Since the beginning of plastic production, approximately 8.3 billion tonnes of plastic have been produced worldwide, of which 60 % ended up in the environment. Microplastics (MPs) are defined as plastic particles ≤5 mm and pollute aquatic ecosystems worldwide, often entering freshwaters in urban areas. MPs affect the environment by releasing toxic compounds, accumulating pollutants, harming organisms, and accumulating in the (human) food chain. The understanding of MPs in freshwater systems is crucial, and rivers are acknowledged to transport MPs into marine ecosystems.
The comparability of different study results for MP assessment in the environment is difficult because studies are carried out using widely different methodologies. Difficulties for standardising or harmonising methodologies for MP sampling, sample processing and analysis are manifold. Various aspects influence the quality and the results of studies, such as: diverse sampling locations, heterogeneous samples, available equipment and scientific question. Those aspects are often interdependent, hampering easy and straight forward standardisations. Hence, an underlying concept of standardisation, namely to agree upon certain ways to proceed and accomplish tasks has not fully been developed for MP assessment in freshwaters. However, there is ample consensus in the scientific (and non-scientific) community for the need to develop standards for MP assessment.
Nets/trawls for large volume surface water sampling are popular, and the use of a filter-cascade pump set-up for water column sampling has many advantages. Collecting sediment with plastic-free grabbers or corers are suitable. Appropriate sample processing steps (digestion, density separation), and combination/repetition of procedures depending on the sample content (inorganic/organic matter) are necessary, followed by MP particle size fractionation. To comprehensively analyse MPs (determine shape, size, colour, polymer type), a coherent combination of available, complementary techniques (FTIR, Raman, optical microscopy, Pyr-GC-MS) is essential. Quantification (mass, concentration, MP sizes) is important to better understand MP pollution, hence, the acquired data should be reported meticulously in a transparent way using consistent units. Methods for MP assessment depend on the study design, the characteristics of the targeted MPs, the available equipment and more. Therefore, we advise ensuring to provide all data necessary to comprehend the research results and applying good practices along all procedures. Although it is not possible to always apply the same methods for MP assessment in freshwaters among different projects, the recommendations given in this report allow for better comparability of the research results.
The project LimnoPlast (2020-2023) aims to assess the sources, fate and sinks of MP in urban freshwater environments and link social, technical and environmental science. Fifteen Early Stage Researcher (ESR) projects have been designed to investigate the MP contamination in Europe’s freshwater systems and tackles the various knowledge gaps. Three of the projects (ESR 3, 4 and 15) will directly assess MP in the water systems in Aalborg/Aarhus (Denmark), the Greater Paris catchment of the Seine river (France) and the City of Amsterdam catchment (The Netherlands), respectively.
In the frame of the LimnoPlast project, we conclude that it is inevitable to agree on certain SOPs, while the individuality of each executing project team is taken into account. This report (Deliverable 2.1) contains a method review for sampling, sample processing and analysing MPs in urban freshwater systems, focusing on sediment and water samples. We discuss different procedures regarding flexibility, suitability and representativeness and draw conclusions for recommendations for harmonized strategies (or SOPs) regarding the objectives of the three LimnoPlast projects.
The main questions and aspects that are touched upon in this report are:
- What are the different steps needed for reliable quantification and characterization of MP?
- Which criteria are essential to choose a suitable method?
- Where to sample MP (sampling locations)?
- How does the targeted size of MP influence the sampling methods?
- What influences the matrix has on the sample processing, and how could this be harmonized? What quality assurance (QA) and quality control (QC) processes should be implemented?
- What MP characteristics should be analysed and reported (particle size, shape, type of polymer, mass)?