Preferential deposition of macroplastic on floodplains

Floods have a large impact on the mobilization, distribution, and retention of plastic in rivers. During floods plastic transport in rivers increases, due to flushing of additional material into the river and (re-)mobilization of macroplastic previously deposited on floodplains and buried in the riverbed. Some of the transported macroplastic is deposited on riverbanks and floodplains, either by entanglement in inundated riparian vegetation or by deposition on the ground. However, the specific deposition patterns, i.e. where precisely macroplastics are deposited across the floodplains are poorly investigated. Here, we show the deposition patterns of macroplastic along two major Dutch rivers following flood events. We sampled 25 floodplain sites along the Meuse following the 2021 summer flood, and 23 floodplain sites along the IJssel following the 2024 winter flood. For each found item we recorded the landcover element (such as grass, herbaceous, bushes, tree, debris pile) it was deposited in. We found that along the IJssel, the majority of macroplastic were deposited in debris piles (>50% by item count, 12.7 items/m²). Along the Meuse, the highest macroplastic was found in trees (3.0 items/m²) and in debris piles (5.67 items/m²). The macroplastic composition in different landcover elements varies. The composition of macrolitter deposited on the ground in grass, also varies from plastic deposited off the ground, entangled in previously inundated vegetation. Out of the macroplastic deposited in riparian vegetation, 84% were soft plastics or textiles, compared to 53\% of items deposited on the ground. Of the items deposited on the ground, 31% were rigid plastic and these same categories only made up 7% of items deposited in vegetation. Along the Meuse, we found that items deposited in trees and bushes were of significantly larger average size and mass (17.7 cm and 13.4 g) than items deposited on the ground (13.3 cm and 12.2 g). Along the IJssel, debris piles contained the highest macroplastic count and mass concentration (12.7 #/m² and 25.0 g/m²). Debris depositions covered 2% of the sampling area, yet contained 58% of the plastic items and 32% of the plastic mass. Macroplastic entangled in vegetation were more likely to be soft and flexible, compared to macroplastic deposited on the ground. Floods play an important role in plastics pathway through the environment. This paper sheds additional light on the role of vegetation and debris piles on plastic retention caused by floods. Our results can be used to help improve the monitoring, mitigation and reduction of river scale plastic pollution.