Rainwater, as it flows over urban surfaces (such as roads, rooftops, or recreational areas), picks up multiple pollutants that have accumulated there. If left untreated, these pollutants are discharged into aquatic ecosystems.

A study coordinated by the Catalan Institute for Water Research (ICRA), the University of Barcelona (UB), and the University of the Basque Country (EHU) (CityPoll project; https://citypoll.eu/) analysed urban runoff waters, identifying a long list of pollutants. Among the most prominent are heavy metals and compounds derived from tyre wear, detected at high concentrations. It has been shown that this urban runoff impacts aquatic organisms, affecting their growth, reproduction, or vital functions, which can have consequences for the functioning of freshwater ecosystems connected to urban drainage networks.

The study found large differences between areas within the same city and also between rainfall events, making the detection and analysis of urban runoff more challenging. The efficiency of green filters as mitigation structures for these impacts was also evaluated. Based on the results obtained, the study proposes using  diphenylguanidine — a chemical marker associated with tyre wear — as an indicator of urban runoff pollution. The study also points out that fully separating rainwater and urban wastewater collection networks is essential to minimise the impact of pollutants. Furthermore, it highlights the need to promote nature-based mitigation structures as treatment measures for urban runoff.

In the context of climate change, where rainfall events are less frequent but more intense, the effects of urban runoff pollution on receiving aquatic ecosystems may be exacerbated. Therefore, it is essential to minimise its impact by adapting our urban systems accordingly.