Impacts Of A Single Untreated Domestic Effluent Discharge On Water Quality In An Amazonian Urban Stream: An Integrated Assessment Using Wqi And Legal Compliance Indicators

Urban streams are highly vulnerable to localized sources of pollution, particularly untreated domestic effluent discharges, even in regions characterized by high freshwater availability. This study evaluated the impacts of a single untreated domestic wastewater discharge on water quality in an urban tropical stream located in the Amazon region, using an integrated assessment framework combining the Water Quality Index (WQI) and a Legal Compliance Index (LCI). Physicochemical and microbiological parameters were monitored at three sampling points—upstream, at the effluent discharge, and downstream—during rainy and dry seasons. The results revealed severe degradation of water quality at points directly and indirectly influenced by effluent discharge, with recurrent exceedances of legal thresholds for dissolved oxygen, biochemical oxygen demand, total phosphorus, turbidity, and microbiological indicators. WQI values classified water quality as poor to very poor at all impacted points, with minimal variation between hydrological periods, indicating the dominance of continuous point-source pollution over seasonal effects. The LCI analysis confirmed widespread legal non-compliance, demonstrating that a single discharge point controlled the overall regulatory condition of the stream. The combined application of WQI and LCI provided a robust and complementary diagnosis of environmental and legal water quality conditions, highlighting limitations of relying solely on natural dilution capacity in water-abundant regions. The findings emphasize the need for targeted wastewater management interventions and spatially explicit monitoring strategies to protect urban streams. The integrated assessment framework applied in this study is transferable and can support water quality monitoring and management in other urban watersheds affected by localized pollution sources, particularly in rapidly urbanizing tropical regions.