Integrating stable isotopes and factor analysis to delineate the groundwater provenance and pollution sources in the northwestern part of the Amman-Al Zarqa Basin, Jordan

Globally, groundwater contamination by nitrate is one of the most widespread environmental problems, particularly in arid and semiarid areas, which are characterized by low amounts of rainfall and groundwater recharge. The stable isotope composition of groundwater (δ²H-H₂O and δ¹⁸O-H₂O) and dissolved nitrate (\({\delta ^{15}}{\rm{N}} - {\rm{N}}{{\rm{O}}_{{3^ - }}}\) and \({\delta ^{18}}{\rm{O}} - {\rm{N}}{{\rm{O}}_{{3^ - }}}\)) and factor analysis (FA) were applied to explore groundwater provenance, pollution, and chemistry evolution in the northwestern part of the Amman-Al Zarqa Basin, Jordan. In this study, we collected 23 samples from the Lower Ajloun aquifer in 2021, including 1 sample from a groundwater well and 22 samples from springs. These samples were tested for electrical conductivity, total dissolved solids, pH, temperature, dissolved oxygen, the concentration of major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, \({\rm{HC}}{{\rm{O}}_{{3^ - }}}\), Cl⁻, \({\rm{S}}{{\rm{O}}_4}^{2 - }\), and \({\rm{N}}{{\rm{O}}_{{3^ - }}}\)), and the stable isotope composition of groundwater and dissolved nitrate. The results revealed that groundwater in the study area is mainly Ca–Mg–HCO₃ type and can be classified as fresh water, hard water, and very hard water. The range and average concentration of \({\rm{N}}{{\rm{O}}_{{3^ - }}}\) were 3.5–230.8 and 50.9 mg/L, respectively. Approximately 33% of the sampling points showed \({\rm{N}}{{\rm{O}}_{{3^ - }}}\) levels above the maximum allowable concentration of 50.0 mg/L set by the World Health Organization (WHO) guidelines for drinking water quality. The values of δ¹⁸O-H₂O and δ²H-H₂O showed that groundwater in the study area is part of the current water cycle, originating in the Mediterranean Sea, with significant evaporation, orographic, and amount effects. The values of the stable isotope composition of \({\rm{N}}{{\rm{O}}_{{3^ - }}}\) corresponded to \({\delta ^{15}}{\rm{N}} - {\rm{N}}{{\rm{O}}_{{3^ - }}}\) and \({\delta ^{18}}{\rm{O}} - {\rm{N}}{{\rm{O}}_{{3^ - }}}\) values produced by the nitrification process of manure or septic waste and soil \({\rm{N}}{{\rm{H}}_4}^ + \). The FA performed on the hydrochemical parameters and isotope data resulted in three main factors, with Factor 1, Factor 2, and Factor 3, accounting for 50%, 21%, and 11% of the total variance, respectively. Factor 1 was considered human-induced factor, named “pollution factor”, whereas Factor 2, named “conservative fingerprint factor”, and Factor 3, named “hardness factor”, were considered natural factors. This study will help local researchers manage groundwater sustainably in the study area and other similar arid and semiarid areas in the world.