Page - 141 - in Freshwater Microplastics - Emerging Environmental Contaminants?

assumed to be uniformlymixed within each section, and the transport processes advection, settling, and size-dependent resuspension from the sediment bedwere taken into account. Microplastic properties were defined by dimension and density. The study assumed plastic particles were pristine, that is, effects of biofouling were not takenintoaccount.Themodelshowedthat the transportofmicroplastics isstrongly relatedtoflowregimes,especiallyfor thelarger (>0.2mm)particles.Thetransport dynamics were more influenced by size than by density, which confirms the findings by Besseling et al. [35, 36]. Average retention of particles was size dependent, decreasing with decreasing particle size and starting with 90–100% retention for particles>0.2mm.Particles<0.2mmwere lesswell retained, and a large portionwas expected to endup in themarine environment. Theparticle size range of the simulated particleswas 0.05–0.7mm; densities ranged from1,000 to 1,300kgm 3.Themodeldidnot includebiofouling,aggregation,orfragmentation. These processes influence the hydrodynamic behavior and size distribution of the particlesbutaccordingtotheauthorsshouldbebetterunderstoodbefore theycanbe included in themodel.Nanoparticles are also not included in themodel yet [37]. Comparison of the Besseling (DUFLOW) and Nizzetto (INCA-Plastic) Models Both the DUFLOW and INCA-Plastic models were in accordance with their design criteria and study aim. TheNanoDUFLOWmodel seemsmore com- plete as it includes aggregation, which has been shown to be a crucial process, especially for submicronparticles [59, 60]. Themodel byBesseling et al. [35, 36] also accounted for biofouling, which also has been shown to affect the settling behavior of plastic particles.Given the study aim,Besseling et al. did not provide long-termsimulations that accounted for the impacts ofweather conditions.How- ever, in principle DUFLOW can accommodate point and diffuse sources like WWTPs, tributaries [96], or runoff [111]. The latter processes were already accounted for in the INCA-Plastic implementation byNizzetto et al., which is a relevantmerit of that study.Bothmodeloutcomesagreeon the important effect of particle size on retention and on a high retention for particles >0.2 mm. A contrasting conclusion, however, is that the INCA-Plastic model predicted that smaller particleswould be lesswell retained in the river and thus exported to sea, whereas the NanoDUFLOW model reported an increased retention again for particles smaller than 5 μm. This difference can be explained from the fact that NanoDUFLOWaccounted for aggregation of these small plastic particles, which allowed for the simulation of the increased sedimentation of these small plastic particles captured in heteroaggregates. This emphasizes the need to include this process. Ithasbeenshownthatbecauseheteroaggregationcapturesvirtuallyall free nanosized particles, uncertainty with respect to the exact parameterization of heteroaggregation is ofminor importance [73, 96, 112]. The conclusions of both studies depend on themodeled scenario’s and parameters’ variability. Also labo- ratory experiments have shown that processes like biofouling and aggregation [57, 62, 84] and particle properties like density, size, and shape [52, 55] signifi- cantly influenceparticle fate. Modeling theFate andTransport of PlasticDebris inFreshwaters:Reviewand. . . 141