Trapped microplastics within vertical redeposited sediment: Experimental study simulating lake and channeled river systems during resuspension events.

Plastic waste and its fragments (microplastics; <5 mm) have been observed in almost all types of environments. However, the mechanisms underlying the flow and transport processes of plastics are unknown. This is particularly valid for river sediments, where complex interactions occur between particles and influence their vertical and horizontal distribution patterns. In this study, we investigated the vertical redistribution of 14 pristine microplastics (MPs) with different densities, sizes, and shapes within disturbed sediment without lateral transport (i.e., low-velocity flow). MPs were spiked into sediments (height: 8 cm) in a column with a height of 1 m (diameter: 6 cm) filled to the top with water. The sediment was perturbed by turning the column upside-down to simulate remobilization and the subsequent deposition of sediment. After the complete sedimentation of the particles, the water column was filtered and the sediment was cut into vertical sections. MPs were then extracted from the sediment using sieves and a density separation method, and were counted under a stereomicroscope. Low-density polymers were mainly recovered in the water column and at the surface of the sediment, whereas high-density polymers were found within all sediment sections. The vertical distribution of high-density polymers changes primarily with the sediment grain size. The distribution of each polymer type changes depending on the size and/or shape of the particles with complex interactions. The observed distributions were compared with the expected distributions based only on the vertical velocity formulas. Overall, the formulas used did not explain the sedimentation of a portion of low-density polymers and predicted a lower distribution in the sediment than those observed in the experiment. In conclusion, this study highlights the importance of considering MPs as multi-dimensional particles and provides clues to understand their fate in low-velocity flow systems, considering that they undergo scavenging in sediments.

The missing ocean plastic sink: Gone with the rivers.

Plastic floating at the ocean surface, estimated at tens to hundreds of thousands of metric tons, represents only a small fraction of the estimated several million metric tons annually discharged by rivers. Such an imbalance promoted the search for a missing plastic sink that could explain the rapid removal of river-sourced plastics from the ocean surface. On the basis of an in-depth statistical reanalysis of updated data on microplastics-a size fraction for which both ocean and river sampling rely on equal techniques-we demonstrate that current river flux assessments are overestimated by two to three orders of magnitude. Accordingly, the average residence time of microplastics at the ocean surface rises from a few days to several years, strongly reducing the theoretical need for a missing sink.

Extraction of microplastics from sediment matrices: Experimental comparative analysis.

Microplastics are small (<5 mm) fragments of plastic debris that are ubiquitous in oceans and terrestrial ecosystems. Studies on microplastics in sediment and soil matrices are particularly challenging because of the need to separate the plastics from the sediments. We investigated the efficiencies of 18 combinations of six extracting solutions (ESs) (oil, water, oil-in-water, NaCl, oil-in-NaCl, and NaI) and three isolation methods (IMs) (hand stirring, centrifugation, and aeration) for fine and coarse sediments, with low and high density polymers. IMs did not affect the extraction efficiency. Except in case of oil, all ESs enabled good extraction (84 ± 17%) of light polymers (PE and PE-ABS). NaI presented the best extraction efficiency (71 ± 17%) for the densest polymers (PET, PES, and PA). For these ESs, fibers were extracted at a lower efficiency than pellets and fragments, and sediment gran size did not affect the extraction. For other ESs, mean extraction rates ranged from 5% to 48%. Overall, the extraction efficiencies were lower than those found in the literature, despite repeating the separation process three times. The collection of floating materials remained a problem, as plastics tended to adhere to the glass wall. Our work will help the comparability between previous and future monitoring results and the selection of the most suitable protocols for future studies. This work clearly demonstrates also that there is no robust protocol for extracting all types and forms of microplastics from fine sediments and that research efforts to arrive at a reliable method remain by taking account the interaction of MPs with other particles as well as the electrostatic properties of MP.

To What Extent Can Micro- and Macroplastics Be Trapped in Sedimentary Particles? A Case Study Investigating Dredged Sediments.

Plastic wastes and their fragments (microplastics, MPs < 5 mm) represent a global, persistent, and ubiquitous threat to ecosystems. Their sources, transfers, and fates are still poorly understood, especially in rivers. To fill this gap, sediments were collected from two dredging disposal sites along the Aa River (France). Four pits were dug, and triplicate samples were obtained at four depths (down to 140 cm). The sediments were sieved to 5 mm to collect macroplastics (MaPs). MPs were separated from the sediment based on density using a NaI solution (1.6 g/mL). Suspected plastics were analyzed with Fourier transform infrared spectroscopy. The studied sediments were found to be widely contaminated with concentrations ranging from 0.97 to 77 MaPs/kg and from 0.78 to 2800 MPs/kg, which were 1-4 orders of magnitude lower than those in most polluted European riverbeds. The MaPs were principally polyethylene, polypropylene, polystyrene, and polyvinyl chloride films, whereas the MPs were mainly polyamide and polyester fibers. The plastic concentrations and features of the two sites, which were filled at two different times, differed. Several factors occurring before and after dredging operations may explain these discrepancies. Nevertheless, no relationships with the sediment features were noted, and thus, one major driving force could not be identified. At the site scale, more than 1 ton of plastic could be stored. In conclusion, this study highlights the importance of dredged sediments for past plastic pollution studies and global plastic budget estimations.

Microplastic fluxes in a large and a small Mediterranean river catchments: The Têt and the Rhône, Northwestern Mediterranean Sea.

This paper aims at quantifying current riverine fluxes of microplastics (MPs) in two Mediterranean river catchments, a large one and a small one, namely the Rhône and the Têt, which are discharging to the Gulf of Lion in the Northwestern Mediterranean Sea. MP fluxes change markedly through time and space in both river systems. However, no clear relationships between MP concentrations and hydroclimatic conditions have been observed. In the Rhône River a non-linear dilution pattern of MPs in total suspended matter (TSM) during flood conditions could be observed. Although dilution is important, samples during floods exert a strong control on average MP fluxes. Compared to the Rhône River, average MP concentrations in the Têt River were throughout greater and more variable in shape and polymer composition. However, as the study year was exceptionally dry, the average specific MP flux, 76 g km-2 y-1, is only slightly larger than the non-flooding value of the Rhône River. We further monitored MP concentrations in shoreline sediments at the mouth of the Têt River to test whether these sediments can represent MP transport in the river. Besides fibers, which probably are easily washed out and transported offshore, MP concentrations and compositions are in agreement with MP loads upstream the river. We also examined the potential role of atmospheric deposition as a source of MP to the Têt River. The average atmospheric MP deposition of 6 kg km-2 y-1 exceeds by far the river average specific MP flux. Moreover, all MPs in atmospheric deposits were fibers, which in terms of mass are of minor importance in the bulk river fluxes. Atmospheric MP deposits may either have been overestimated and/or may be removed from surface waters by efficient removal processes (such as waste water treatment plants).

Beached microplastics in the Northwestern Mediterranean Sea.

Microplastics are small (<5mm) fragments of plastic debris that are ubiquitous in coastal areas and in open ocean. We have investigated the occurrence and composition of microplastics in beach sediments from the micro-tidal Northwestern Mediterranean Sea. Samples were collected on two beaches (northern and southern site) of the western Gulf of Lion showing markedly different characteristics. Sampling was performed along depositional lower, mid and upper beaches and repeated after 1month. Concentrations of microplastics in the northern and southern site were highly variable, ranging from 33 to 798 and from 12 to 187 microplastics per kg of dry sediment, respectively. Highest concentrations were found at three specific locations: nearby a local river mouth, within an accretionary area and in a depositional upper beach. The spatial and temporal distribution of beached microplastics seems to be directly dependent on external forcing such as wind, swell, precipitation, outflow and river mouth proximity.