Microplastics in rivers: a new mathematical model

Environmental plastics are a growing ecological concern. A new model, developed in collaboration with researchers at Oxford University, has advanced the understanding of how microplastics move through rivers.

In March, the UK Government’s Environmental Audit Committee launched an inquiry into the environmental impact of microplastics; earlier in December, the US Government passed legislation outlawing the use of plastic microbeads in toiletries by July 2017. These developments reflect growing concerns of the environmental impact of microplastics, the result of an increasing body of evidence highlighting various detrimental ecological impacts.

Plastic production currently exceeds 300 million tons per year and is steadily increasing. Microplastics are tiny fragments of plastic smaller than a few millimeters, such as microbeads used in exfoliators and injection moulding, or plastic debris resulting from the fragmentation of larger plastic objects. A fraction of these products is released directly into the sea, with the impact on the marine environment estimated to be around $13 billion per year.

Microplastics. Image: MPCA Photos. Licensed under CC BY-NC 2.0.

While the effect of microplastics on the marine ecosystems are well documented, little is known about the release and retention of microplastics in rivers. Households, industry, transport and poorly managed landfills generate a large volume of microplastic debris, which travel from these sources to river networks where they are partly transported to the sea and partly retained by the riverbed. A fraction of the microplastics released into the sewage network by households and industry is processed at wastewater treatment plants and retained within sewage sludge. This sludge is in turn used as agricultural fertiliser, releasing microplastics into the soil.

Researchers at Oxford University’s School of Geography and the Environment, Dr. Gianbattista Bussi and Prof. Paul Whitehead, helped advance the first theoretical assessment of the transport and diffusion of microplastics through a river network, as part of a NIVA (Norwegian Institute for Water Research)-led research collaboration.

The framework is based on the assumption that microplastics can be treated conceptually like sediment i.e. the mathematical equations governing the movement of microplastic particles are conceptually similar to those used for the assessment of sediment transport by water. The study adapted equations from an existing hydrological and sediment model, the INCA (Integrated Catchment Model) model, developed by Prof. Paul Whitehead, to assess the transport and depositions of microplastics in the River Thames (UK). In the absence of quantitative information on microplastics transport from the Thames, the study was conceived to provide a purely theoretical, nevertheless rigorous, assessment of microplastics transport across the pedosphere and hydrosphere.

The findings, published in Environmental Science: Processes and Impacts, a journal of the Royal Society of Chemistry, show that soils have a great potential to accumulate microplastics released by sewage sludge application. Furthermore, sediments of river sections experiencing low stream power are possible hotspots for the accumulation of plastics. On the other hand, particles smaller than 0.2 mm are predicted to be poorly retained in the catchment, regardless of their density, and will eventually be conveyed to the marine environment.

While research on microplastics in river environments is still at the very early stages, this study represents a novel approach that can be used to assess the potential of river catchments to retain microplastics. Given that the only realistic mitigation measures to curb the release of microplastics to the sea are those focusing on managing emissions and transport processes on land, it is hoped that this study will drive the efforts of researchers and catchment managers towards an integrated assessment of the presence of microplastics in rivers.