Unexpected River Water Quality Deterioration Due to Stormwater Management in an Urbanizing Watershed

Abstract Sustainable stormwater management (SSM) has gained global interests to solve both on‐site and off‐site downstream water problems. However, current structural SSM practices are often limited to small scales and their impacts on river water quality have not been fully understood. In this study, we have quantified watershed responses to SSM via multiyear monitoring and integrated modeling in an urbanizing watershed in China, while five scenarios (baseline, current SSM, improved SSM, future traditional urban development, and future improved SSM scenarios) were comparatively analyzed. The results showed that water quality of ammonia nitrogen (NH3‐N), total phosphorus (TP), chemical oxygen demand (COD), and suspended solid (SS) would deteriorate by both traditional urban development and current SSM design. Compared to other pollutants, NH3‐N and TP showed greatest deterioration by current SSM due to disproportionate management of on‐site runoff quantity‐related and quality‐related problems. Comparatively, the improved SSM design, which incorporated watershed responses into consideration, would reduce current concentrations of NH3‐N, TP, COD, and SS by 0–53.85%, 8.16–38.84%, 23.07–61.03%, and 68.82–77.82%, respectively; and improve water quality by 60–80% in future SMM scenario. Priority management and stricter objectives should be given to those urbanizing regions at the watershed scale. The results of this study should be used as references for future SSM design as well as water resource protection.