柳江盆地浅层地下水硝酸盐背景值研究

A study of nitrate background level of shollow groundwater in the Liujiang Basin

为探索地下水硝酸盐背景值获取方法,文章以柳江盆地为研究对象,在对比分析国内外研究方法的基础上,首先采用绝对含量和毫克当量百分位数双因子法从宏观上剔除硝酸盐异常数据,然后再利用层次聚类分析结合主成分分析法,分析地下水水化学特征及识别异常分类,进一步剔除异常数据。最后剩余数据进行分布类型检验,采用浓度累计频率法确定地下水硝酸盐背景值范围。研究结果表明,绝对含量和毫克当量百分位数双因子法虽然不能完全剔除异常值,但可为后续层次聚类分析异常识别减少异常信息和子集;层次聚类分析法注重对各亚类的水化学特征分析来识别分析异常数据,具有识别人为异常和天然异常的优势。对比分析常用的数理学方法计算表明,2种方法结合,更能有效识别异常,计算出的地下水硝酸盐背景值更合理。异常数据剔除分析表明,柳江盆地浅层地下水硝酸盐异常与农业化肥的超量施用和居民生活污水与垃圾粪便的下渗污染具有密切的关系。

The method of calculating groundwater nitrate background level was explored in Liujiang Basin as a research object.Based on a comparison and analysis of methods adopted both in China and abroad, the authors firstly used an absolute concentration and milliequivalent percentile double factors method to eliminate outliers macroscopically, and then employed hierarchy clustering analysis combined with principal component analysis to investigate hydrochemical characteristics and identify anomaly category for further candidate selection. The last remaining data were examined for distribution type with the range of groundwater nitrate background level being determined by the cumulative frequency method. The results show that, although the absolute concentration and milliequivalent percentile double factors method cannot completely eliminate outliers, it helps to reduce abnormal information and subset for subsequent hierarchical clustering analysis identifying outliers. Hierarchical clustering analysis method takesadvantage of identification of artificial and natural anomaly in that it emphasizes groundwater hydrochemical characteristics analysis. Compared with the mathematical methods, it is more efficient to identify outliers and more reasonable to calculate groundwater nitrate background level by using the two methods in combination. The outliers analysis shows that the high nitrate content in shallow groundwater is closely related to overuse of agricultural fertilizer and living sewage and faeces infiltration in Liujiang basin.