Mechanism of Water Pollution of Urban Streams and Methods for Recovering the Water Quality

流经城市地区的河流从其小的排水区域接收高污染物负荷。因此，城市水系水质的变化通常对城市化和污水系统的建设很敏感。本研究通过对流经长江市的典型城市河流——卡基河的水质观测，探讨了卡基河对水的净化作用机制。在观测结果的基础上，提出了描述河流BOD、DO和SS的数学模型。通过对河流水质的反复观察，揭示了两个重要的净化过程。一是悬浮物的沉降是表面净化的主要过程。二是泥沙需氧量（SOD）影响河水的氧平衡。利用岩心取样沉积物对SOD进行了实验。结果表明，SOD是沉积物BOD和速率常数的乘积，与水温呈arrhenius型关系。建立了河流BOD、DO和SS的模拟模型。为了区分沉降过程，将颗粒BOD和溶解BOD作为模型变量。将超氧化物歧化酶、沉积物的再悬浮和溶解BOD的释放作为模型过程。通过实验计算SOD的速率常数等模型常数。该模型成功地再现了卡其河BOD、SS和DO的分布。模型分析证实，颗粒物的沉降是河流表面净化的主要过程。清除沉积的沉积物以及减少污水建设造成的污染物负荷对改善城市水系水质是有效的。

Streams flowing through urban areas receive high pollutant loads from their small drainage areas. Therefore, the water quality of urban streams is generally vary sensitive to the urbanization and the construction of a sewerage system. In this study, we carried out the water quality observation of the Kaki River, which is a typical urban stream flowing through Nagaika City, to investigate the mechanism of water purification in the river. On the basis of the observational results, we proposed a mathematical model to describe BOD, DO and SS in the river.Two important processes of the purification are revealed from repeated observation of the water quality of the river. One is that the settling of the suspended materials is the dominant process of the apparent purification. The other is that the Sediment Oxygen Demand (SOD) affects the oxygen balanc of the river water. Experiments on the SOD were made using core-sampled sediments. The results showed that the SOD is expressed as a product of the sediment BOD and the rate constant which shows an Arrhenius-type dependence on the water temperature.A simulation model was developed to describe BOD, DO and SS in a river. In order to distinguish the settling process separately, the particulate BOD and the dissolved BOD are considered as model variables. The SOD, resuspension of the sediments and release of the dissolved BOD are taken into account as model processes. All model constants including the rate constant of the SOD were evaluated by experiments. The model successfully reproduced the distribution of BOD, SS and DO in the Kaki River.The model analysis confirms that the settling of the particulate matter is the dominant process of the apparent purification in the river. Cleaning of deposited sediments as well as reducing pollutant loads due to sewage construction is effective for the improvement of the water quality in urban streams.