Seite - 128 - in Freshwater Microplastics - Emerging Environmental Contaminants?

2 Modeling theTransport ofParticles inAquaticSystems Fewmodelsexist that simulate the transport and fateofplasticdebris in freshwater systems. Plastic debris includes buoyant macroplastic items like bottles, food wrappers and containers, plastic cutlery, and expanded polystyrene (PS), larger polyethylene(PE),orpolypropylene(PP) itemsthatfloatat thesurface [13,38]and willbe transportedunder the influenceofwaterflowandwind(discussed inSect.4 andFig.1).Non-buoyantplasticsorbuoyantplastics thatbecomemoresusceptible to vertical mixing due to their small size (i.e., microplastic and nanoplastic) will become submerged andmay be subject to settling in a fashion similar to that of natural colloids and suspended solids (Fig. 2). Hence, such natural particlesmay serve as a proxy for some classes of plastic debris, and models simulating the transport of such natural particles can form the basis for the development of transport models for plastic debris. In this section, we summarize modeling methods for (submerged) particles in freshwater systems in general. This includes howdifferentmaterials,aquaticsystems,processes,andscalescanbemodeled. It is beyond the scopeof this review to strive for completenesswith respect to the large number of specific particle transport models that have been presented before, especially since excellent reviews on transportmodels already exist for sediment [39–42], algae [43], microorganisms [44], and nanomaterials [45, 46]. These reviews describe the present top models such as SWAT, WASP, HSPF, ANSWERS, andWEPP, all ofwhich include suspended solids [39, 42]. Key Processes Affecting Particle Transport in Freshwater Systems Particles can enter an aquatic system via external inputs including, for example, WWTP Fig. 1 Schematic representation of the different processes playing a role in the transport of macroplastic in a river and lake. Turbulent watermovement below aweir can “capture” plastic debris for a certain period of time.The scalingof the different components is not representative, andnot all processes happen to eachplastic piece or in afixedorder 128 M.Kooi et al.