Monitoring plastic debris in urban stormwater: fluxes and management issues

Sewage systems may be the preferred pathways for plastic debris from urban areas to the natural environment during wet periods. Some French local authorities are trying to prevent this leakage into the environment by equipping combined sewer (mixed of stormwater and wastewater) or stormwater outfalls (separate sewer systems) with nets. More than a curative solution, these devices represent a unique opportunity to monitoring urban litter, including plastic debris, as close as possible to their source of emission, i.e., urban areas. Since 2020, nets are being (or have been) in used in French cities. In several cities, anthropogenic litter from the nets was collected, washed, air dried and sorted according to the J-list classification (Fleet et al., 2021), which is the updated European classification first developed for marine and riverine litter (MSFD Technical Subgroup on Marine Litter, 2013). Results show that urban waters are a major source of macroplastics for rivers, with mass flows per capita within the orders of magnitude of those estimated in French rivers (1-10 g/cap/yr). In addition, mass flows and items categories differ relative to the type of sewage systems, land use and local specificities. In combined sewer, wipes are by far the main waste found in nets often followed by tobacco-related products and sweet wrappers from roadways. In stormwater run-off, tobacco-related products and sweet wrappers are the main categories by numb, but bottles (in metal, glass and plastic) rank TOP 5 by mass. Acquiring those data is a very harsh task and a dedicated technical platform is under development to extend monitoring at the national level (or beyond) over the long term.