for the best understanding of the behavior of the aquifer system, a series of field research are necessary. In this study the hydrodynamic properties of aquifer were studied and simulated using MODFLOW based on the available data for wells, springs, precipitation, geophysical surveys for a period of 15 years from 1374 to 1389. The results show good agreement between the calculated and observed groundwater static heads. In the next section, the quality of groundwater in Najafabad plain including EC, TDS and SAR were simulated using MODFLOW software code MT3D and compared with Ayers and Wescot criteria and Wilcox diagram. The results indicate that this type of chemical parameters in the area were in moderate mode except for the southeast region which needs appropriate management actions. The pollution in the area may be originated from chemical fertilizers and pesticide use municipal and industrial waste water discharges into rivers in low land. These pollution are mainly located in the center of region. Finally, a comparison was made between MODFLOW with DRASTIC method. Both models estimated the vulnerability of aquifer is low to moderate mode. Key words: Groundwater, Qualitative simulation, Najafabad area, MODFLOW
