Establishment and application of the reverse source apportionment method for readily biodegradable organic matter-examples of urban rivers in Southern China during the rainy season.

In many urban rivers, such as those in Shanghai, intensifying hypoxia is becoming a critical environmental issue. The increased concentration of readily biodegradable organic matter (RBOM) and seasonal temperature increase are significant contributing factors to the hypoxia of urban rivers, often resulting in a series of environmental problems. Clarifying the primary sources of RBOM and analyzing the interaction between seasonal temperature increase and RBOM are of great importance in resolving the issues of the water system. Given the inaccuracies inherent in forward source apportionment techniques for RBOM, this study devised a reverse source apportionment approach based on the carbon isotopes of dissolved inorganic carbon (DIC), a degradation product of RBOM during the oxygen-consuming processes. The source apportionment of RBOM was subjected to analysis using a carbon isotope (δC) coupled Bayesian stable isotope mixing (BSIM) model with an analysis error rate of less than 5.5 %. Subsequently, this method was employed to the RBOM source apportionment from four urban rivers with hypoxia in Shanghai during the rainy season. The results demonstrated that hypoxia was primarily caused by elevated RBOM concentration (67.4 %) and seasonal temperature increase (32.6 %). Source apportionment indicated that RBOM was mainly derived from domestic sewage (Ds, 37.37-66.89 %) and phytoplankton (Phy, 33.11-62.63 %), as confirmed by fingerprinting analyses of particulate organic matter (POM) and dissolved organic matter (DOM), which further validated the accuracy of the method established. To mitigate urban river hypoxia, efforts should focus on controlling RBOM sources by integrating infrastructure upgrades, ecological restoration, and optimized sluice dispatching. Future work should incorporate multi-depth and longitudinal monitoring, and microbial degradation kinetics of RBOM to better support targeted and refined management strategies for the hypoxia of urban rivers.