Tracking the spread of microplastics

“Microplastic pollution in the marine environment is one of our biggest environmental challenges. It can affect critical ocean currents such as the Gulf Stream, alter ice melt and impair plankton behaviour and survival,” says Bijan Dargahi , Associate Professor of Hydraulic Engineering and visiting researcher at the Department of Sustainable Development, Environmental Science and Technology at KTH.

Microplastics in the oceans come from many sources – everything from households and sewage treatment plants to industrial emissions. The particles can float long distances with the ocean currents, accumulate in large concentrations or sink to the bottom.

To better understand these movement patterns, Dargahi has developed a modelling tool that tracks each individual particle and how it is affected by water movements and wave forces.

First step

The research has not yet been published, but is based on his previously published and acclaimed studies on similar modelling tools.

“Understanding the physics behind how plastic is transported in water is the first step in reducing or controlling microplastic pollution,” he says.

Using data on currents, waves and temperature, the model can simulate what happens to microplastics in the sea over time. It takes into account that plastic can be washed ashore, covered by organisms, broken down, split into smaller pieces, sink to the bottom or be swirled up again.

“All the forces that affect microplastics are included in the model. This makes it possible to track how individual particles actually move, unlike previous research, which has often focused on measuring microplastic levels rather than their path through the sea”.

Protect the Baltic Sea

The tool is specially adapted for the Baltic Sea, where the aim is to protect a unique and highly sensitive marine environment. The goal is to be able to map how microplastics spread along the coasts, around islands and on the seabed – and how much floats and how much sinks into deeper water.

At the same time, Dargahi emphasises that technical solutions for cleaning the sea, such as filtering, breaking down or binding microplastics, are only of limited use.

“It is certainly impossible to predict how useful technological developments will be. But cleaning the oceans of microplastics on a large scale is extremely difficult. The most important thing is to reduce the amount of plastic entering the oceans in the first place,” he says.

The aim is for the research to contribute to increased awareness and stronger commitment, both among the general public and decision-makers.

“Plastic must be prevented from ending up in the sea. This requires strong policies and measures that stop pollution at source”.

Text: Christer Gummeson ( gummeson@kth.se )

How much microplastic is there?

The Earth's oceans contain an enormous amount of plastic particles, but there are still uncertainties in the research because it is difficult to measure. Dargahi refers to estimates that the global ocean contains at least 5.25 trillion plastic particles, weighing approximately 250,000 tonnes, floating on the surface. There may be up to 10,000 times more particles on the seabed. These figures are estimates and may change as methods improve.

All types of plastic break down over time from macroplastics to micro- and nanoplastics through various processes. Larger plastic fragments gradually break down into smaller and smaller particles. Microplastics (less than 5 millimetres) easily mix with marine sediments and are difficult to separate, while nanoplastics (less than 1 micrometre) can penetrate living cells and are virtually impossible to control.

The research in brief

A new three-dimensional model has been developed to track how microplastics move in the ocean. The model is known as Lagrangian, which means that it tracks individual plastic particles as they are transported by currents and waves. It has several special features, including that it is based on theoretical sub-models that describe biofouling, i.e. how bacteria and other microorganisms grow on the surface of microplastics and affect their behaviour in the water. The model also uses microplastic properties developed in a laboratory environment.

In addition, the model takes into account the circular movements of waves and how the buoyancy of microplastics changes when the water temperature varies. The model has been used to study the Baltic Sea and to track how microplastics spread from discharges in ten major rivers. Previously, there have only been a few studies modelling microplastics in the Baltic Sea, two of which use a so-called Eulerian method, where concentrations in fixed areas are studied, and one that uses a Lagrangian method.