白洋淀流域浅层地下水硝酸盐分布及来源的区域分异特征

Regional characteristics of nitrate sources and distributions in the shallow groundwater of the Lake Baiyangdian watershed

白洋淀位于雄安新区规划的核心范围,地下水是白洋淀流域主要的用水水源。由于白洋淀上游工业、生活污水的排放和农田肥料过量施用等农业面源引起的硝酸盐污染来源多样,使得流域内地下水硝酸盐污染较为普遍。然而,目前对全流域尺度地下水硝酸盐分布特征及来源仍不明晰。本文在分析过去近10年地表水和地下水硝酸盐数据的基础上,于2016年12月采集了平原区浅层地下水样品,结合水化学和硝酸盐氮同位素,从全流域尺度解析浅层地下水硝酸盐污染分布的时空差异和不同来源氮对地下水硝酸盐影响的程度。研究表明:山区典型流域河谷沉积带地下水硝酸盐浓度高值主要受农村厕所粪污水和局地污水排放影响(最高达313 mg·L−1),而历史时期农田有机肥施用是近年来地下水硝酸盐普遍升高的原因;雨季降水淋滤作用使地下水硝酸盐浓度明显升高,硝酸盐超标率大于旱季的2倍以上。平原区地貌类型控制着不同来源地下水硝酸盐的空间分布和迁移转化。2016年12月平原区130个浅层地下水硝酸盐超标率为21.5%,从上游到下游不同地貌类型地下水硝酸盐浓度中值呈现下降趋势:洪积扇(42.4 mg·L−1)>冲洪积扇(24.1 mg·L−1)>冲洪积平原(6.0 mg·L−1)和河道带(6.2 mg·L−1),而硝酸盐氮同位素中值呈现上升趋势:洪积扇(12.8‰)和冲洪积扇(11.3‰)<冲洪积平原(16.7‰)<河道带(20.9‰),说明从上游到下游地下水硝酸盐反硝化作用增强。其中山前平原洪积扇和冲洪积扇地区渗透性较好,地下水硝酸盐超标率高达33.3%和34.0%,主要来源于污水和有机肥。湖泊洼淀区典型生活和工业污水河周边,地下水硝酸盐则存在工业、生活和化肥多污染源并存的特征,且随着地表治污措施的影响地表水和地下水硝酸盐浓度变化较大,污水侧渗导致河道周边地下水硝酸盐浓度较高,距河道较远含水层强烈的还原条件使地下水硝酸盐浓度降低(<10 mg·L−1),污染风险较低。鉴于以上不同区域地下水硝酸盐脆弱性程度和风险水平的差异,提出了对白洋淀流域上游山区、山前平原洪积/冲洪积扇区、湖泊洼淀污水影响区等硝酸盐脆弱区实施区域分异农田面源污染和水环境整治及管理的建议,为雄安新区水环境安全保障提供科学依据。

Lake Baiyangdian is located in Xiong’an New Area, China, where groundwater is the primary water supply. Groundwater nitrate(NO3-) contamination is common in the Baiyangdian Lake watershed because of industrial and domestic wastewater discharge and over-application of agricultural fertilizer. However, the source characteristics andNO3- distribution across the entire watershed are still unclear. In this study,NO3- samples collected from rivers and shallow groundwater over the past decade were analyzed. Samples were also collected in December 2016 from the Lake Baiyangdian watershed area, and the spatio-temporalNO3- distributions of groundwater and the effects of various sources on groundwaterNO3- were analyzed using water chemical ions and stable nitrate nitrogen isotopes(δ15 N-NO3-). The results showed that theNO3- concentration in shallow groundwater differed, and the nitrogen sources had variable effects, particularly from the hills to the plains. In the hilly area, highNO3- concentrations measured in the alluvial valley groundwater were attributed to local rural sewage, with the highest NO3- concentration of 313 mg·L-1;while the regional farmland manure application over several decades was the main cause of commonly high groundwaterNO3- concentration in recent years. Rainy season leaching led toNO3- concentrations two times greater than that during the dry season, which exceeded the World Health Organization’s(WHO) standard(50 mg·L-1) and threatened downstream water quality safety. Of the shallow groundwater samples collected in the plains in December 2016, 21.5% hadNO3- concentrations exceeding the WHO standard. The median groundwater nitrate concentrations trended downward from upstream to downstream in geomorphological type(proluvial fan: 42.4 mg·L-1>alluvial-proluvial fan: 24.1 mg·L-1>alluvial-proluvial plain: 6.0 mg·L-1 and river zone: 6.2 mg·L-1), but the median δ15 N-NO3 isotopes trended upward(proluvial fan: 12.8‰ and alluvial-proluvial fan: 11.3‰<alluvial-proluvial plain: 16.7‰<river zone: 20.9‰), indicating that denitrification increased from upstream to downstream. High aquifer sediment permeability in the proluvial fan and alluvial-proluvial fan regions increase the risk of nitrate leaching into the aquifer. Sewage(33.3%) and manure(34.0%) were primary sources of groundwater nitrate and caused the deviation from the WHO standard rate. In regions with lakes and depressions, groundwater nitrate was affected by industrial and domestic sources and fertilization, and, compared to other regions, groundwater nitrate was higher near the domestic and industrial wastewater river(but also had drastically different surface pollution control measures). However, the reduced conditions in other lake and depression regions lowered the groundwater nitrate concentration(<10 mg·L-1). This study provides suggestions for managing nonpoint source pollution in the Lake Baiyangdian watershed shallow groundwater based on the regional source characteristics and nitrate distribution, particularly for vulnerable places, such as hilly areas, the proluvial/alluvial-proluvial fan region of the piedmont plain, and wastewater influence areas.