Microplastics (MPs) are an environmental problem of great concern. Agricultural fields are believed to be hotspots of this pollution. Contaminated biosolids and sewage sludge used as fertilizer (along with other sources) can be important inputs of MPs to these environments. Before the beginning of this project very little information was available on the occurrence, behaviour and impacts of MP in soils. IMPASSE represented therefore a seminal research to solve these knowledge gaps. The project was a collaborative effort xiresearch institutes NIVA (Norway, Coordinator), IMDEA (Spain), SLU (Sweden), University of Amsterdam (the Netherlands), University of Windsor (Canada) and University of Ljubljana (Slovenia). The objectives of the study were to assess the inputs of MPs from the use of sewage sludge into representative agricultural fields, assess MP accumulation in soil, the possible release to freshwater ecosystems and the impact on soil and freshwater organisms.
In order to carry out this study we first developed a method to analyse MP in soil and sludge samples. This is a very challenging task. IMPASSE substantially contributed to advance knowhow in this area through developing a validated method for measuring MPs down to 50 micron in size. The method was applied to several hundreds of samples throughout the project; an effort that required over 3500 hours of laboratory work. We analysed samples from two case studies in Canada and Spain with very different climate and soil characteristics. In both cases, we found that the sewage sludge used as fertilizer contained large amounts of MPs (e.g. > 10,000 MP per kilogram of dry weight). In both studies we found that soils that had a long history of sludge application contained more MPs than soils with only recent treatments or soils that were never treated with sludge, suggesting that this is an important source of MPs and that farming soils can retain MPs over multiple years leading to a progressive increase in soil contamination. MPs can however also be washed out from soils by intense rain that also lead to soil erosion. During the study in Canada intense rain events occurred. We observed that they caused the release of a majority of recently added particles to soil. Baseline precipitations instead generally tend to mobilize only a very small fraction (i.e. <1%) of the particles present in soil. Even background leaching of MP from agricultural soils can however result in the release to freshwater of tens or hundreds of tonnes of MPs every year in Europe and North America. Plastic that remain in soils can undergo vertical translocation and accumulate differently at different depths in soil. This can be important for the interaction with organisms inhabiting different soil habitats.
We investigated that river sediments and stream water in agricultural catchment contains considerable amounts of microplastics. Polyethylene fragments and fibers are most commonly found, in agreement with the profile observed in the sludge and soil samples.
We have conducted a number of tests on fresh water and soil organisms that confirmed ingestion of MPs by terrestrial and aquatic invertebrates. No observable effects on mortality were detected at environmentally relevant concentrations. In some soil organisms, however the presence of MPs had a marginal effected on the organisms biomass development and reproduction. In addition, some organisms could modify MPs characteristics. For example, we found that earthworms produced faeces containing polyester microfibers shorter compared to those added to their food during the test, suggesting they can break them during digestion and release smaller and possibly more mobile MPs.
We use the data from the studied agricultural environments to create a mathematical model of microplastic behaviour in soil and freshwater systems. The model is the first and only of its kind and showed the ability of predicting the order of magnitude of MPs levels in river sediments with the available information on inputs from agricultural soils and realistic climate and hydrological conditions. The model however, revealed some important knowledge gaps and research needs to enable more accurate predictions. These are the need of a better mechanistic understanding of the physical processes controlling remobilization form soil of MPs with different size and shapes, and the need of larger and more detailed dataset for model calibration and validation. The project has delivered several important scientific and popular science publications. Some with a high number of citations. More papers will be released based on the collected data even after the end of the project.
We contributed to include plastic pollution in soil into Norwegian, European and Canadian policy agenda.
We have impacted the research agenda in Europe. The 2020 EU H2020 work programme included the first call for research on the topic. IMPASSE developed into a larger consortium of 20 European and Chinese institutes and successfully competed for this grant. This success places the Norwegian research institute sector in a central international position for influencing policy and innovation agenda in the agricultural sector.
We have developed the first and unique mathematical model for mechanistic simulation of transport of microplastics in catchments. It is now employed in 2 capacity building projects funded by the ministry of foreign affair of Norway.
We developed a new line of microplastic reference materials. This development already resulted in a successful business spinoff. Supplying consortia of research institutes and the California Environmental Protection Agency.
While it is widely known that microplastics (MPs) in the ocean are a serious environmental problem, the threat posed by MPs in agricultural lands is almost entirely unknown. As much as 90% of MPs produced in industrialized countries may end up in sewage sludge. A sizeable fraction of this sewage sludge is spread on agricultural lands. We estimate the MP input to agricultural lands in Europe to be between 50000 and 175000 tonnes/year. This is especially alarming given the high concentrations of toxic compounds and endocrine disrupting substances with no safe level commonly found in MP. Effectively, sewage sludge application may be causing persistent, pernicious and almost totally ignored contamination of agricultural land. In IMPASSE, we propose to develop and communicate the new understanding of MP behavior in agrosystems which is urgently needed to avoid the potential of serious and long lasting environmental contamination. The highly interdisciplinary project includes risk communication, stakeholder engagement, ecotoxicology, catchment modeling, development of decision support tools, monitoring and experimental work needed to understand and then minimize threats associated with MPs in agrosystems.