Regarding urban and industrial development, greater Tehran, with a population of more than 6.8 million, suffers continuously from lock of water. In addition, because of the geographical characteristics of Tehran, which is located in an arid and semi-arid region, the critical role of groundwater resources in supplying drinking water has doubled. Groundwater has a major and outstanding role in responding to the drinking water demand of Tehrans inhabitants a 45% share of which has been contaminated by human activities over a period of several years.
In this paper, nitrate has been considered as the best criterion for showing ground water contamination, especially due to anthropogenic activities. In order to study nitrate contamination in groundwater in Tehran, there are several problems such as lack of data, and the large area of the case study, as well as the complexity of nitrate activity. Hence, some methods, such as complicated numerical models, which need detailed data, were useless. All of the methods were developed in the GIS base using ArcGIS software. This study focuses on contaminant sources, unsaturated zone nitrogen activity and the amount of nitrate discharged into groundwater. At first, contaminant sources were studied and major sources were determined. As a result, it was found that wastewater (95%), parks and agriculture activities (4%) and contamination due to air pollution (1%) were the major sources of nitrogen discharge in ground water.
Furthermore, a simulation of the nitrogen compounds, activity was done, in order to assess the discharged amounts of nitrogen and nitrate activity in the unsaturated zone, as well as the in - depth removal of nitrogen compounds. Modeling of nitrate activity in the unsaturated zone was done by a lump parameter method in a one-dimensional model as well as by a mass balance method. Utilizing these two methods lead to an understanding of nitrate and nitrogen compound activities in the unsaturated zone. According to the results, nitrate will be produced in depths of between 2 and 3 meters. Although the ammonia amount decreases very fast, in the first meters the rate of ammonia removal decreases gradually. In depths of between 20 and 30 meters, the rate of removal is almost fixed and very low. Based on the results of the mass balance method, 55% of nitrogen compounds are averagely removed in the unsaturated zone. These methods had good correlations in the results, which show their versatility in the study of cases such as Tehran, in which there are not enough and reliable data.