Reservoir NO pollution and chemical weathering: by dual isotopes of δN-NO, δO-NO and geochemical constraints.

Reservoir dams alter the nutrient composition and biogeochemical cycle. Thus, dual isotopes of δO-NO and δN-NO and geochemical signatures were employed to study the NO pollution and chemical weathering in the Three Gorges Reservoir (TGR), China. This study found that the TGR dam alters the δN-NO composition and is enriched in the recharge period. Values of δN-NO varied from 4.5 to 12.9‰ with an average of 9.8‰ in the recharge period, while discharge period δN-NO ranged from 3.2 to 12.5‰, with an average of 9.3‰. δO-NO 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. δO-NO and δN NO confirm that sewage effluents, nitrification of soil organic material, and NH fertilizers were the primary sources of NO in the reservoir. Carbonate weathering mainly provides ions to the reservoir. HCO + SO and Ca + Mg represent 90% of major ions in the TGR. Downstream sampling sites showed low solute concentration during the recharge period, indicating the dam effect on solute concentration. Ca-Mg-Cl, Ca-HCO and Ca-Cl were the main water types in the TGR. The average percentage of solutes contribution revealed the carbonate weathering, evaporites dissolution, silicate weathering, and atmospheric input were 51.9%, 41%, 7.8%, and 1.7% for the recharge period. In contrast, the discharge period contributed 66.4%, 29.2%, 10%, and 4.3%, respectively. TGR water is moderately suitable for irrigation, and hardness is high in drinking water. This study provides new insight into the dual isotopic approach and geochemical signatures to interpret the NO cycle and chemical weathering process under dam effects in the TGR. However, this isotopic application has some limitations in source identification, isotope fractionation, and transformation mechanisms of nitrate. Thus, further studies need to be done on these topics for a better undestanding.