Water quality evaluation and trend analysis during typical operation of large-scale long-distance open water diversion project

Water resources in northern region of China are severely scarce all year round, and water supply in the region is not guaranteed, and regional ecological environment is deteriorating day by day. In order to alleviate this phenomenon, national and local governments have implemented many cross-basin and cross-regional water resource allocation projects, the most famous of which is the China South-to-North Water Diversion Projects. This has also raised concerns from Ecological Environment Department about of water quality safety when implementing water diversion, especially under condition of connected river systems. This article takes Jiangsu section of the East Line of South-to-North Water Diversion Project as a research case. Based on water quality monitoring stations set up and monitoring data along the water conveyance rivers during water diversion operation from 2019 to 2020, single index method and comprehensive water quality identification index method are used to evaluate the water quality status of each water conveyance rivers, so as to two water source areas of Yangtze River and Hongze Lake. Trends of different water quality indicators along the line are given, and reasons for water quality changes are analyzed. Results show that during typical annual water diversion operation period, overall water quality of each water conveyance river meets the Surface Water Class III standard, rivers such as Sanyang River, Tonghe River, Jinbao Channel, et al., which are sourced from Yangtze River, are Class III and Class II. Xuhong River, which is sourced from Hongze Lake, is slightly worse than other rivers, and there is a Class IV situation in December 2019 and May 2020, because CODMn is not meeting Class III standard. Water quality evaluation of Yangtze River is classified as Class II all year, while Hongze Lake is classified as Class IV in some months, with main exceeding indicator being TP. Overall water quality in the line shows a trend of gradually deteriorating from south to north, which is closely related to the actual water quality background of Yangtze River and Hongze Lake, as well as the actual pollution load entering water conveyance rivers along the line. One of the most important reasons is that water conveyance river also plays as a drainage channel, which needs to undertake water withdrawal and drainage of regional agricultural production activities.