典型山地城市河流营养元素空间分布特征及影响因素分析:以重庆市清水溪为例

Spatial Distribution and Influential Factors of Nutrients in Rivers of a Typical Mountainous City:A Case Study of the Qingshuixi River in Chongqing

城市地表水水环境问题严重制约城市的进一步发展和市民对美好生活的向往,因此广受社会各方关注.由于地形特殊,与平原城市相比,山地城市地表水水环境特征及污染问题有其自身特点.在2019年8月和10月两次通过采集位于重庆市主城区典型山地城市河流——清水溪干支流地表水和干流河底沉积物,并监测地表水、沉积物样品中碳、氮和磷等营养元素以及其它基本理化指标.结果表明,现阶段清水溪地表水属劣Ⅴ类,在综合污染指数评价体系中属严重污染级别.在所有水质指标中,NH_(4)^(+)和TP超标最为严重.清水溪地表水水质呈现出支流优于干流、降雨期优于非降雨期的时空分布特征.污染源、尾水/支流汇入和河流自净能力共同决定了营养元素在清水溪干流地表水中的空间分布规律.源头污水直排和中下游的污水溢流等点源污染是清水溪干流地表水最主要的污染源,面源污染对水质的影响有限,河底沉积物在河流处于不同的水量条件下会发生营养元素“汇”与“源”间的相互转化,体现出内源污染的复杂性.支流/污水处理站尾水汇入的稀释作用,以及由于山地河流地表水中高DO导致其较高的自净能力,使得干流地表水水质向下游有逐步改善的趋势.综上所述,建议通过在重点区域市政管网的布设减少点源污染的输入,对污水处理站提标改造为河流提供更低营养元素浓度的水源,在DO较低河段进行河道清淤等多种方式,对清水溪进行水环境综合整治.

Surface water pollution seriously restricts the development of the city and results in the citizens yearning for a better life.Mountainous cities have their own characteristics in surface water environment and pollution compared with those of plain cities due to their unique topography.In August and October 2019,surface water and sediments were collected in a typical river(Qingshuixi River)in the main urban area of Chongqing.These samples were analyzed for nutrients such as carbon,nitrogen,and phosphorus as well as other basic physicochemical indicators.The results showed that the surface water of the Qingshuixi River was inferior to Category V at present,and it was also at a serious pollution level according to the comprehensive pollution index evaluation system.Among all the water quality indices,NH_(4)^(+) and TP showed the most serious pollution.The tributaries of the Qingshuixi River exhibited higher surface water quality than that of the main stream,and the water quality during the rainfall season was better than that during the non-rainy season.Pollution sources,tailwater/tributary inflow,and river self-purification capacity determined the spatial distribution of nutrients in the surface water of the mainstream of the Qingshuixi River.Point sources such as the direct discharge of source sewage and pollution overflow in the middle and lower reaches were the most important sources of surface water pollution in the mainstream of the Qingshuixi River.Non-point source pollution had a limited impact on water quality.The“sink”or“source”role transformation of river sediments would occur under different water volume conditions in the river,reflecting the complexity of endogenous pollution.Surface water quality of the main stream showed a gradual improvement tendency along the downstream due to the dilution of the tail water/tributary inflow and the increased self-purification capability of the surface water in the mountainous rivers due to the increased DO content.To summarize,this study suggests that the water environment of the Qingshuixi River should be improved in comprehensive ways,which might be reducing the input of point source pollution by the deployment of the municipal pipe network in key areas,upgrading the sewage treatment plants to decrease pollutant concentrations or fluxes or channel dredging at the reaches with low DO content,etc.