Microplastics in sediment and sediment cores – Sedi-PLAST

The Sedi-PLAST project, a multi-disciplinary project bringing together chemists, sedimentologists, geochemists and polymerists, proposes to combine a sedimentological and geochemical approach to the analysis of microplastics on a catchment scale. The aim is to analyse microplastic in different sediment types in order to understand the relationships between sedimentary deposition environments (granulometry, organic matter, etc.) and the content, size and morphology, or chemical nature of microplastic found in sediments. The coupling and characterisation of microplastic by spectroscopy (µ-IRTF and FPA-IRTF) and by Pyr-GC-MS will provide a wealth of innovative information on the chemical nature of microplastic (quantity vs. number of particles, size distribution, ageing) for particles down to 25 µm.

The results of the project confirm that sediments are major reservoirs of microplastics. On the scale of the three rivers studied, levels vary between a few thousand and several tens of thousands of microplastics per kilogram of sediment. Of the factors that explain this contamination, the fraction of fine sediment (< 63 µm) and the organic carbon content seem to be the most important in explaining the spatial variability observed between the different stations, but they alone cannot explain this variability. The geomorphology of the sites and ultimately the deposition environments and the connectivity of these environments with the river also have an influence on the concentration of microplastics in the sediments, implying conditions that favour their accumulation. In the Loire, it has been shown that semi-active channels are more contaminated than the banks and sandy banks of the minor bed. A similar observation was made on the Rhône between sediments taken from the main channel and the hydraulic appendages.

To establish the chronology of this plastic contamination, a total of six cores were sampled, upstream and downstream of Paris (J.S. Barbier's thesis), upstream and downstream of Lyon (Dhivert et al., 2024) and two archives in the downstream Loire before the start of the estuary (C. Croiset's thesis) in different depositional environments (a high bank and a river annex). Spatial comparison of the sediment records shows a sharp increase in contamination levels and polymer diversity downstream of the conurbations (Paris and Lyon). For these stations, however, the sediments show a decrease in PM levels since the 2000s for polymers associated with industrial sources and since the 2010s for ubiquitous polymers. This trend diverges from global assessments, which assume uncontrolled pollution and suggest an increase in contamination levels correlated with the exponential production of plastics around the world. It suggests that European wastewater policies and regulations on industrial discharges have had a positive influence on water quality as far as microplastics are concerned.

The results of this project have revealed significant stocks of microplastics in sediments, particularly in secondary channels. Under certain hydrological conditions and/or management operations, these stocks can be remobilised, thereby further altering the quality of aquatic environments. The conditions under which this remobilisation occurs could provide a perspective for this work. It also raises issues for the management of river sediments and the conservation/restoration of aquatic environments.

A venir

Plastic pollution has become highly publicized and is of increasing concern. From a scientific standpoint, attention is mostly directed towards microplastics (MP) (i.e. with sizes ranging from 1 µm to 5 mm) and this topic involves many societal actors. The European Marine Strategy Framework Directive targets this pollution by establishing litter reduction actions and MP monitoring in environments.

With an overwhelming majority of the works published in this field dedicated to the marine environment, research on the continental environment only started much more recently. A recent review of these works highlighted that although the sediment may act as a sink of MP, it has remained relatively ignored and insufficiently considered. For the vast majority of studies reviewed, sediments are sampled without further investigation of the hydro-sedimentary processes affecting MP levels, nature and/or distribution and their recordings over various spatial scales and temporal scales. This is of prime importance to provide an accurate comparison of MP contents and fluxes at different spatial scales within river catchments. It would be expected that MP accumulation and archiving in sediments depends on the properties of the MP and the hydro-sedimentary processes occurring.

In this context, the Sedi-PLAST project entitled “Microplastics in continental sediments and sedimentary cores”, and its multidisciplinary consortium involving chemists, sedimentologists, geochemists and polymer scientists, proposes combining a sedimentological approach with an MP content analysis within both surface sediments (bed sediments and flood deposits) and the sedimentary column. It is intended to assess the relationships between sedimentary deposits and MP contamination settings in order to develop, in close collaboration with stakeholders involved in the management of aquatic ecosystems, a new tool for monitoring MP-related risks. For the MP characterization, coupling spectroscopy approaches (µ-FTIR and FPA-FTIR) and Pyr-GC-MS is strongly innovative and will provide novel information on the chemical nature of MP (quantity vs. number of particles, size distribution, aging, etc.) for particles below 50 µm.

By analysing sediment cores representative of sediment deposit accumulated over the last ~60 years, this project also seeks to evaluate MP evolution over time, in terms of quality, quantity and conservation. This approach to studying MP in sedimentary recordings is particularly innovative and will provide novel insights into: i) determining the temporal trajectories of plastic pollutions at the basin scale; and ii) providing the lacking reference values in order to compare contemporary and historical MP contents and iii) better understanding factors controlling MP pollution and their potential variation throughout time. Through the dating of multiple sediment cores with radionuclides, this effort will provide the first chronostratigraphic assessment of plastic pollution in the largest metropolitan drainage basins.

In this project, three hydrosystems are considered, i.e. the Seine, the Loire and the Rhône Rivers. The consortium teams have a strong expertise on the hydrosedimentary processes occurring in these catchments through the innovative coupling of various approaches from geophysics to geochemistry.

The objective of Sedi-PLAST project consists in providing guidelines and recommendations of MP monitoring in sediment and co-constructing, in close collaboration with aquatic ecosystem managers, a tool for monitoring MP-related risks in freshwater. The data compiled, recommendations and the management tool developed in this project will provide a surrogate to long-term observations and monitoring of MP pollution in freshwaters and will contribute to evaluate the efficiency of measures mitigating plastic pollution.

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