摘要:地表水硝酸盐（NO₃^-）污染会造成水生态环境损害，引起水生态系统发生退化。通过溯源研究能够及时发现隐患和危害的源头，对保障水生态环境安全具有重要的意义。本研究选取渭河流域关中段13个子流域为研究对象，运用同位素方法结合水化学研究土地利用类型与地表水NO₃^-含量的关系，准确识别NO₃^-的主要污染来源。研究结果表明，整个流域NO₃^-含量范围为4.2-150.1 mg/L，平均含量38.2 mg/L。约有35.3%的样品NO₃^-含量超过《地表水环境质量标准》（GB 3838-2002）中硝酸盐的含量。渭河干流污染较支流严重，南岸支流较北岸支流污染严重。渭河干流、支流的源头及上游区域NO₃^-浓度普遍较低，说明受人类活动影响较小，沿着河流流向，NO₃^-浓度逐渐升高。耕地和城乡/工矿/居民用地面积与地表水NO₃^-浓度呈显著正相关（P < 0.05），相关系数分别为0.627和0.830。而草地和林地与NO₃^-浓度呈显著负相关（P < 0.05），相关系数分别为-0.775和-0.695。流域内，NO₃^-污染来源主要为动物排泄物及生活污水和工业废水的排放。农业活动中化肥的施用也是NO₃^-升高的一个重要原因。根据流域污染溯源结果，建议规范建设用地，加强污水排放和畜禽粪便的管理，同时提高化肥的使用效率，以达到减少水生态环境损害的目的。



Abstract:Land-use patterns changes dramatically have effects on the river water quality by altering the natural appearance, material circulation, and energy distribution of landscapes within watershed ecosystems. Nitrogen pollution is due to alteration of runoff, non-point source pollution production, and nutrient transportation driven by land-use changes. Nitrate pollution in rivers is becoming a serious problem,and excessively high levels of nitrate in rivers could lead to more virulent diseases and cause environmental and ecological problems. The pollution sources of river nitrate can be identified by the nitrogen isotopic composition of nitrate. In this study, thirteen subwatersheds are selected with surface water samples collected from Guanzhong area in the Weihe River basin in January 2016. Hydrochemical assessment and nitrogen isotopic measurement are made to determine the pollution levels of nitrate, investigate the effects of land-use types on river nitrogen pollution, and identify the principal sources of nitrate pollution in surface water. For the whole watershed, the nitrate concentration of surface water ranges from 4.2 mg/L to 150.1 mg/L, with the mean value of 38.2 mg/L. Approximately 35.3% of the surface water samples exceed the maximum contaminant level (MCL) of nitrate for the environmental quality standards for surface water. The nitrate concentration of the Weihe River main stream, northern tributaries, and southern tributaries range from 9.6 mg/L to 82.5 mg/L, 4.2 mg/L to 86.4 mg/L, and 8.0 mg/L to 150.1 mg/L, with the average nitrate concentration of 47.9 mg/L, 33.1 mg/L, and 43.8 mg/L, respectively. About 40%, 33.3%, and 33.3% of surface water samples from the Weihe River main stream, northern tributaries, and southern tributaries, respectively, exceed the MCL. Nitrate content in the main stream is obviously greater than that in the tributaries. Nitrate pollution in the surface water around the mountainous areas was much less than that in other areas. The proportions of land-use types differ greatly among the subwatersheds, and the proportion of cropland is the largest, with the average value of 47.5%. The proportion of build-up land increases from upstream to downstream With rivers receiving municipal sewage along the flow directions. A correlation analysis shows that four land-use types (build-up land, cropland, forestland, and grassland) correlate well with nitrate variables, with the correlation coefficients of 0.830, 0.627, -0.695, and -0.775, respectively, an indication that farmland and built-up land uses have positive contribution to river nitrate pollution, whereas grassland and forestland use types have negative contribution to river nitrate pollution in the surface water. There is no significant effect of denitrification on the shift in N isotopic values. The δ¹⁵N composition ranges from 0.7‰ to 19.1‰, with the average value of 10.3‰. Analyses of hydrochemistry and nitrogen isotopic composition indicate that the nitrate pollution in the Weihe River basin is significantly affected by human activities. Manure and sewage are considered to be the dominant sources of NO₃^- pollution of surface water in the whole basin, especially in the lower reaches (Xianyang, Xi'an, and Weinan sections). Chemical fertilizer is the other important nitrate contributor to rivers in the agricultural areas. It is suggested by this research that regulating build-up land uses, strengthening the management of manure and sewage, and improving the application rate of fertilizer should be the effective methods for controlling river nitrogen.





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