At present nitrate contamination of groundwater is a significant problem. In the past decades, nitrogen fertilizers have been widely used without considering negative impacts on the soil and ecology. The nitrate easily reaches the surface and subsurface water. The objective of this study was to investigate nitrate contaminant distribution using Geographical Information System (GIS). Accordingly, 10 observation wells were selected out of 500 operational wells and water sample collected during (12 months) of July 2007 to June 2008. In total, 120 water samples were analyzed for nitrate concentration. The results show that nitrate concentration in the central parts of the plain is lower than northern and southern parts and did not change during the year. The most contaminated region of the aquifer is located in the southern part where it is contaminated by the urban wastewater wells in the vicinity of the aquifer. In the summer, the nitrate concentration is increased due to high agricultural activities and consumption of nitrogen fertilizers. The results indicate that the nitrate leachs during irrigation and so it affects the water quality.