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likely due to the combined contribution of bothwastewater and runoff. The levels observedinCSOsarehigher thanthelevels instrictseparatedrunoff.For twoof the threesamples,concentrationswerehigher than theconcentrations inwastewater.A settlementofparticlesduringdryweatherperiodsand their re-suspensionwhen the flow increases was described on the literature . A similar behavior could be expectedfor thefibers.The leveloffibersalsodependson theprevious rainevents: thefirst samplingwasconductedafter a longperiodofheavy rainfall,whichmight have induced adecrease in the amount offibers in the sewer system. Levels of fragments vary between 35 and 3,100 fragments L 1. The levels are especiallyhighevenif theyvarybytwoordersofmagnitude.Lowerconcentrations of fragments in comparison tofibers canbeobservedexcept for the event present- ing thehighest runoff contribution andvolume. 3.3 FiberFluxes inDifferentCompartments As fibers are of utmost importance concerningMP pollution in freshwater, it is especially interesting to assess the relative contribution of the various sources. In thissection, thenumberoffiberfluxesisestimatedat thescaleof theParisMegacity (surfaceareaaround2,500km2–10millioninhabitants).ThenumberofMPfibers is used to estimate themassfluxes.Because thefibersweremeasured, the cumulated total lengthof thefiberswascalculated.The lengthwascoupledwith their approx- imated diameter to evaluate the volume. It was estimated that diameters ranged between5and100μmwithanaveragediameterof25μm.Therefore,with the total volume and specific densities of the plastic polymers (1 g cm 3 for the PA and 1.45 g cm 3 for the PET, corresponding to polymers widely used in the textile industry), totalmasses canbe estimated.The results are summarized inFig. 2. • Atmospheric fallout.According to theaverageatmosphericfluxof totalfibers at theurbanandsuburbansites (110and53fibersm 2day 1),wecanestimate that at the scale of theParis agglomerationbetween1.2 and2.5 1011fibers could originateannually fromtheatmosphere.Accordingtoouranalyses,wecalculate that 30% of these fibers are plastic polymers. Therefore, between 3.5 and 7.6 1010MPswould fall peryear fromtheatmosphereon theParis agglomer- ation.Themasses of plastic fibers are likely between6and17metric tons. • Gray water. Based on the water consumption for washingmachine in France (14.4L inhab 1 day 1) and 10millions of inhabitants, we assess that between 4 1014 and 2 1015 fibers are discharged annually into the wastewater. However, the washing machine fibers were not chemically characterized in thiswork. Two hypotheses can be assumed. For the first one, we can consider that, at a global scale, 60% of these fibers are synthetic, according to the Europeans’ uses and supposing also that both categories of fibers tear off similarly fromclothes. The secondhypothesis is based on a talk at theSETAC Europe2016conference indicating thatMPsaccount for 5%of thefibers at the 76 R.Dris et al.
Freshwater Microplastics Emerging Environmental Contaminants?