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

Abstract

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^- + SO₄^2- and Ca²⁺ + Mg²⁺ 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-HCO₃^- 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 NO₃^- 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.

Keywords: Aquatic geochemistry; Chemical weathering; Dual isotopes; Nitrate pollution; Three Gorges Reservoir.