Buttertubs spilled during the July 2021 flood as plastic transport tracers in the Dutch Meuse river

Rivers play a substantial role in plastic pollution transport and storage but the transport processes that determine macroplastic fate in the riverine environment are not fully understood yet. Usually it is unknown when and where specific plastic litter items entered the environment, therefore macroplastic transport is often studied via e.g. GPS trackers. However, the July 2021 flood provided an unique opportunity of spilled macroplastic items, with clearly known time and space of emission.
In July 2021 severe floods affected multiple European river catchments, including the Meuse catchment in Belgium. A dairy company located at the Meuse tributary Vesdre was flooded, with parts of their facilities and a lot of material washed away. Among the washed away material was also ~8 million empty dairy packages ("buttertubs"), which have a printed ID code that can be traced to their emission point. During macroplastic sampling immediately after the flood event, and in the following two years, we found 617 of these buttertubs along the Dutch section of the Meuse river (~66 - 328 km downstream of the dairy company). We used the buttertubs as tracers for macroplastic transport in the period that includes the flood event, and the following two years. Within 20 days of the flood event, some of the buttertubs were transported ~328 km and were found close to the Rhine-Meuse-Delta. However, the majority of buttertubs was transported less than 100 km within these 20 days, with an average transport distance between 9.75 - 18.25 km/day. Over the following two years the average transport distance decreased to 0.23 km/day. Which could imply that the buttertubs either were only transported across smaller distances in the following two years, or even not remobilized at all after being deposited during the flood event. Some of the buttertubs we also collected, and we investigated their mass and fragmentation development over time. 
In this unique opportunistic study, we found that the buttertubs mean transport distance moved downstream over the course of two years. The majority of them however, was deposited rather close to their emission point, even given the extreme flood situation.