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

Abstract

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 ( $${{\rm{\delta}}^{15}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ and $${{\rm{\delta}}^{18}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ ) has been applied to identify $${\rm{N}}{{\rm{O}}_{{3^ - }}}$$ 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. $${\rm{N}}{{\rm{O}}_{{3^ - }}}$$ 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 $${\rm{N}}{{\rm{O}}_{{3^ - }}}$$ (5.0–10.0 mg/L). The δ2H-H2O and δ18O-H2O values indicated that the groundwater is meteoric, and of Mediterranean origin, with a strong evaporation effect. The $${{\rm{\delta}}^{15}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ values ranged between 6.0‰ and 11.3‰ with an average of 8.7‰, and the $${{\rm{\delta}}^{18}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ 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 $${{\rm{\delta}}^{15}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ and $${{\rm{\delta}}^{18}}{\rm{N - N}}{{\rm{O}}_{{3^ - }}}$$ 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.