Study On The Influence Of Mainstream Of The Three Gorges Reservoir On Water Quality Of Daning River Backwater Area

The Three Gorges Project is one of the largest water conservation and hydropower project around the world.The Three Gorges Reservoir（TGR）has brought much environmental impacts since its initial impoundment in 2003.The most serious environment problem was eutrophication and algae blooms in the tributaries of the TGR.The water from the mainstream flowing back to tributaries because of impounding of the TGR changed the tributary sensitive area hydrodynamic and water quality conditions,and increased the risk of eutrophication and algae blooms in this area.The Daning River and its adjacent Three Gorges Reservoir（TGR）mainstream were chosen to be the research area in this study.A significant difference in both theδD andδ¹⁸O values（p<0.01）between the TGR mainstream and the upstream of Daning River confluence zone（UCZ）was found during both impounding period and high water level stage,which can be used to distinguish the water mass of TGR mainstream from that of UCZ.Thus,the hydrogen and oxygen isotope traers is suitable for water source analysis.The results of hydrogen and oxygen isotope tracers showed that water from the TGR mainstream is the main source of Daning River backwater area during the impoundment period.The water mixing ratio calculated by the oxygen isotope values was 0.79.The nitrate from the TGR mainstream contributed 88%of the Daning River backwater area.The dynamics of vertical distributions of major ions and water contributions of the TGR mainstream indicated that the water mass from the Yangtze River mainstream flowed backward from the confluence zone up to the tributary along the surface and upper-middle layers,whereas water from the tributary flowed into the mainstream through the lower-middle and bottom layers.The results of EFDC model gave a fomula to indicate how The TGR mainstream’discharge(Q_CJ),the Daning River’discharge(Q_DNH)and the water level change（?H）of the mainstream affected the backward coefficient（Mix）.The quantity relationship between the nutrient salt backward coefficient（f _D）,the TGR mainstream’nutrient concetration(C_CJ),the Daning River’nutrient concetration(C_DNH)and Mix was deduced from the formula.EFDC water quality simulation results show that the mainstream intrusion mainly concentrated in the impoundment period and low water level stage.The water quality influence distance of Daning River during impoundment period and low water level stage are 20 km and 8 km,respectively.The water quality influence distance of Daning River during high water level stage and drawdown period are 3 km and 5 km,respectively.Becaue of the TGR mainstream intrusion,the average nitrate nitrogen concentrations of Daning River backwater area in impounding period,low water level stage,high water level stage and drawdown period were 1.03,0.91,,0.94 and 0.99 mg/L,respectively,the total dissoved phosphorus concentrations were 0.11,0.08,0.09 and 0.10 mg/L.Reducing the TGR’s inpouding speed was an effective measure to cotrol the risk of eutrophication and algae blooms in the TGR’s branches.