The Dongjiang River Network is located in the eastern part of the Pearl River Delta, of which the water quality and ecological health are of pivotal importance for regional development. With the advancement of pollution control, the pollution level of rivers has continued to decrease. However, the low dissolved oxygen has become the major factor restricting the further improvement of water quality in this area. Using continuous observations from multiple automatic online sampling stations in the region and the validated numerical model covering the Pearl River network and the nearshore area of Guangdong Province, this study explored the contribution of natural and anthropogenic pollution factors to hypoxia.

Due to the warm weather throughout the year, especially high water temperatures in summer,the saturated dissolved oxygen in the Dongjiang river network area in summer is about 7 mg/L. Considering the effect of oxygen-consuming substances, the dissolved oxygen is prone to fail below water quality standards. The longer transportation time in the Dongjiang River Network is also supposed to be an influential factor in hypoxia.

The numerical model simulates the water mass transport process during the rain pollution period in 2019 and 2020, and quantfied the contributions of water temperature rise and pollutant oxygen consumption to the hypoxia phenomenon. In the spring when the water temperature in the river network continues to rise, the large amount of runoff brought by rainfall interrupted the increasing trend, thus elevating the dissolved oxygen. However, in summer, rainfall-runoff with warmer water and the pollutants further reduce dissolved oxygen. Furthermore, this study explores the data-driven prediction of dissolved oxygen in this area. Combing natural and anthropogenic factors, suggestions for the setting of dissolved oxygen assessment standards are also discussed.