Reservoir NO3− pollution and chemical weathering: by dual isotopes of δ15N-NO3−, δ18O-NO3− and geochemical constraints

Reservoir dams alter the nutrient composition and biogeochemical cycle. Thus, dual isotopes of δ18O-NO3− and δ15N-NO−3 and geochemical signatures were employed to study the NO3− pollution and chemical weathering in the Three Gorges Reservoir (TGR), China. This study found that the TGR dam alters the δ15N-NO3− composition and is enriched in the recharge period. Values of δ15N-NO3− varied from 4.5 to 12.9‰ with an average of 9.8‰ in the recharge period, while discharge period δ15N-NO3− ranged from 3.2 to 12.5‰, with an average of 9.3‰. δ18O-NO3− varies (1.2–11.3‰) with an average of 6.5‰ and (2.4–12.4‰) with an average of 7.5‰, in the recharge and discharge periods, respectively. Stable isotopic values sharply decreased from upstream to downstream, indicating the damming effects. δ18O-NO3− and δ15N NO3− confirm that sewage effluents, nitrification of soil organic material, and NH4+ fertilizers were the primary sources of NO3− in the reservoir. Carbonate weathering mainly provides ions to the reservoir. HCO3− + SO42− and Ca2+ + Mg2+ represent 90