Pipeline misconnections in separated sewer systems worsen urban river ecological status through stormwater discharge

The widespread issue of pipeline misconnections in urban drainage networks has posed escalating threats to urban aquatic ecosystems. Urban river ecosystems face mounting pressure from sewage-contaminated stormwater discharges, yet the ecological response mechanisms and recovery potential remain poorly understood. Here, we conducted a comparative analysis of three urban rivers experiencing different levels of overflow pollution during a monsoon season, examining both immediate impacts and subsequent recovery. We monitored overflow discharge characteristics, tracked changes in nutrient profiles and emerging contaminants, and analyzed shifts in benthic microbial communities. Results showed that sewage contamination in stormwater systems significantly amplified pollutant loads, with ammonia nitrogen concentrations up to 2.34 times higher than in uncontaminated overflows. Post-rainfall monitoring revealed that rivers receiving sewage-contaminated overflows exhibited persistent elevation in nutrients and emerging contaminants, particularly antibiotics, alongside marked alterations in microbial community structure. These impacted rivers showed reduced microbial diversity, disrupted species interactions, and compromised nitrogen cycling functions. Notably, even after two weeks without overflow events, microbial communities in sewage-impacted rivers demonstrated limited recovery, suggesting lasting damage to their ecological resilience. Network analysis revealed that prolonged exposure to sewage-contaminated overflows drove microbial communities toward simplified, stress-tolerant assemblages with diminished functional diversity. This study provides the first systematic evidence linking sewage-contaminated overflows to impaired ecological recovery potential in urban rivers, highlighting the urgent need for enhanced infrastructure management to protect urban water ecosystem resilience.