Evaluation of the Hydrochemical and Biogeochemical Mechanism Driving Water Discoloration in an Urban River

This study investigated two discoloration events in the Bo-Ju Trench through comprehensive analyses of water quality, microbial communities, and algal assemblages. The first discoloration event, which exhibited a grayish-green appearance, was attributed to a bloom of Oscillatoria tenuis ( 56,260 cells / L ), belonging to the division Cyanophyta. During the second discoloration event (occurrence of a milky-white hue), the concentrations of linear alkylbenzene sulfonic acid (LAS) and grease were measured at 1.2–1.6 and 0.9 – 1.3 mg / L , respectively. Although humic substances, containing surfactant-like components, can induce the natural formation of milky emulsions, their levels in this event were relatively low ( 6.9 – 7.4 mg - C / L ). Next-generation sequencing showed that the microbial community in the Bo-Ju Trench was dominated by facultative or obligate anaerobic bacteria, many of which are capable of producing biosurfactants. This suggests that naturally occurring biosurfactants may have contributed additional surfactant components to the system. However, because the concentrations both of surfactants and of oil and grease were low, the formation of a stable milky emulsion layer through natural emulsification likely was minimal. Water quality analyses revealed that the elevated sulfate ( 1,123.5 mg / L ) and calcium ( 160.9 mg / L ) levels originated from tidal seawater rich in sulfate and from upstream trench water containing calcium-bearing minerals after storm events, respectively, approaching the solubility threshold of calcium sulfate at 33°C. Elemental analysis of the precipitates confirmed calcium sulfate formation, indicating that the milky-white appearance was caused primarily by natural calcium sulfate precipitation induced by tidal intrusion rather than by anthropogenic pollution. This finding advances scientific understanding of biogeochemical interactions in tidally influenced urban waterways. These contributions offer new insights and tools that enhance both environmental engineering practice and the broader understanding of urban aquatic system dynamics.