密云水库上游流域地下水中氮素污染特征及影响因素

为分析密云水库上游流域地下水中氮素的污染情况,于2014年7月和2015年1月进行了地下水样品的采集,应用域法和地质统计学方法等多元统计方法识别流域地下水中不同形态氮的时空分布特征,并解析土地利用类型、地下水埋深以及地表水对地下水中氮素的影响。结果表明:区域地下水的氮素污染不容乐观,29.73%的样品中硝态氮含量超标(10 mg·L-1≤NO_3~-≤20mg·L~(-1)),27.03%的样品出现严重超标(NO_3~--N≥20 mg·L~(-1))。从空间来看,地下水氮素具有空间自相关性,其中氨氮空间变异的随机性较大,硝态氮最小,硝态氮的污染主要发生在城镇人口密集区域;从时间来看,硝态氮污染呈逐年升高趋势,硝态氮的超标样品百分比从2008年的2.30%增长为2015年的25.71%,且年内变化表现为丰水期高于枯水期。各种土地利用类型中,城镇的氮污染最严重;硝态氮、亚硝态氮的含量随地下水埋深增加呈减小趋势;地下水氮污染浓度与流向有一定的联系,从上游至下游呈升高的趋势。

Nitrogen pollution and its influencing factors in groundwater in upper-river basin of Miyun Reservoir

The Miyun Reservoir is the sole source of drinking water for Beijing residents, and the groundwater is the primary water supply source of the Miyun Reservoir. Therefore, nitrogen in the groundwater would critically affect the water quality of the Miyun Reservoir. We collected groundwater samples in an upper basin of the Miyun Reservoir in July, 2014 and January, 2015. Spatial-temporal distributions of NO₃^--N, NO₂^--N and NH₄⁺-N in the groundwater and influencing factors, such as land use and groundwater level, and relation between surface water and groundwater of nitrogen were investigated using domain identification method and geo-statistic method. Concentrations of NO₃^--N in 29.73% of the total samples ranged between 10 mg·L^-1 and 20 mg·L^-1, and those in 27.03% samples exceeded 20 mg·L^-1, implying N pollution in the studied area. The spatial variation of NH₄⁺-N in the groundwater had little pattern while NO₃^--N pollution mainly occurred around towns or villages with dense population. The percentages of water samples containing NO₃^--N above the national standards increased from 2.3% in 2008 to 25.71% in 2014. Seasonally, NO₃^--N concentrations were higher in the wet season than in the dry season. In addition, NO₃^--N and NO₂^--N concentrations in the groundwater decreased with increasing depth of groundwater level, and NO₃^--N concentrations in groundwater increased from upstream to downstream.