Monitoring litter typology and loading in stormwater: Implications for waste management and mitigation policies

Sewer systems are an overlooked pathway of litter to rivers. We monitored six combined and four separate sewer outlets in France using retention nets over several years to quantify and characterize over 30,000 items > 5 mm, including 91% plastic debris. Total median loading rates were 155 g ha^-1 yr^-1 (120 g.ha^-1.yr^-1 for plastics) and 8 g cap^-1 yr^-1 (6 g cap^-1.yr^-1 for plastics), consistent with the few previous studies available in stormwater and rivers, although litter typology is different. Combined sewers were dominated by sanitary products like wet wipes and towels (46% of plastic mass), while separate systems were dominated by cigarette butts and food packaging related to on-the-go consumption. Sewer monitoring proved complementary to river monitoring, with fewer unidentified items (∼15%) and clearer source attribution. Management strongly influenced maintenance costs: internal management and incineration by public authorities was far cheaper (800 € per net and per year) than outsourced landfill disposal (16,000 € per net and per year). Despite operational limits of nets, sewer monitoring provides robust data for litter flux models and for evaluating upstream mitigation policies. Expanding such monitoring, supported by sustained funding and international coordination, will afford reliable and homogeneous method for litter loading estimates into rivers and their source identification:an essential step toward an effective plastics treaty.