Research On Inflow And Outflow River Pollution Characteristicsin Lake Taihu Basin

Dealing with water quality deterioration of lakes should start from dealing with that of rivers. Rivers are most important pass ways for connecting lakes and their watershed. In order to understand spatiotemporal variation patterns and their controlling factors of nitrogen, phosphorus and other pollutants from outflow and inflow rivers of Lake Taihu, we analyzed the variation of different forms of nitrogen and phosphorus concentration with water samples collected at 53 inlets or outlets of the main outflow and inflow rivers during 2008-2010. The field works were carried out to monitor the hydrological, biochemical characteristics of those rivers. By using multiple statistics analysis method and buffer analysis method we also analyzed the relationship between river water qualities and their watershed land-using patterns to find the governing factors on spatiotemporal variation patterns of river water quality. The annually-averaged water balance between inflow and outflow to the lake and annual amount of nutrient loading were also calculated. The results showed that:1) Only 36.9% of the main outflow and inflow rivers passed the National Environmental Quality Standards for Surface Waters (NEQSSW) for the pollution control, and the failing of those rivers to pass the NEQSSW was because of ammonia nitrogen exceeding. The passing percentages of rivers in upstream regions (Wuxi region and Wujin of Changzhou region) were much lower than their counterparts in downstream regions (Huzhou region and Suzhou region).2) River water qualities were mainly in water quality level gradeⅢ(27%) and gradeⅣ(31%) during the study period. Water qualities of outflow rivers were much better than that of inflow rivers, and for seasonal variations, water qualities were the worst in winter and the best in summer;3) Lake water balance calculation showed that annual water input from rivers was 9.40×109 m3 and annual output from rivers was 1.02×1010 m3. Annual inputs of TN and TP were 5.11×104 tons and 1.9×103 tons, while annual outputs of TN and TP were 3.6×104 tons and 1.1×103 tons respectively. The retention rates of dissolved forms of nutrients were much high than that of floating particle forms of nutrients, which was thought to be because of the ecosystem consuming in the lake. The input and output amounts of nutrients have been increasing since 2000. The artificial water loading from Yangzi River to the lake has effectively improved the water quality of the lake. The in-lake water qualities were significantly correlated with water qualities of nearby rivers, which indicated that the in-lake water qualities are strongly influenced by their nearby rivers;4) Multiple statistics analysis showed that nitrogen was the leading pollution factor, and phosphorus the second one for the water pollution status in the inflow rivers, while COD was the leading pollution factor, and phosphorus the second one in the outflow rivers;5) Investigation of spatiotemporal variation patterns of nitrogen showed that there were clear seasonal variations for every forms of nitrogen, the concentrations of nitrogen decreased in a direction of winter, spring, summer and autumn. The concentrations of nitrogen were higher in rivers on the West-North part of the lake, and lower in the East-South part. DTN was the major form of TN (77.81%), within which NH4+-N,NO3--N and S-org-N occupied 35.37%,41.15% and 23.48% of the DTN respectively.6) Investigation of spatiotemporal variation patterns of phosphorus showed that there were not clear seasonal variations for phosphorus. Similarly with that of nitrogen, the concentrations of phosphorus were higher in rivers on the West-North part of the lake, and lower in the East-South part. DTP occupied 40-50% of TP, the percentages in the inflow rivers were higher than that of outflow rivers, PO43-P is the major form of DTP (61.51%). Adversely, PP occupied 50-60% of TP, and the percentages in the inflow rivers were lower than that of outflow rivers.7) Buffer zone analysis showed close relationships between river water qualities and their watershed land-using patterns, displaying distance scale effects with positive-to-negative correlation conversion points. This study provided a theoretical basis for water pollution control and management in regions of floodplain river network in Lake Taihu Basin.