Tracking nitrate sources and transport pathways in riparian wetlands using a multi-tracer approach combined with a Bayesian mixing model

Abstract

Identifying nitrate (NO₃⁻) sources and key transport pathways is essential to manage diffuse NO₃⁻ pollution, particularly in riparian wetlands where nitrate pollution is a major contribution to eutrophication. In this study, we used multiple tracers to assess the contribution of multiple NO₃⁻ sources and transport pathways in the riparian wetlands along the lower Yellow River during four hydrological seasons. The results revealed that NO₃⁻ concentrations in approximately 12.2 % riparian wetland water along the lower Yellow River were higher than the Chinese government and WHO allowed threshold (10 mg/L), particularly in wetlands outside the dykes during high flow seasons. The signature of hydrochemical parameters and stable isotopes (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻ and δ¹⁸O-H₂O) values illustrated multiple sources recharge NO₃⁻ levels in riparian wetland water, rather than biogeochemical processes. A Bayesian mixing model based on dual nitrate isotope values further revealed that chemical fertilizer (35 %), soil organic nitrogen (33 %), and manure/sewage (26 %) served as the main NO₃^– source recharging riparian wetlands water. Of the main nitrate transport pathways, groundwater (34 %), the Yellow River (33 %), and canal water (28 %) contributed more to riparian wetlands water pollution that did atmospheric deposition (precipitation). However, both source and the relative importance of certain transport pathways of NO₃⁻ varied both spatially and temporally. These results are critical for better informing the management and restoration of riparian wetlands.