Assessing the microplastics in urban effluents and in the Seine River (Paris)

Most studies are dealing with the issues of microplastics in the marine environment. On the contrary, their behaviour in freshwater is largely unknown. However, the issue of the abundance of the microplastics in the continental environment is very important as they constitute a potentially major source of marine ones. For instance, studies suggest that a large proportion of microplastic fibres found in the marine environment could originate from sewage as a consequence of washing machine effluents1. In this context, this study aims at providing a first assessment of the abundance of microplastics in the continental environment. It was conducted at the scale of the Seine River catchment, and more especially on the Paris urban area. Surface water, sediments as well as waste water treatment plant effluents were collected and microplastics evaluated. The natural samples were collected in the Seine River bank, upstream of Paris. The wastewater effluents were collected from the Seine-Aval wastewater plant, downstream Paris. For liquid matrices, samples were filtered. For sediments, the samples were treated with H2O2 to remove natural organic debris. A sequential density separation was then employed, with NaCl followed with ZnCl2. The supernatant was filtered. Microplastics were accounted on the filters with a stereomicroscope. Whatever the matrix considered, microplastics encountered in all types of samples are almost fibres. For surface water, between 62 and 101 fibres/l were found. This results are very high compared to other studies in the marine environment2. The sediments also presented very high concentrations varying between 405 and 750 fibres/kg of dry weight.This is in, our knowledge, the highest concentration ever found in a study3. Wastewater exhibits very high concentrations (469 fibres/l) of microplastics while treated water presents much lower concentrations (31 fibres/l). Based on these first results, this could suggest that microplastics are removed with a great efficiency during the treatment. Further investigations are required in order to investigate for all microplastics sources, to confirm these first results and to gain a better understanding of the distribution and sources of the microplastics in the terrestrial habitats.