Page - 127 - in Freshwater Microplastics - Emerging Environmental Contaminants?

[23], plastic deposited in river sediments [15], and fragmentation increase the uncertaintywith respect to loads even further. Besides microplastics, nanoplastics are likely to be present in the freshwater environment [24].Noformal sizedefinitionhasbeenset fornanoplastics, resulting in different classifications such as<100 nm [24, 25],<1 μm [26], and<20 μm [16].Hereafter,wewill use<100 nmas a size cutoff for nanoplastics, to comply with the definition of engineered nanoparticles [24]. Nanoplastics can be either directly released into to the environment (e.g., as a by-product of thermal cutting, 3Dprinting)or indirectlyvia thedegradationof largerplastics [24,27–29].Several studies have shown that nanoplastics can be ingested by a variety of organisms, althoughsystematiceffects remainunknown(summarized in [24,30]).Despite the attention toplasticpollutionand thepotentialharmitcauses in theenvironment, to datenoproperenvironmentalriskassessment(ERA)frameworkisavailablefor this anthropogenic pollutant. So far, microplastics have been found to be ingested by freshwater organisms such asfish [31–33] andmud snails [34] (see [8] for further detail). However, effect assessments are scarcely done for freshwater species [16,17].Retrospectiveexposureassessmentshavealsonotbeendoneyetforplastic debris, because of the difficult, time-consuming, and costly detection methods currently available. However, exposure assessments can also be based on quanti- tativemodel estimates ofplastic debris loads anddistributions.Toourknowledge, only one transport-fatemodel has been developed for plastic debris fromnano- to 1-cm-sized particles [35, 36], one for microplastics [37] in rivers, and none for lakes. However, other types of models simulating particle transport in rivers do exist, and theycanbeusedas inspirationfornewplasticdebris transportmodels for the freshwater environment. Theaimsof this revieware (a) to identifyhowexistingparticle transportmodels canserveasexamplesfornewplastic transportmodels, (b) toidentifytheproperties and processes that are relevant for the modeling of plastic debris in freshwater systems, (c) to review the existingmodels that (to some extent) already take into account thesepropertiesandprocesses,and(d) toproviderecommendations for the furtherdevelopmentof thesemodelsandguidanceofhowthesemodelscanbeused in the frameworkofanERA.Wefirstbrieflydiscussexistingparticle transport and fatemodels fordifferentparticle typessuchassedimentororganicmatter (Sect.2). Weidentifywhatcharacterizesplasticdebris fromatransportmodelingperspective andhowthisdiffers fromother (traditional,natural)particles (Sect.3), followedby a critical review of the fatemodels for freshwater systems published in the peer- reviewed literature (Sect. 4). In Sect. 5, we include a short review on data and knowledge gaps in relation to plastic modeling and discuss what kind of model categoriesarehighlyrelevant forplasticdebris.Wealsodiscuss thepossibleroleof fatemodeling inafuture riskassessment frameworkforplasticdebris in freshwater systems. The terms “plastic debris,” “plastics,” and “plastic particles” are used interchangeably in this review and do not refer to a specific size class. Macroplastics, microplastics, and nanoplastics refer to particles>5mm in size, particles between5and100nm in size, andparticles<100nm, respectively. Modeling theFate andTransport of PlasticDebris inFreshwaters:Reviewand. . . 127