ANALYSIS OF WATER QUALITY OF THE YANGTZE RIVER WITHIN THE THREE GORGES RESERVOIR AREA DURING CONSTRUCTION PERIOD

Water quality in the Yangtze River within the Three Gorges Reservoir area before and after the impoundment was analyzed.In general,water quality after the impoundment was better than that before the impoundment.Average percentage of water quality(according to monthly evaluation results)met or better than III standard increased from 62.3% before the impoundment to 78.3% after the impoundment.The main parameters not meeting the standards were total phosphorus(TP),permanganate index(CODMn)and plumbum(Pb),etc.New characteristics in spatial and temporal distribution of water quality have been found.The concentrations of CODMn,TP,Pb of water column near the dam decreased significantly compared with that of pre-impoundment,especially for water area near the dam site.Concentrations of CODMn,TP,Pb were still wet seasoneven,dry season.It is also showed that concentrations of the above parameters decreased along the direction from tail to head of the reservoir,especially in the wet season.Above changes of water quality features were mainly due to sedimentation of sands in the reservoir resulting from the increase of water level and the decrease of velocity,which meaned that part of pollutants settled with deposition of suspended solids and entered into sediment.