Chemical Identification and Environmental Fate of Fluorescein during Celebratory Surface Water Dyeing: A Field Evaluation in the Chicago River

Abstract A growing number of cities dye waterways to celebrate significant cultural or social events. However, the ecological safety of these practices has not been evaluated in the open literature. The Chicago River has been dyed annually since 1962, yet the identity, fate, and impacts of the applied dye have not been independently verified. Field measurements combined with chemical analyses identified the dye as fluorescein. Surface water fluorescein concentrations reached 63±4 μg/L within 2 h of application and declined to near-background levels within days with an apparent attenuation half-life of ∼15 h, consistent with photochemical transformation. Measured concentrations were well below predicted no-effect concentrations, indicating low ecological risk. These results provide an empirical evaluation of widely cited environmental claims regarding the safety of a well-known celebratory dyeing event.