Soil and water

Microplastics aren’t just an ocean problem – they’re in our soils too


Microplastics aren’t always that easy to see. Photo: Chayanuphol / Shutterstock


Inertia

One of Covid-19’s environmental legacies may well be large amounts of plastic pollution. Since the start of the pandemic, scientists have warned of the long-term threat of throwing face masks and other PPE into the environment.

These large plastic objects break down into microplastics, which dump into our waterways and farmland. Microplastics are generally defined as solid plastic particles or synthetic fibers with a size between 1 and 5000 µm, or micrometers.

The problem is of course much bigger than face masks: industrial and private activities release microplastics into the environment in different ways, where they have a negative impact on ecosystems. Microplastics are found in many ecosystems and are even ingested by many species.

Microplastic pollution is best known as a pollutant in oceans, lakes and rivers. But microplastics also have an impact on the earth: they can break down the structure of the soil and harm the creatures that live there, for example when ingested by earthworms.

The two environmental problems are linked: microplastics in soils can also be transported from agricultural fields to rivers, lakes and the ocean by erosion and surface runoff.

In both water and soil, microplastics can have an extremely long lifespan. In soil, plastic particles can last up to a hundred years or more, depending on the characteristics of the plastic particles and the environmental conditions of the soil.

Count microplastics

We still don’t know much about how and how much microplastic pollution enters the environment.

One of the challenges is to determine the microplastic content of the soil. The difficulty is that the chemical and physical characteristics of microplastic particles are similar to those of organic matter, which makes it very difficult to separate the plastics from the soil.

This means that it is very expensive to analyze individual soil samples and the prohibitive cost limits the number of samples that can be examined at a time. The small number of samples available is normally not sufficient to derive regional information on emissions or pollution.

But we can use modeling to build a picture of the environmental situation on a regional scale. This can help identify pollution hot spots and potentially useful sampling sites.

Our recent study presents a regional environmental model to analyze microplastic emissions in German agricultural soils.

Three pollution paths

In Germany, microplastics in agricultural soils come from sewage sludge, compost and plastic films used in agricultural production (for example, mulch films in market gardening).

Farmers apply sewage sludge to use its organic matter and nutrients, including nitrogen and phosphorus, as organic fertilizer for crops. Microplastics such as textile fibers in clothing or microbeads in cleaning and cosmetic products enter sewage sludge via wastewater. This means that high concentrations are found near areas with a high population density and with a high rate of land application of sewage sludge in agriculture, such as Hanover in the center-north.

Microplastics enter the compost as fragments of plastic food packaging in collected biological waste or as plastic waste along parks or highways. Compost is applied for the same reason as sewage sludge. High concentrations of microplastic pollution from compost are identified in densely populated areas where bio-waste is used in agriculture, for example around Cologne.

The microplastic in plastic films penetrates the ground when the film is damaged, for example during its removal or change. Mulch film is applied in different vegetable production systems to protect fruit, maintain soil temperature and moisture, and prevent erosion and weeds. Pollution from plastic films is very high in areas where vegetables are grown under mulch such as areas south of Berlin or north-east of Munich.

Maps showing simulated regional microplastic pollution

Maps showing regional microplastic pollution simulated in agricultural soils by sewage sludge (left), compost (middle) and plastic mulch film (right). Image: Martin Henseler et al, Provided by the author

Our modeling shows that the concentrations of microplastics from the applied sewage sludge are higher than those found in the areas where they come from compost and plasticulture. Depending on their location, some hot spots might be more critical than others, for example if they are close to rivers and lakes. These results can help identify high risk areas as target regions for further soil sampling and analysis.

Building a Clearer Picture of Microplastic Pollution

In situations where empirical information on microplastic pollution is scarce, model-based analysis is a complementary tool that we can use to assess the situation, both current and future. But the results require continuous updating based on new knowledge on the quantities and impacts of microplastics in soils.

The regional results can also feed into other simulation models, which, for example, simulate the transport of microplastics through river systems to the sea. This approach is already being used for other environmental pollution problems, notably emissions of nitrogen and phosphorus.

Our general ignorance of the extent of microplastic pollution requires rapid progress in research.

More evidence is needed to assess the situation and decide whether it is necessary to put in place new environmental policies to tackle the problem. Until then, farmers will face the conflict of recycling bio-waste as fertilizer for the benefit of the circular economy, while polluting their fields with unknown substances.The conversation

This article was written by several authors: Martin Henseler, Research Engineer, EDEHN – Le Havre Normandie Economic Team, Le Havre Normandie University; Elke Brandes, scientist, Thünen Institute, and Peter Kreins, associate researcher, Thünen Institute. This article is republished from The Conversation under a Creative Commons license. Read the original article.