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Nitrogen and Oxygen Isotopic Tracing of Nitrate Pollution in the Dianchi Lake Basin TEXT SIZE: A A A

LI Qing-guang1, 2, WANG Shi-lu1

(1. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;
2. Graduate University of the Chinese Academy of Sciences, Beijing 100039, China)

Abstract:The water quality of the Dianchi Lake showed an increasing deterioration due to nitrate pollution, and hence there is an urgent need to identify its sources for the purpose of pollution control and improvement. We have collected rain water, river water, lake water, and well water samples from the Dianchi Lake watershed for the determination of inorganic nitrogen concentrations and nitrogen (δ15N) and oxygen (δ18O) isotopic ratios of nitrates. The results displayed that the nitrate varied over a large range from below the detectability to 13.44 mg-N/L, demonstrating different degrees of nitrate pollution in different areas. The highest concentration was detected in the well water located on the farmlands to the south of the lake proper. The plot of δ15N-NO3- against δ18O-NO3-pointed out fertilizer and atmosphere deposit sources, showing their influence on the groundwater in the superficial zone. This also implied that these two sources might indirectly affect the Dianchi Lake. The rivers had larger ranges of nitrate concentrations, and were polluted more severely than the Dianchi Lake. Domestic sewage and manure would be the main nitrate sources in most rivers as indicated by δ15N-NO3- and δ18O-NO3- values. This argument also seems true for the Dianchi Lake. From the south to the north of the Dianchi Lake, the pollution aggravated gradually, corresponding to the distribution of rural farmlands in the south and urban areas in the north. In general, nitrates in the Dianchi Lake mostly resulted from domestic sewage and manure, and agricultural fertilizers and atmosphere deposits indirectly contributed nitrates in the southern part. Further research should focus on the quantification of relative contributions of different nitrate sources.

Key words: Dianchi lake watershed; nitrogen pollution; nitrate; nitrogen isotope; oxygen isotope

EARTH AND ENVIRONMENT Vol. 40, No.3, Tot No.289, 2012, Page 321-327

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