River plastic transport and deposition amplified by extreme flood

Abstract

Plastic pollution in the world’s rivers and ocean is increasingly threatening ecosystem health and human livelihood. In contrast to what is commonly assumed, most mismanaged plastic waste that enters the environment is not exported into the ocean. Rivers are therefore not only conduits but also reservoirs of plastic pollution. Plastic mobilization, transport and retention dynamics are influenced by hydrological processes and river catchment features (for example, land use, vegetation and river morphology). Increased river discharge has been associated with elevated plastic transport rates, although the exact relation between the two can vary over time and space. However, the precise role of an extreme discharge event on plastic transport is still unknown. Here we show that fluvial floods drive macroplastic (>2.5 cm) transport (items h−1) and accumulation (items m−2) in river systems. We collected unique observational evidence during the July 2021 flood along the whole Dutch part of the Meuse. Plastic transport multiplied by a factor of over 100 compared with non-flood conditions (3.3 × 104 versus 2.3 × 102 items h−1). Over one-third of the modelled annual plastic item transport was estimated to occur within 6 days of extreme discharge. Between Maastricht and Ravenstein (291 km and 131 km from the river mouth), plastic transport during the flood period decreased by 90%, suggesting that the dispersal of plastic mobilized during the flood is limited due to the entrapment on riverbanks, in vegetation and on the floodplains. Plastic transport and accumulation on the riverbanks decreased significantly along the river, corroborating the river’s function as a plastic reservoir. Using new observational evidence, we demonstrate the crucial role of floods as drivers of plastic transport and accumulation in river systems. Floods amplify the mobilization of plastics, but the effects are local, and the river-scale dispersal is limited. We anticipate that our findings will serve as a starting point for improving global estimates of river plastic transport, retention and export into the sea. Moreover, our results provide essential insights for future large-scale and long-term quantitative assessments of river plastic pollution. Reliable observations and a fundamental understanding of plastic transport are key to designing effective prevention and reduction strategies.

Modelling plastic fluxes with INCA-macroplastics in the Imus catchment: impacts of long-term accumulation and extreme events

“…Here, the sensitivity analysis points to two processes governing plastic loads variability: overland transport and the detachment from riverbanks (table 3). This is also in line with current research priorities proposed by van Emmerik et al (2023) to focus on quantifying mobilization thresholds, plastic transport velocities in response to varying driving forces, and deposition thresholds. Note that INCA-Macroplastics is designed to take these thresholds as input parameters.…”

Macroplastic Fate and Transport Modeling: Freshwaters Act as Main Reservoirs

“…In summary, we estimated that about three-quarters of the input macroplastic emissions were retained through long-term accumulation in river environments. This contrasts with microplastic retention behavior, which was found to be dominated by lakes, e.g., refs and – but is supported by field observation of macroplastic litter on river banks, e.g., refs – .…”

Modelling future coastal water pollution: impacts of point sources, socio-economic developments & multiple pollutants

“…Fourth, we acknowledge limitations in our modelling approach for pollutant retention calculations (see SI 3). For example, microplastic retention in rivers depends, amongst others, on discharge [44] and extreme weather events [45,46], which are not considered in our model because our model provides annual average pollution levels at the sub-basin scale. Fifth, our scenario assumptions may have resulted in over-or underestimations of future water pollution.…”