The concentration of floating microplastics on some Barcelona beaches is very high, reaching up to 44 fragments per square metre—levels higher than those recorded in major marine litter accumulation zones such as the South Pacific Gyre.  

It is estimated that, in recent years, the amount of micro- and macroplastics entering the sea from the Llobregat and Besòs rivers and from combined sewer overflows (CSOs), which discharge both rainwater and wastewater into the sea, totals around 40 billion particles per year, equivalent to approximately 7 tonnes. Most of this waste comes from the Llobregat River, which contributes around 28 billion particles annually (4.4 tonnes), followed by the Besòs River, with 11 billion particles (1.7 tonnes). Another 550 million particles (1.2 tonnes) come from CSOs.

Approximately 90% of the litter discharged by rivers and CSOs returns to the coastline within roughly 28 days. Currents, wave action and coastal orientation determine how this waste is transported and where it accumulates on beaches. A large quantity of floating litter is also observed depending on discharge flow rates, meaning more waste is detected after episodes of heavy rain, when river and sewer flows are higher.

The most common types of plastic found are transparent fragments and films, mainly made of polyethylene and polypropylene. Other identifiable sources include wet wipes and fibres from artificial grass, which together represent about 12 million particles per year.

These are some of the findings of a study carried out by researchers from the Maritime Engineering Laboratory (LIM) of the Universitat Politècnica de Catalunya - BarcelonaTech (UPC) and the Marine Geosciences research group of the University of Barcelona (UB), as part of the project TRACE (Tools for a better management of marine litter in coastal environments to accelerate the tRAnsition to a Circular plastic Economy).

The project is funded by the Ministry of Science and Innovation and by the Spanish Agency for Strategic Projects Aimed at the Ecological and Digital Transition. Its aim is to develop prediction and monitoring tools for floating litter to support the conservation, sustainable use and management of coastal areas. To this end, Barcelona has been used as a case study. Over the course of two and a half years, the research team conducted sampling campaigns along the Barcelona coastline using drifting buoys, water turbidity sensors and specially designed nets for collecting micro- and macroplastics. Samples were also collected at the mouths of the Llobregat and Besòs rivers and at the outlets of several CSOs.

A citizen science project
The study included a citizen science initiative in which volunteer groups collected microplastic samples on various Barcelona beaches, both directly from the sand and using microplastic trawl nets deployed from paddle boards or kayaks. The sampling was coordinated with the Spanish delegation of Surfrider Europe.

The samples were analysed in the laboratories of the University of Barcelona, where the waste was categorised by plastic type, colour and shape to help trace its origin.

Based on the collected data and the analysis of variables such as wave conditions, currents and beach type, UPC researchers developed a numerical modelling tool to simulate the journey of plastic waste from river mouths and outfall points into the sea, and to identify the areas where it is most likely to accumulate.

The final results of the project were presented at an event held in the auditorium of the Vèrtex building on the UPC’s North Diagonal Campus on 20 May. (Watch the video of the event.)