Spatial and temporal characteristics of pollution loads in Tuojiang River watershed located in Sichuan Province, Southwest of China

Tuojiang River watershed is the water supply area of Chengdu plain which is located in southwestern Sichuan Province, China, its safety of water environment directly impacts on social–economic sustainable development. In this study, we firstly used pollution discharge coefficient method to estimate the pollution loads of chemical oxygen demand (COD), ammonia nitrogen (NH3-N), total nitrogen (TN) and total phosphorus (TP) of main pollution sources in 28 districts (counties) of Tuojiang River watershed from 2007 to 2017. We then adopted principle component analysis and k-means cluster analysis to determine categories of pollution sources. We finally recognized pollution risk areas through global and local spatially autocorrelation analysis. The results suggested that the COD was the primary pollutant whose pollution load value can be up to 44.99 × 104 t. The livestock and poultry breeding, and urban life pollution sources were main pollution sources. The pollution loads were correlated to climate and intensive agricultural activities. The pollution sources types related to agricultural activities changed into pollution sources types related to urbanization levels because of changes in population and agricultural driving factors. The Yanjiang district located in the middle of study watershed was always in higher pollution risk, and the pollution of Fushun county located in the downstream of watershed had a spillover effect on its surrounding counties because of its high pollution risk. Therefore, it is vital to help environment decision makers make strategies of water environment management across watershed through quantification of temporal and spatial characteristics of main pollution loads and their driving factors.