Abstract: Water pollution is a serious problem in the Taihu Basin. The key problems for water environment improvement in the future are as follows: (1) the total amount of pollutants into the lake remarkably exceeds the water environmental capacity of the lake, (2) there remains a high Cyanobacteria biomass in the lake, and (3) the industrial structure transition and upgrading along the lake are very slow-moving. As industry and population increase rapidly, the pollution of water environment has been a main factor influencing the sustainability of economic development in the Western Taihu Basin. Water environmental problems of main inflowing rivers were analyzed according to the monitoring data of 2010. Through the investigation on the distribution of the industry pollution source, urban and rural sewage source, agricultural and breeding pollution source in studied region, total amount of pollutant drainage into the rivers was calculated. After 2007, the State Council ratified Master Plan of Integrated Regulation of Water Environment of Taihu Basin and Water Function Zoning of Taihu Basin, embarking the comprehensive treatment of water environment in the basin. In this context, water quantity and quality model have been established to predict regional water environmental capacities. Based on the water function and water areas, the water environmental capacities were assigned to every county or district. After the implementation of the existing and planned engineering measures, a spatial and temporal reduction rates for pollution load and a controllable target were proposed. The results indicate that the water quality in the Taihu Lake was worse than Grade V, aand was mainly influenced by TN, and the index could only reach Grade IV in rivers and influenced mainly by NH₃-N. The western Taihu Basin has a total pollution load of 25 410.2t/a COD, 2 795.4 t/a NH₃-N, 4 646.4 t/a TN, and 313.8 t/a TP. The urban sewage source has the largest pollution load contribution with the proportion of 35%~50%. The pollution load in Yixing and Wujin were bigger than the other two. Forecasted pollution discharge into river for most of the water exceeds the water environment capacity. Recently, reduction rate of COD, NH₃-N, TN and TP are 8.0%~56.0%, 8.0%~62.1%, 6.0%~41.8%, 8.0%~59.9%, respectively, and are expected to be higher in future. Using the simulation result of the model, an average COD, NH₃-N, TN and TP value of 4.50, 0.80, 3.50 and 0.08 mg/L are proposed as the water quality controllable target for 2015 in the western Taihu Lake, furthermore, the value achieves to 4.50, 0.60, 3.00 and 0.07 mg/L in 2020. The study provides the technical support and the scientific basis to water pollution control in western Taihu Lake.

Key words: Western Taihu Basin, water environmental capacity, allocation, water quality controllable target, Water quantity and quality model