Spatio-temporal distribution of phosphorus pollution in the upper reaches of Beijing-Hangzhou Canal and its source analysis

To reveal the reason why total phosphorus failed to consistently meet the Class Ⅲ standard of Environmental Quality Standards for Surface Water (GB 3838-2002) in Tongxiang segment of Beijing-Hangzhou Canal, 24 sampling sites were set up in Tongxiang section of the mainstream and 18 sampling sites were set up in the tributaries to carry out water quality monitoring and study the spatio-temporal changes of phosphorus pollution. The main sources of phosphorus pollution in key river segments were analyzed by the principal component analysis based on routine water quality indexes, and the correlation analysis of strong load index in each principal component factor and three-dimensional fluorescence component. The quantitative evaluation of the contribution of major pollution sources was conducted using the absolute principal component-multiple linear regression model. The research results revealed that the total phosphorus concentration originating from Tongxiang segment of Beijing-Hangzhou Canal's upper reaches was 0.14-0.20 mg/L, while the water quality along the monitoring sites 5-7, 9 and 21-24 tended to deteriorate significantly, with the highest concentration reaching 0.40 mg/L. Certain tributaries into the mainstream exhibited poor water quality, and the total phosphorus concentration reached 0.44 mg/L. Three main factors were obtained by the principal component analysis. Factor 1 was mainly loaded with ammonia nitrogen and dissolved phosphorus, and was significantly correlated with protein-like component, representing production and domestic pollution. Factor 2, with permanganate index, dissolved phosphorus and particulate nitrogen as the main loads, was significantly correlated with humic-like component, representing agricultural sources. Factor 3, with particulate phosphorus and nitrogen as the main loads, was significantly correlated with turbidity, representing the dock and sediment source. The phosphorus pollution in the upper reaches of the canal mainly occurred at the monitoring sites 5-7 and 9, mainly from the dock and sediment sources, with contribution rates of 65.9% and 31.8% during the high and flat water periods, respectively. At monitoring sites 21-24, agriculture was the primary source of phosphorus pollution, contributing at rates of 34.0% and 32.1% during the high and flat water periods, respectively. Furthermore, the production and domestic pollution exhibited a significant influence during the high water periods. The contribution rates of the prodution and domestic to the monitoring sites 5-7, 9 and 21-24 were 42.6% and 31.8%.