The Plastic Underground: Are Microplastics in the Subsurface a Ticking Time Bomb for Soil and Groundwater Ecosystems?

Increasing volumes of mismanaged plastic waste have resulted in millions of tons of plastics entering the environment. While recent research has made substantial progress in determining the fate and transport of microplastics (MP) in river systems and their subsequent discharge to the worlds oceans, much less is known about the subsurface fate of MP as they enter soils, (riverine) sediments and global groundwater resources. Initial studies have identified MP in selected groundwater samples and there is great interest to understand entry pathyways of MPs into soils, in particular through agricultural pathways. The mechanisms of MP release from agricultural sources such as seed and agrochemical encapsulations or sewage sludges as well as the total quantity, spatial distribution, residence time scales as well as the impact of MP on soils and subsequently groundwater ecosystems are completely unknown. There is hence a critical need to study the role of soils and groundwater systems as a long-term sink for plastic pollution, including the development of legacy risks.

Here we identify the significance of various entry pathways for MP into subsurface ecosystems, integrating experimental and model based quantifications of MP transport through streambed sediments with quantifications of MP inputs into agricultural soils through irrigation with river water. We present first results of MP impacts on the functioning of subsurface ecosystem services, by the particular example of MP exposure impacts on the behaviour of bioturbating invertebrates and the subsequent consequences for sediment biogeochemical cycling in order to draw attention to the potential risks for vital soil and groundwater ecosystem services.

We complement this site specific mechanistic process understanding with global estimates of mismanaged plastic waste accumulation in river basins to quantify MP catchment wide loads available for leakage into soils and present first results of our recently started participatory approach that aims to develop a baseline of MP pollution in aquifers across the world. Such baseline data is imperative to increase our understanding of MP fate and transport processes, MP uptake by groundwater organisms and the interaction of MP with nutrients and potential co-contaminants. Our specifically tailored protocol allows for standardized MP sampling in boreholes, springs or wells across a wide range of geological settings and land cover classes. We invite and encourage the community to contribute to this global effort in order to enable estimates of the magnitude and expected time scales of soil and groundwater MP contamination.