关键词: 永安江, 硝酸盐污染, 氮氧同位素, 来源示踪, IsoSource模型, 污染贡献率

Abstract:

A case study of the Yongan River, a major inflow of the Erhai Lake, was conducted to identify sources of nitrate-N in the river water with the δ¹⁵N-NO₃^-and δ¹⁸O-NO₃^- double isotope technique. Along the river, 9 monitoring sites were set up, responsible for characterizing nitrate pollution of the water, and measuring nitrate δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- in the water samples after treatment with ion exchange resin. Results show that the nitrate sources along the Yongan River are responsible for about 50% of the total nitrogen pollution load in the water. Nitrate concentration in the water samples at the monitoring sites varied in the range of 0.07-5.22 mg·L^-1, with mean being in the range of 1.00-2.39 mg·L^-1. The isotope test reveals that the mean δ¹⁵N-NO₃^- of the water samples at the monitoring sites ranged from 6.12‰ to 13.88‰, and the mean δ¹⁸O-NO₃^- did from 8.24‰ to 11.72‰. The findings demonstrate that the nitrate pollutant in the river comes mainly from chemical fertilization, livestock manure, domestic sewage, and nitrification of organic nitrogen in the soil in the valley. Quantification of contributions of the four sources with the IsoSource mixing model tells that 37.3% was attributed to chemical fertilization, 34.6% to disposal of livestock manure, 18.2% to discharge of rural domestic sewage and 9.9% to nitrification of organic nitrogen in the soil. The experiment shows that the use of the IsoSource mixed model to quantify sources of nitrate pollutant in the river may provide future studies with a new train of thought and that ratio of the contributions of the sources to the total nitrate pollutant in the river is related to location of the villages the river runs through and type of land use therein.

Key words: Yongan River, nitrate pollution, nitrogen and oxygen isotope, source tracing, IsoSource model, pollution contribution rate