title: A theoretical assessment of microplastic transport in river catchments and their retention by soils and river sediments.
abstract: The presence of microplastics (MPs) in the environment is a problem of growing concern. While research has focused on MP occurrence and impacts in the marine environment, very little is known about their release on land, storage in soils and sediments and transport by run-off and rivers. This study describes a first theoretical assessment of these processes. A mathematical model of catchment hydrology, soil erosion and sediment budgets was upgraded to enable description of MP fate. The Thames River in the UK was used as a case study. A general lack of data on MP emissions to soils and rivers and the mass of MPs in agricultural soils, limits the present work to serve as a purely theoretical, nevertheless rigorous, assessment that can be used to guide future monitoring and impact evaluations. The fundamental assumption on which modelling is based is that the same physical controls on soil erosion and natural sediment transport (for which model calibration and validation are possible), also control MP transport and storage. Depending on sub-catchment soil characteristics and precipitation patterns, approximately 16-38% of the heavier-than-water MPs hypothetically added to soils (e.g. through routine applications of sewage sludge) are predicted to be stored locally. In the stream, MPs < 0.2 mm are generally not retained, regardless of their density. Larger MPs with densities marginally higher than water can instead be retained in the sediment. It is, however, anticipated that high flow periods can remobilize this pool. Sediments of river sections experiencing low stream power are likely hotspots for deposition of MPs. Exposure and impact assessments should prioritize these environments.
authors.affiliation: Norwegian Institute for Water Research, NO-0349, Oslo, Norway. Luca.nizzetto@niva and Research Centre for Toxic Compounds in the Environment, Masaryk University, 62500, Brno, Czech Republic.
authors.forename: Luca
authors.initials: L
authors.lastname: Nizzetto
authors.affiliation: School of Geography and the Environment, University of Oxford, South Parks Road, Oxford, OX1 3QY, UK.
authors.forename: Gianbattista
authors.initials: G
authors.lastname: Bussi
authors.affiliation: Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden.
authors.forename: Martyn N
authors.initials: MN
authors.lastname: Futter
authors.affiliation: Norwegian Institute for Water Research, NO-0349, Oslo, Norway. Luca.nizzetto@niva.
authors.forename: Dan
authors.initials: D
authors.lastname: Butterfield
authors.affiliation: School of Geography and the Environment, University of Oxford, South Parks Road, Oxford, OX1 3QY, UK.
authors.forename: Paul G
authors.initials: PG
authors.lastname: Whitehead
mesh_terms: D000383:Agriculture; D004784:Environmental Monitoring; D019015:Geologic Sediments; D008962:Models, Theoretical; D010969:Plastics; D045483:Rivers; D012987:Soil; D012989:Soil Pollutants; D014873:Water Pollutants
chemical_list: D010969:Plastics; D012987:Soil; D012989:Soil Pollutants; D014873:Water Pollutants