Assessment of water quality in the Three Gorges Reservoir using hydrodynamic and water quality mddels

The Three Gorges Reservoir is located in the middle reach of the Yangtze River ofChina. It is the largest water conservation project and biggest hydropower plant in theworld. Many experts expected that the environmental protection at the reservoir areawould face tough challenges ahead with the slowdown of the stream flow after waterstorage. Water pollution would become severe not only in the Three Gorges Reservoirbut also in the entire Yangtze River.This thesis used hydrodynamic and water quality models to predict the water qualitysituation and to develop the pollution load reduction measures.One and two dimensional hydrodynamic and water quality models for the Three Gorgesreservoir were set up by applying MIKE Zero model code. The models simulateddynamic relationships between the hydrological regime, pollution sources, and waterquality of both river and reservoir situations.Observed data on several major water quality indexes for the past 6 years (2000-2005)were collected from 4 observation stations along the reservoir. The dominant pollutantsin the reservoir, CODMn, BOD5, NH3-N and TP were simulated by the models.The modelling results show that water quality of the reservoir would not evidentlydegrade after the reservoir is formed, compared to the natural river condition. On thecontrary, the pollution carrying capacity would increase to some degree in the dryseason because the increased water volume dilutes the pollutants and the prolongedresidence time is beneficial for the decay of pollutants. During the flood season,however, water quality would be slightly worse.Sensitivity analyses were carried out to identify the dominant factors for the ThreeGorges reservoir pollution control. Industrial and municipal pollution are the mainsources of pollution in the dry season and the non-point pollution is the dominant factorin the flood season.Comparing the effectiveness of the control measures applied at the upstream area withthat in the reservoir area, an integrated approach in the whole river basin was found theonly way to protect the water resources in the reservoir. According to the results of thesimulation, in order to reach the water quality targets set by 2009, about 50% of totalpollution load in the upstream area, 60% of the diffuse pollution load in reservoir areaand 16% to 60% of point pollution loads for different locations in the reservoir area willhave to be reduced