[Spatial-Temporal Variation of Water Environment Quality and Pollution Source Analysis in Hengshui Lake]

To study the spatial-temporal variation of water environment quality in Hengshui Lake and determine the associated pollution sources, we used historical water monitoring data (from 2000 onwards) and data from 17 sites sampled in 2019 to determine the trophic level index (TLI), comprehensive water quality index (WQI), and water environment quality index (EQI). The results showed that the proportion of monitoring points reaching level Ⅲ increased from 2000 to 2019. The permanganate index, total nitrogen, and total phosphorus were the main water environmental indicators. Spatially, TLI, WQI, and EQI all generally decreased from the south to the middle and west of the lake, and then further decreased towards the northeast. After the establishment of the Hengshui Lake National Nature Reserve, a series of water body protection policies and measures were implemented. These interventions are reflected in reductions in the TLI, WQI, and EQI between 2000 and 2019 by 20.9%, 53.4%, and 49.2%, respectively. However, side seepage and sewage discharge, agricultural non-point source pollutants transported by water diversion, and the decay of plants in the lake present significant challenges for water quality in Hengshui Lake.