北京半城子水库水体营养物质表观沉降速率

Apparent settling velocity of nutrients in Banchengzi Reservoir,Beijing

表观沉降速率是流域非点源污染模型中反映水体营养物质通过不同过程,输移进入水库沉积物净效果的参数。选择位于密云水库上游的半城子水库进行水体营养物质监测,计算营养物质表观沉降速率,分析其影响因素,主要得出以下结论:半城子水库TN表观沉降速率平均为55.5mm/d,TP表观沉降速率平均为25.2mm/d,相对于世界其它地区的湖泊水库而言,TP表观沉降速率较慢。水体营养物质表观沉降速率受表层水温和水流条件共同影响。TN表观沉降速率均随表层水温的升高而递增。上游来水后,水体中P元素则经历了内源磷快速释放,之后快速沉降的变化过程。研究成果可为水污染防治和流域环境综合治理提供理论依据。

Beijing is suffering a severe water shortage in recent decades, and this problem will be more serious in future under the influence of urban expansion and population increasing. Several studies have been conducted on the nutrients problem in the water body or upper-rivers of Miyun reservoir in past 20 years. However, the studies concerning this issue have long ignored the connection of nutrients delivery between the upper-rivers and Miyun reservoir. Without this connection, it is difficult to simulate the nutrients delivery processes from rivers to reservoir. The apparent settling velocity can quantitatively represent the net effect of different processes that deliver nutrients to sediments. To estimate the apparent settling velocity of nutrients, in this present study, 213 water samples were collected from Banchengzi reservoir and the upper-river watershed in 2009-2010. The total nitrogen （TN）, nitrate （NO3--N）, ammonium （NH4＋-N）, total phosphorus （TP） and index of potassium permanganate （CODMn） were analyzed to estimate the flux of nutrients. Based on the analysis of mass balance and seasonal variations of nutrients, the apparent settling velocity was calculated and the potential affecting factors were also analyzed. The results showed that the apparent settling velocity for nitrogen and phosphorus of the Banchengzi reservoir were 55.5mm/d and 25.2mm/d, respectively. Compared with many of lakes and reservoirs aboard, the apparent settling velocity for phosphorus was slower. This indicated that endogenous phosphorus was slowly released. The apparent settling velocity of the reservoir was influenced by surface temperature and conditions of disturbance of water body. Moreover, the seasonal variation of apparent settling velocity can be obviously found. The apparent settling velocity for nitrogen was increased with the increasing surface temperature of water body; however, the increasing trend of phosphorous was not obvious. The apparent settling velocity for nitrogen was rapidly increased when the river water flowing into the reservoir and the speed was 3 times faster than before. Moreover, after the river water flowed into the reservoir, the apparent settling velocity for phosphorus showed a rapidly increasing trend in the first month and then decreased rapidly for the resuspended sediments settled. The results of this study can provide scientific guidance in developing the conservation plan for the water quality of upper-river basin of Miyun Reservoir.