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.

Micro- and Nanoplastic Pollution of Freshwater and Wastewater Treatment Systems

“…Hydrodynamic effects can have greater influence in MNP distribution than population density, industrial activities, or sewage discharge and MNP concentrations in river shores, as observed in a case study in Germany [55]. As such, transport models can be useful tools to simulate MNP behaviour in riverine systems and evaluate the factors that control their transport and distribution; however, little focus has been given yet to investigate modelling approaches [73]. To examine riverine and wastewater inputs, sources and flow or discharge can be used as with other contaminants to predict MNP loading.…”

“…Fundamentally, if plastics behave in the same way as sediment, available hydraulic models can be easily applied to MNP load studies, and if they behave differently, the models can be fine-tuned to get their behaviour in properly. For example, use of a modified INCA-Contaminants simulator, utilising catchment hydrology, soil erosion and metereological controls for prediction of microplastic accumulation and distribution, revealed strong hydrolological controls in transport and storage of microplastics [73]. Heavier and larger microplastics ([2.0 mm) were more likely to be retained in hotspots for sediment deposition, but high flow events caused their remobilisation [73].…”

“…Physical drivers for sediment transport can be tested to build models for MNP transport and storage, and identify areas of high deposition [73]. Fundamentally, if plastics behave in the same way as sediment, available hydraulic models can be easily applied to MNP load studies, and if they behave differently, the models can be fine-tuned to get their behaviour in properly.…”

“…For example, use of a modified INCA-Contaminants simulator, utilising catchment hydrology, soil erosion and metereological controls for prediction of microplastic accumulation and distribution, revealed strong hydrolological controls in transport and storage of microplastics [73]. Heavier and larger microplastics ([2.0 mm) were more likely to be retained in hotspots for sediment deposition, but high flow events caused their remobilisation [73]. A similar approach can be used with other available models, for example the Delft hydraulics model (Delft 3D suite) for rivers and estuaries.…”

Microplastics in freshwater and terrestrial environments: Evaluating the current understanding to identify the knowledge gaps and future research priorities

“…To our knowledge, to 337 date there are no studies which quantify microplastic presence at terrestrial field sites. Based on the 338 above evidence, however, it is highly likely that microplastics will be present within terrestrial 339 environments and, if investigated in detail, may be found to be as equally pervasive as they are in 340 freshwater and marine environments (Nizzetto et al, 2016a). 341 .…”