Seite - 131 - in Freshwater Microplastics - Emerging Environmental Contaminants?

lower the flow velocity, enhancing sedimentation of suspended solids [64]. Also, water withdrawal for various human uses [65, 66], which is highly different for different regions [67], influences thefateofplasticsas theseabstractedparticlesare no longer carried to the ocean. Particle Type With respect to modeling natural particles in freshwater systems, different particle types can be distinguished, such as sediment [39, 68, 69], algae [43], microorganisms [44], particulate organic matter [70, 71], nanoparticles [59, 63, 72, 73], and seeds [74]. The properties of these materials, such as size, shape, density, porosity, fractal dimension, and attachment efficiency, influence their hydrodynamic behavior and thereby their fate [75]. Some of them approach the properties of categories of plastic particles, which may cause them to have similar hydrodynamicbehavior andamutual applicabilityofmodelingapproaches and results. For instance, some plastic particles may become captured in low-density aggregates or flocs, as has been shown for the marine environment [57,76],whichaffects thehydrodynamicsof the resultingnewcompositeparticles [77, 78]. This implies that the transport of the plastic-inclusive floc or aggregate may become indistinguishable from that of a fully natural floc or aggregate. The implications of similarities and differences of plastic compared to natural solid materials for fate and transportwill be further discussed in thenext section. 3 PlasticDebris:Properties andProcessesRelevant forFateModeling Key Properties Relevant for Fate Modeling Plastic debris comprises a highly diversemixture of particle sizes and shapes,made out of different polymers. The size ranges from>10 cm for fishing nets, bottles, and plastic bags to nanosized particles<100nm.Nanoplasticshavesofarnotbeendetected innaturalwatersbut are likely tobepresent [24, 36].Thedensityofplastics ranges from50kgm 3 for extruded polystyrene foam to 1,400 kgm 3 for PVC. It can be expected that the compositionofplastic in rivers is related to theproductionvolumesof thedifferent polymers, of which polyethylene (38%), polypropylene (24%), PVC (19%), and polystyrene(6%)areproducedmost[1].Recentdatapartlyconfirmedtheserelative proportionsof polymers in river sediments of the riverRhine [15], in the reservoir of theThreeGorgesDam[79],andfloatingontheriverSeine [38].Besides thesize and density, the shape of plastics is also highly variable, ranging from small lines and fibers to irregular fragments to granules [80].Microplastics have often been classified as fragments, fibers, spheres, pellets, lines, sheets, flakes, and foam [13, 15, 22, 79, 81], ofwhich fragments aremost abundant [15, 22, 79]. The size, shape,anddensityofparticleswill influence their transportbehaviorandfate in the aquatic environment. Modeling theFate andTransport of PlasticDebris inFreshwaters:Reviewand. . . 131