Urban rivers at wet seasons exhibit predominant self-purification capacity in pollutant concentrations than dry seasons at subtropical climate

Abstract

River self-purification is considered the most sustainable and cost-efficient method for controlling and improving water quality. However, there has been limited research on the temporal variation of river self-purification capacities. By incorporating probability distribution analysis, the Water Quality Index (WQI), and other effective statistical analyses, we accurately assess river self-purification capacities in terms of contaminant concentration reductions and water quality improvements. Results demonstrate that wet seasons consistently exhibit stronger self-purification capacities compared to dry seasons, resulting in significant reductions in pollutant concentrations and improved water quality conditions. Specifically, from May to October, the concentrations of solids (total solids, total volatile solids, and turbidity) and dissolved solutes (conductivity, salinity, and orthophosphate-phosphorus) were significantly reduced in high-magnitude and low-frequency concentrations. Moreover, the improvement in water quality conditions during wet seasons can be attributed to the reductions in 5-Day Biochemical Oxygen Demand (BOD5) and Ammonia-Nitrogen (Ammonia-N) concentrations, coupled with the ability to tolerate high dissolved oxygen (DO) levels. However, at finer temporal scales, flow rates have the potential to promote acidification, future water degradation, and pollution from solid contents. Therefore, it is crucial that future management strategies aimed at reducing river water pollutants and strengthening self-purification processes should be constructed at a seasonal or monthly level, rather than at a finer temporal scale.