滇池流域硝酸盐污染的氮氧同位素示踪

滇池流域硝酸盐污染严重,厘清其来源对硝酸盐污染治理至关重要。本研究在滇池流域收集河水、湖水、井水、雨水样品,分析了无机氮浓度和硝酸根氮、氧同位素比值。总体上,硝酸盐浓度变化范围较大,从低于检测限到高达13.44mg-N/L,显示流域硝酸盐污染存在较大的空间差异。最高浓度出现在流域南部农田区的井水中,井水样品的氮、氧同位素数据大部分落在化学肥料和大气干湿沉降区,表明农业面源和大气输入对流域浅层地下水产生了污染,污染的浅层地下水又是湖泊水体的一个潜在污染源。流域内河流硝酸盐浓度变化范围较大,总体污染程度高于滇池湖泊水体,氮、氧同位素组成表明大部分河流中硝酸盐来自生活污水和人畜粪便。滇池水体的硝酸盐氮、氧同位素组成和河流的相似,说明人畜粪便和生活污水是主要来源。湖泊水体硝酸盐浓度从南向北有逐渐增加的趋势,这与滇池北部紧邻城区(生活污水)、流域南部主要为农田区(面源污染)的空间格局是一致的。总体上,滇池水体的硝酸盐主要来自城市生活污水,农业面源和大气输入。通过地下水途经进入湖泊主要发生在流域南部地区,具体的贡献份额还需要进一步的计算。

Nitrogen and Oxygen Isotopic Tracing of Nitrate Pollution in the Dianchi Lake Basin

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-NO-3 against δ18O-NO-3 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-NO-3 and δ18O-NO-3 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.