Urban areas generate large quantities of surface runoff, which is loaded with dissolved organic matter (DOM), hydrocarbons, nutrients, heavy metals, pesticides, etc. The composition of urban surface runoff varies significantly at temporal and spatial scales. Limited knowledge of the types and concentrations of pollutants can lead to severe pollution of aquatic ecosystems. This study aims to evaluate the temporal and spatial variation in surface runoff fluorescent DOM due to its potential impact in water systems, on active microbial populations in sediments or on the transport of pollutants, as DOM has the ability to bind heavy metal ions and other persistent pollutants. Samples were collected, over 24 months (2023 – 2025), from six points, in the south of Bucharest, Romania. The sampling location has a combination of urban and peri-urban areas, with different traffic surface cover and anthropogenic activities (moderate road traffic; heavy road traffic; residential, moderate road and pedestrian traffic; agricultural–residential; low road traffic, park, private and public buildings, partially paved; and residential, ongoing construction, unpaved, partially agricultural). Fluorescence peaks corresponding to humic and microbial fractions were identified. Samples taken from high and moderate traffic areas showed fluorescence peaks with similar intensity and trends. The concentration of humic matter increased from spring to late autumn. In winter, all samples from these locations were dominated by microbial matter. The results could be explained by the accumulation of plant matter towards autumn, which led to increased microbial activity. Also, an increase in humic matter was observed during summer. All fluorescence peaks revealed a significant difference (p < 0.05) between agricultural samples and high and moderate traffic samples. Also, the samples from unpaved surfaces showed a different fluorescent signature compared to samples with road traffic.

This research was funded by the Ministry of Education and Research, grant number OPTRONICA VII PN23 05 (11N/2023).