Influence of the south-to-north water transfer and the Yangtze River mitigation projects on the water quality of Han River, China

The Middle Route of China's South-to-North Water Transfer (MSNW) and Yangtze-Han River Water Diversion (YHWD) Projects have been operated since 2014, which may deteriorate water quality in Han River. The 11 water samples along the middle and down streams of the Han River watershed were collected monthly from July 2014 to June 2016 to investigate the major pollution types and main variables influencing the water quality in the Han River using factor and cluster analysis approaches. The factor analysis identified three main pollution types (agricultural nonpoint source, organic, and phosphorus point source pollution) in the Han River while the cluster analysis classified all sampling sites into four groups and determined seasonal (dry and wet) pollution sources. The water quality at the sites nearby the city was dominated by point source pollution while nonpoint source pollution from Jianghan Plain prominently contributed to the water quality at the downstream sites except for city sites. Variations of water qualities are influenced by hydrological conditions that are dependent on operations of engineering projects and seasonal variability, especially in the dry season. Good water quality (defined as Class III or low levels) mainly occurred when the flow rate is greater than 800 m(3)/s during the dry season. The average flow rate below 583 m(3)/s may degrade water quality as Class V appeared at all monitoring sites under this condition. Given low flow conditions, thus, an increase in flow discharges from the MSNW and YHWD Projects may enhance the water quality for the Han River and further prevent algal blooms. The relationship between water quality and flow rate investigated in this study may provide useful information to develop sustainable water quality management under various flow conditions.