Source identification of nitrate in the upper aquifer system of the Wadi Shueib catchment area in Jordan based on stable isotope composition

Groundwater forms the main freshwater supply in arid and semi-arid areas, and contamination of this precious resource is complicated by the slow rate of recharge in these areas. Nitrate contamination of groundwater is a global water quality problem, as it entails threat to human health as well as aquatic ecosystems. Source identification of contamination is the cornerstone and a prerequisite for any effective management program of water quality. Stable isotope composition of the dissolved nitrate (δ¹⁵N-NO₃^- and δ ¹⁸O-NO₃^-) has been applied to identify NO₃^- sources and the main transformation processes in the upper aquifer system (A1/2, A4, and B2/A7 aquifers) in the Wadi Shueib catchment area, Jordan. Moreover, the stable isotope compositions of the groundwater (δ²H-H₂O and δ1⁸O-H₂O) in conjunction with the groundwater hydrochemistry were integrated to investigate the origin and evolution of the groundwater. Results revealed that groundwater in the study area is fresh and hard-very hard water, and mainly a Ca-Mg-Cl type. NO₃^- concentration was in the range of 7.0-74.0 mg/L with an average of 37.0 mg/L. Most of the samples showed concentration higher than the natural background concentration of NO₃^- (5.0-10.0 mg/L). The δ ²H-H₂O and δ¹⁸O-H₂O values indicated that the groundwater is meteoric, and of Mediterranean origin, with a strong evaporation effect. The δ¹⁵N-NO₃^- values ranged between 6.0‰ and 11.3‰ with an average of 8.7‰, and the δ¹⁸O-NO₃^- values ranged between 1.6‰ and 5.9‰ with an average of 3.4‰. These values are in conformity with the stable isotope composition of nitrate derived the nitrification of wastewater/manure, and soil NH₄. Nitrification and denitrification are the main transformation processes affecting nitrogen species. Statistical analysis revealed no significant differences in the δ²H-H₂O and δ¹⁸O-H₂O values, and δ¹⁵N-NO₃^- and δ ¹⁸O-NO₃^- values for the three aquifers (A1/2, A4, and B2/A7), indicating that the groundwater of these aquifers has the same origin, and a common source of pollution.