Small rivers and streams as pathways for urban litter to the marine environment

Resumen

Recent studies have highlighted the importance of large rivers as contributors to the global issue of plastic accumulation in the marine environment. However, data on plastic input from small rivers and streams are scarce, possibly misleading the relevance of a large pool of entry points along coastal areas all over the world. This study is focused on assessing floating macro plastic input from 11 small rivers in an urban coastal area of Portugal. Frequent visual observations allowed identifying the most common plastic items and calculating plastic flux into the sea in the long term. Plastic and polystyrene pieces, bags and bottles ranked among the top items, representing 90% of total plastic items. The observed average plastic flux was 1.73 items per hour, with individual monitoring values ranging from zero to 74 plastic items per hour. Extrapolating the average plastic flux for the 11 monitoring sites selected in this study, this coastal section (approximately 20 km length) could be receiving ca. 167,000 plastic items per year through small rivers and streams. These results provide a novel view on the role of small rivers as pathways for plastic in populated coastal areas that is of interest for environmental management and urban planning.