Macroplastic Fate and Transport Modeling: Freshwaters Act as Main Reservoirs.

Macroplastic fate and transport in the freshwater environment are of great concern due to the potentially harmful effects of macroplastic on plants, animals, and humans. Here, we present a modeling approach to simulate macroplastic fate and transport at the country scale based on an existing plastic release model. The fate model was parametrized through available monitoring data and results from field experiments and applied to Swiss rivers and lakes. We found that almost all (98%) macroplastic emissions into freshwater remain within Switzerland. After exploring the influences of weirs, retention in rivers, and retention in lakes through a sensitivity analysis, we found a high retention variability across different catchments and within rivers. In all 22 analyzed scenarios for continuous retention along each river bank (i.e., beaching), we found that at least 70% of input emissions into the water bodies would be retained long-term in the catchments (about 200 g per river km and year). Across all catchments, we found a dominance of "continuous retention" through beaching along the entire river length compared with "point retention" at weirs or lakes. Thus, by modeling macroplastic fate and transport on a country level for the first time, we were able to confirm the concept of "rivers as plastic reservoirs" through modeling.