Determining nitrate pollution sources in the Kabul Plain aquifer (Afghanistan) using stable isotopes and Bayesian stable isotope mixing model

The Kabul urban aquifer (Afghanistan), which is the main source of drinking water for Kabul city's inhabitants, is highly vulnerable to anthropogenic pollution. In this study, the geochemistry of major ions (including reactive nitrogen species such as NO3-, NO2-, and NH4+) and stable isotope ratios (δ15N-NO3-, δ18O-NO3-, δ18O-H2O, and δ2H-H2O) of surface and groundwater samples from the Kabul Plain were analyzed over two sampling periods (dry and wet seasons). A Bayesian stable isotope mixing model (BSIMM) was also employed to trace potential nitrate sources, transformation processes, and proportional contributions of nitrate sources in the Kabul aquifer. The plotting of δ15N-NO3- against δ18O-NO3̄ (δ15N-NO3- and δ18O-NO3- values ranged from +4.8 to +25.4‰ and from -11.7 to +18.6‰, respectively) suggests that NO3- primarily originated from the nitrification of sewage rather than artificial fertilizer. The plotting of δ15N-NO3- versus NO3-/Cl- ratios also supported the assumption that sewage is the dominant nitrate source. The results indicate that denitrification did not influence the NO3- isotopic composition in the Kabul aquifer. The BSIMM model suggests that nitrate in the dry season originated mainly from sewage (~81%), followed by soil organic N (10.5%), and chemical fertilizer (8.5%). In the wet season, sewage (~87.5%), soil organic N (6.7%), and chemical fertilizer (5.8%) were the main sources of NO3- in the Kabul aquifer. Effective land management measures should be taken to improve the sewage collection system in the Kabul Plain.