Water Environmental Capacity And Water Quality Management In Huzhou District, Taihu Basin

Water pollution control of Taihu Lake is one of the national key projects in our country. With the rapid development of socio-economic of the watershed, more and more pollutant was discharged into the lake and causing water pollution crisis. In order to improve water quality and to rebuild a healthy aquatic ecosystem, it is necessary to improve capability of pollutants control and reduce pollutants load into Taihu Lake. Huzhou city, located at the south of Taihu Lake, is a key place to reduce the quantity of pollutant. Chosing the rive-network plain as objective region, the characteristic of water pollution was analysised, the water environment capacity of CODcr and ammonium nitrogen of the rivers surrounding Taihu Lake was estimated by using WASP7.3 model (Water Quality Analysis Simulation Program, Version 7.3), and a comprehensive water management mode integrated with regional management and TMDL (Total Maximum Daily Loads) was developed in the paper. The main results are summarized as follows.(1) It was estimated that the loads of CODcr and ammonia nitrogen were 14116.9 t/a and 2415.33 t/a respectively in 2005. The main sources of pollutant loads were domestic pollution sources and industrial pollution sources. The order of source contribution ratio from high to low as following: domestic pollution sources > industrial pollution sources > farm-lands runoff > aquaculture pollution sources > livestock pollution sources. NPS (Non-point pollution source) was the main cause for water quality deterioration. The contribution ratio of CODcr and ammonia nitrogen of NPS were reached 63% and 81% respectively.(2) According to the parameters validation of WASP7.3 model, the mean relative errors of influent flow which flow into Taihu lake was 18.3% for CODcr while thatwas 17.0% for effluent flow which flowing from the Lake. The results also showed that the mean relative errors of ammonia nitrogen reached 40.9% and 25.9% for influent flow and effluent flow respectively. Thevalidaiton results showed that the model could be used for water quality simulation for the rivers of study region. (3) An integrated regional management mode including factors of natural, social and economic was built here, and Geni coefficient was used to assess fairness of water environment distribution. In order to meet the needs of regional management, the reduction rates of CODcr and ammonia nitrogen must reached 19.7% and 22.7% on the level of 2005 in contract to corresponding water environment capacities, which were 11334.4 t/a and 1866.0 t/a under the 90% hydrographic confidence.(4)A comprehensive watershed management mode based on TMDL, dealt with water environment capacity distribution between point and non-point sources, as well as margin of safety was constructed at last. The results showed that CODcr of many rivers in the watershed met the function needs, while ammonia correspond was opponent. In order to meet the function needs of the whole watershed, the reduction rate of point and non-point CODcr reached 1465.1 kg/d and 3670.6 kg/d respectively and that of ammonia correspond reached 100.0 kg/d and 1860.1 kg/d.