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湖库富营养化已成为全球范围内的环境问题[1-3]。由于水体接纳过量的氮、磷等营养物质使得浮游生物大量繁殖,导致水体富营养化,水质逐渐恶化。湖库水体营养盐的来源通常分为内源和外源两个部分。内源一般是指沉积物中累积的营养盐再次释放进入上覆水中;外源则主要是通过地表径流途径进入库区水体[4-5]。入湖库河流是大部分湖库水体营养物质最主要的输入源[6]。
极端降水事件的发生使得河湖等水体扰动加剧,还会通过地表径流携带大量污染物进入水体[7-9],导致河湖水环境质量的变化。因此,研究极端降水对湖库上游河流水质特征的影响过程与规律,对流域水环境治理和水质安全保障具有重要意义。
岩口水库是浙江省义乌市最大的饮用水水源地之一,为义乌市近3.0×105人口提供日常供水。监测数据表明,岩口水库水体大部分水质指标能够满足湖库Ⅱ类标准,但在丰水期总磷为0.025~0.045 mg·L−1,总氮为0.7~1.5 mg·L−1,春、秋季节水体中叶绿素a浓度为4~20 μg·L−1,导致水库每年 4月~10月存在间歇性藻华暴发的风险。岩口小流域集水区为丘陵地形,分布有25个自然村,户籍人口为13 307人,流域内现存1 206 hm2农业用地,其中482 hm2(469~496 hm2)为桃树果园。库区水质极易受到流域内人类活动的影响,同时在降水发生时地表径流对地面的冲刷会导致入库河流污染物通量急剧增加,使得水库农业面源污染加剧,水质污染风险增大。
削减入库河流输入污染物总量是提升库区水质、防止富营养化和藻华暴发的前提条件。通过解析降水过程中入库河流水量水质的动态变化规律,阐明降水过程对入库溪流水质的影响,是控制库区水质污染负荷、保障水质安全的重要基础,也是有效核算水库外源输入污染负荷、制定削减方案的前提。
本研究基于“利奇马”(2019年)和“黑格比”(2020年)2次台风降水后岩口水库主要入库河流流量和水质变化情况的全过程监测数据,探究降水对入库河流不同断面水质、水量的影响过程和规律,解析降水过程中流域污染物流失过程及输出特征,评估极端降水条件下河道径流对库区水质、水量的影响,为科学制定入库污染负荷削减对策提供参考。
Influence of extreme rainfall process on pollution load of the inflow stream of Yankou reservoir and its countermeasures
- Received Date: 08/09/2021
- Available Online: 10/06/2022
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Key words:
- extreme rainfall /
- Yankou Reservoir /
- exogenous load /
- nitrogen and phosphorus flux /
- rainfall runoff
Abstract: The influence process of extreme rainfall events on the water pollution load of upstream rivers and downstream reservoirs in the basin was analyzed, and the contribution of external input to water pollution in the reservoir area under extreme rainfall conditions was evaluated, withthe small watershed of Yankou reservoir as the research object, by monitoring the changes of nitrogen and phosphorus flux of river entering the reservoir during two typhoon rainstorms in 2019 and 2020. The results showed that the whole process of the two rainstorms with obvious differences in rainfall duration and intensity can be divided into four typical stages: local runoff generation period, whole watershed runoff generation period, water volume regression period and watershed restoration period. The heaviest pollution load ocuured in the whole watershed runoff generation period (stage II). More than 50% of the total phosphorus load in the whole stage was transported in stage II and the increasing multiple of the pollution load was significantly correlated with the rainfall intensity. During the two rainstorms of LQM and HGB, TN load of Yankou reservoir increased by 880 kg and 419 kg, TP load increased by 109 kg and 23 kg respectively, compared with that before the rainfalls. It was showed that surface runoff erosion leaded to a large amount of non-point source pollution load into the river,since the increased total nitrogen was mainly dissolved, while the increased total phosphorus was mainly granular. The results of this study proved that extreme rainfall events, especially in the whole watershed runoff generation period, significantly aggravated the pollution contribution of non-point sources to the downstream reservoir area. Therefore, it is suggested to adopt reasonable strategies such as detention, sedimentation and purification to reduce the pollution load of surface runoff in order to reduce the risk of water pollution in the downstream reservoir area.