Groundwater nitrate contamination in an area using urban wastewaters for agricultural irrigation under arid climate condition, southeast of Tehran, Iran

Nitrate contamination of groundwater has been identified as a major environmental problem of the last few decades. In this study, stable isotope ratios (delta N-15-NO3-, delta O-18-NO3-, delta O-18-H2O and delta D-H2O) of groundwater samples from the Varamin aquifer located southeast of Tehran, Iran, were analyzed in two periods, in order to determine the sources of nitrate pollution and nitrogen transformation processes where treated wastewater from the capital city is used for irrigation purpose. The approach in this study was based on isotopic fingerprinting of nitrate contaminant sources of groundwater using delta N-15-NO3- and delta O-18-NO3-. In addition, a hierarchical cluster analysis was applied to classify groundwater samples into three different hydrochemical facieses (groups 1-3). Furthermore, the stable isotope ratios of molecular water showed that most of the groundwater samples were plotted along the local meteoric line and distinguishable isotopic compositions from those of river water and treated wastewater. Stable isotopic analysis together with these hydrochemical dassifications suggested that the main sources of nitrate in groundwater are chemical fertilizers and treated wastewater in the irrigated area (group 1) in addition to domestic wastes in the residential area (group 2) and chemical fertilizers and sewage water recharged by the Shour River (group 3). Denitrification is a major transformation process occurring at the western and southwestern parts of the aquifer. Seasonal variations in the nitrogen and oxygen isotopic compositions of nitrate were more obvious in the wet season compared to the dry season. This study is unique in that it describes how groundwater nitrate contamination is enhanced in the area using treated wastewater for irrigation purposes. The results of the current study are applicable for other areas with similar settings, since treated wastewaters are important water resources for water users in arid to semi-arid regions all over the world.