| The man-made lake is structured through building the dam across the river in the city and is widely used to improve the water ecological system in urban area in northern China.However,the water level of the man-made lake is related to the function of the lake in particular season while the water quality of the lake is affected by the water quality of the up-stream,the in-stream pollutants,the point and non-point sources,the environmental condition in the lake and water exchanging cycle of the lake.The resultant hydraulic condition,such as the velocity field,and the characteristics of water pollution in the man-made lake across the river in the urban area is significantly different from that of the typical rivers and lakes.Therefore,it is necessary to carry out the related study on water pollution and control for the man-made lake across a river. This is crucial to improve the water quality of the man-made lake and ensure its funcation in improving the aquatic ecological system.In this study,the characteristics of water quality of the runoff,the combined sewer overflows(CSOs) and the in-stream pollution were investigated in Jinwei Lake,which is the typical man-made lake in Baoji city,to figure out the factors affecting on the water quality of the lake.The water quality of the lake is assessed using the single pollution index method and the comprehensive pollution index method using the water quality monitoring data that obtained in this study.The water quality models were set up and used to estimate the water quality in the particular duration.The results that obtained from the water quality models could be used in water pollution control in the lake.The sand filtration system was also structured for runoff water treatment on the base of water pollution characterization.The filtration system can potentially be used for water quality control in the man-made lake.The related results and conclusions are as following: 1,The water quality of the runoff and CSOs in Baoji city was investigated.The results showed that the concentration of the typical pollutants were significantly increased and followed by a decreasing stage and a stable stage in the rainfall process.The results showed that the concentration of the suspended solids(SS),chemical oxygen demand(COD),NH3-N and total phosphorus(TP) in runoff were ranged from 989 to 126mg/L,854 to 100mg/L,3.89 to 0.63mg/L and 0.96 to 0.37mg/L,respectively. The SS is the dominating material that related to the COD in the runoff water and the CSOs while the dissolved organic matter contributed insignificant amount of COD.The untreated runoff water that discharge into the lake is the dominating pollution source for Jinwei Lake and results in the decrease in water quality of the lake.Therefore,it is necessary to figure out the pollution control methods according the characteristics of the runoff water in urban area.The results showed that concentration of SS,COD,NH3-N and TP of the CSOs in Baoji city were ranged from 1352 to 154.5 mg/L,1930 to 210 mg/L,57.6 to 9.4 mg/L and 8.66 to 1.7 mg/L,respectively.The COD and SS followed the linear relationship with correlation coefficient r=0.8791.The municipal wastewater was the dominating source of COD and SS while the runoff water contributed insignificant amount of COD and SS in the CSOs.2,Aiming at the phenomenon of excessive growth of the Spirogyra in recent early summer in Jinwei Lake,characteristics of nitrogen and phosphorus pollution of the sediment and the water in lake were investigated and the reasons of the Spirogyra growth were explored.The results showed that the concentration of the total nitrogen (TN) and TP were ranged from 0.35 to 0.78mg/g and 0.11 to 1.17mg/g,respectively. Resuspending and releasing of sediment would lead to eutrophication in lake and promote Spirogyra growth.The concentration of TN and TP of sediment and that of water had showed correlation with the Spirogyra growth status.The results proved that the most essential reason of the Spirogyra recurrence was nitrogen and phosphorus circulation in the Jinwei Lake.Meanwhile,the other important reasons of the Spirogyra overgrowth were include lighting,temperature and water level in the lake.3,The water quality of the Jinwei Lake was assessed using pH,DO,oxygen demand,NH3-N,TP and TN according to the Environmental Quality Standards for Surface Water(GB3838-2002).The results obtained by using the single pollution index method and the comprehensive pollution index method showed that the water quality of the lake was ranged from typeⅣto the level worse than typeⅤwhile the NH3-N and TN were shown to be the dominating pollutant affecting the water quality of the lake. 4,Based on hydraulic characteristics and water pollution characteristic of Jinwei Lake,the water quality models of oxygen demand,NH3-N and TP were established as C=C0 exp(-0.0086·t),N=N0 exp(-0.0067·t) and P=P0 exp(-0.0269·t), respectively.The models were verified using the monitored data while the average error of oxygen demand and NH3-N were less than 15%.The results showed that the models were reliable and can be used in the water quality management.According to the predicting results,it is need to decrease the concentration of COD and NH3-N in the runoff water in the level of less than 50mg/L and 2mg/L,respectively,to ensure the water quality of Jinwei Lake.5,The operation methods of the rainfall storage system and the runoff water treatment system were figured out according to the characteristics of rainfalls and the practically running condition of the rainfall pumping station in Baoji city.The runoff water and the CSOs should be treated using the sand filtration system before any discharge in the case that the rainfall is higher than 30mm.The results showed that the concentration of COD and SS in the effluent was less than 50mg/L while the COD and SS removal efficiency could reach up to 80%.Therefore,it is necessary to link the discharge system,runoff water storage tank and filtration treatment system to reduce the effect of runoff water on the water quality of man-made lake in the urban area.
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