Seite - 79 - in Freshwater Microplastics - Emerging Environmental Contaminants?

the use of the 80μmmesh size is preferred. Fragments on the other hand are less abundant, and sampling higher volumes is mandatory, requiring large mesh size (330μm). In this chapter, bothmethodswere employed.Moreover, becausefibers seem to characterize the area of the case study and are less investigated by the previously published investigations on freshwater, a long-term monitoring from April2014toDecember2015wascarriedoutonfourstations (P2–P5)ontheSeine River fromupstreamtodownstreamParisplusonestationontheMarneRiver (P1). For the fragments, only five different campaigns were carried out on the various sites (Fig. 1). 4.2 Fibers in theSeineandMarneRivers Concentrations through the year in theMarne River (P1) range between 5.7 and 398.0 fibers m 3 with a mean concentration of 100.6 99.9 fibers m 3 (mean SD). From the upstream to the downstream points, the concentrations are48.5 98.5fibersm 3 (P2), 27.9 26.3fibersm 3 (P3), 27.9 40.3fibersm 3 (P4), and 22.1 25.3 fibersm 3.Variations occurred in a parallelway on the different sites. This could indicate that global factors that vary equally for all sites are more likely to affect the concentrations than local factors. The variations in diffusive inputs or seasonal changes could be at cause.We could also suspect a relationwith the river flow variations, but no clear correlationwas found. None- theless, a tendency to always have lowfiber levels duringhigh-water-flowperiods was observed.During low-water-flowperiods, levels aremuchmore variable and couldbe influencedbydifferent parameters suchas the input offibers, either from punctual sources (WWTP,CSO), diffusive sources (atmospheric fallout), or apos- sible re-suspensionoffibers from the sediments. It is possible to assess the annual fluxes of fibers in the Seine River using the 19punctualfluxescalculatedat each site.The increasebetween themostupstream andmostdownstreampoint (P2andP5) isonly6%,which ismuchsmaller than the uncertainty induced by the short-term variability (unpublished data). As a conse- quence, it seems regarding the fibers that the impact of the Paris agglomeration is rather small.Current state of knowledgedoes not allow to understand and explain thisnon-increasingpattern. In fact, betweenP2andP5, two tributaries (Marneand OiseRivers), threeWWTPdisposals (SeineAmont,SeineCentre, andSeineAval), and numerous CSOs join the Seine River. Sinks that counterbalance these inputs couldexplain the fact that similarfluxesare found fromupstreamanddownstream Paris.Wesuspect an important role related to the sedimentation anddepositionon the banks of thefibers. Further researchon the fate of thefibers is still required. Theminimumandmaximumestimatedfluxes for the siteP5are2.8 1010and 6.1 1011fiber/yearwithameanof1.8 1011.Withthehypothesis that65%ofthe fibers are synthetic, we approximate that between 1.8 1010 and 4.0 1011MP Sources andFate ofMicroplastics inUrbanAreas:AFocusonParisMegacity 79