Estimation of Unintended Treated Wastewater Contributions to Streams in the Yangtze River Basin and the Potential Human Health and Ecological Risk Analysis.

"Clean water and sanitation" is one of the United Nations Sustainable Development Goals. One primary objective of wastewater treatment is to remove contaminants such as pathogens, nutrient, and organic matter from wastewater, while not all contaminants could be removed effectively. Wastewater treatment plants would inevitably represent concentrated point sources of residual contaminant loadings into surface waters. This study focuses on the populated Yangtze River Basin where emerging contaminants are frequently detected in the rivers in the recent years. A python-based ArcGIS model is developed to estimate the contributions of effluent discharges in water supply sources and quantify fate and environmental risks of human-derived contaminants in the river network. We find that one-third of the river networks are potentially influenced by the effluents through local or upstream inputs. Average fraction of unintended wastewater reuse in water supply intakes is estimated to be lower than 3% under the average flow scenario with an average traveling time of 0.05 day from the nearest effluent input site to water supply intakes. However, under low flow scenario, the percentage of effluent discharge would increase largely, leading to substantial increases in human health and ecological risks. This study provides a systematic investigation to understand extents of impacts of effluent inputs in river networks as well as identify the opportunities to improve the water management in the densely populated regions.