Assessing de facto wastewater reuse and its implications for water quality in Yangtze Basin (2014-2021).

De facto wastewater reuse is the incidental presence of treated wastewater in a water supply source. Unplanned indirect wastewater reuse, which is also called de facto reuse, occurs when wastewater is discharged into surface water upstream of the intakes of suitable drinking water treatment plants. Although this discharged wastewater may increase the water quality risks for downstream water supplies, they contribute to the water supply source as an additional in-stream flow. Therefore, proper wastewater management is crucial to ensure access to safe water and address the challenges due to urbanization and population growth. There are detailed data on the infrastructure and river flows in some countries, but this study examined the use of a more limited dataset in other countries and specifically assessed the de facto reuse in the Yangtze River Basin in China. GIS modelling was used to calculate the streamflow of Yangtze River from the DEM, while macro water consumption data was used to estimate the wastewater discharges, and this methodology solved the problem that the outcome was limited by the lack of gauging stations at more locations and lack of precise geospatial location of drinking water intakes or wastewater discharges. Under an average-flow condition, Chongqing in the upper reaches had de facto reuses that increased from 0.57 % in 2014 to 0.60 % in 2021; Wuhan in the middle reaches had de facto reuses, which increased from 1.56 % in 2014 to 1.64 % in 2021; Shanghai, which is located in the estuary of the Yangtze River, increased its de facto reuse from 2.35 % in 2014 to 2.51 % in 2021. Under low-flow conditions, the de facto reuse of Chongqing, Wuhan, and Shanghai in 2021 was 1.7 %, 4.9 %, and 7.4 %, respectively. This relatively high level of de facto reuse is consistent with the estimation of unintended treated wastewater contributions to streams and growing risk of drinking water quality in the Yangtze River Basin. This research estimated de facto reuse in the Yangtze River Basin and showed the feasibility of using limited infrastructure datasets to assess de facto reuse and the modeling methods allow identification of river reaches most impacted by wastewater, and perhaps suggest prioritization for investments in additional wastewater treatment in reaches with high de facto reuse.