Contents lists available at ScienceDirect Agricultural Water Management journal homepage: www.elsevier.com/locate/agwat Groundwater nitrate contamination in an area using urban wastewaters for agricultural irrigation under arid climate condition, southeast of Tehran, Iran Zohre Nejatijahromi a,b , Hamid Reza Nassery a, ⁎ , Takahiro Hosono b,c , Mohammad Nakhaei d , Farshad Alijani a , Azusa Okumura b,c a Department of Minerals and Hydrogeology, Faculty of Earth Sciences, Shahid Beheshti University, Evin Ave, Tehran, Iran b Priority Organization for Innovation and Excellence, Kumamoto University, 2-39-1 Kurokami, Kumamoto, Japan c Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto, Japan d Department of Applied Geology, Faculty of Earth Sciences, Kharazmi University, Hesarak, Karaj, Iran ARTICLE INFO Keywords: Groundwater Nitrate contamination Stable isotope ratios Denitrification Cluster analysis ABSTRACT Nitrate contamination of groundwater has been identified as a major environmental problem of the last few decades. In this study, stable isotope ratios (δ 15 N-NO 3 - , δ 18 O-NO 3 - , δ 18 O-H 2 O, and δD-H 2 O) 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 δ 15 N-NO 3 - and δ 18 O-NO 3 - . 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 classifications 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 com- positions 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 irri- gation 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. 1. Introduction Nitrate pollution in groundwater resources is a persistent problem that is typically caused by non-point-source nitrogen loads from a variety of land uses. Identifying the sources of dissolved nitrate and its behavior along water flows is, therefore, the first step in reducing the nitrate pollution of groundwater. Groundwater nitrate can be initiated by a natural or anthropogenic origin. The concentration of naturally occurring NO 3 - –N does not usually exceed concentrations of 3 mg/L in aquatic environments (Burkart and Kolpin, 1993), except in areas with high geological impact sources (Lowe and Wallace, 2001; Min et al., 2003; Stadler et al., 2008). Higher concentrations of NO 3 - –N in groundwater are typically caused by anthropogenic activities including the use of nitrogen compounds in synthetic fertilizers and/or manure in agriculture, leakage from septic systems, and other wastewaters (McLay et al., 2001; Widory et al., 2005; Kendall et al., 2007; Heaton et al., 2012; Wang et al., 2013; Ma et al., 2015). In drinking water, a high concentration of NO 3 - –N (> 10 mg/L) may result in blue baby syn- drome in infants (methemoglobinemia) and may be a contributing factor for some types of cancers (WHO, 2004; Galloway et al., 2008; Ward et al., 2010; Pastén-Zapata et al., 2014; Xue et al., 2016; Nikolenko et al., 2018). In fact, groundwater is often a major source of water for drinking purposes, and thus, the cause and fate of nitrate contamination should be examined properly in addition to regular https://doi.org/10.1016/j.agwat.2019.04.015 Received 1 November 2018; Received in revised form 5 April 2019; Accepted 16 April 2019 ⁎ Corresponding author. E-mail address: h-nassery@sbu.ac.ir (H.R. Nassery). Agricultural Water Management 221 (2019) 397–414 0378-3774/ © 2019 Elsevier B.V. All rights reserved. T