Seite - 139 - in Freshwater Microplastics - Emerging Environmental Contaminants?

Weprovide a first implementation for plastic in SB4N in this review (Fig. 3), whichhasnot beenpublishedbefore. SB4Nwasparameterized for the riverRhine catchment,assuminginitialemissionstothecompartmentssoilandsurfacewaterof 50% of total emission, each. We assumed that no direct emission to sediment occurs, implying that plastic particles reach the sediment only through settling from thewater column. Further assumptions are provided in the caption of Fig. 3. With all parameters at the same value, an increase in particle diameter results in more removal from water and soil and increased concentrations in sediment (more settling, Fig. 3). 4.4 SpatiotemporallyExplicitModels To date, twomodels have been presented that are able to simulate the transport ofplasticdebris in freshwater riverswithhighspatial and temporal resolution [35– 37].Bothmodels are framedby the authors as theoreticalmodels, that is, they are supposed to be valid with respect to the design criteria and in agreement with existing theory, but they are not yet validated against measured data for plastic debris (Table 1). Modeling theTransportofPlasticDebris in theDommelRiver (TheNetherlands) Themodel byBesseling et al. [35, 36] is the firstmodel that simulated the fate of nano-up tocentimeter (i.e.,macroplastic)-sizedplasticparticles ina river (see [24] for review). The model is based on the NanoDUFLOW hydrological model [96, 109] and includes advective transport of particles, their homo- and hetero- aggregation, biofouling, sedimentation/resuspension, degradation of plastic, and burial in the sediment. This implies that all processesmentioned in Sect. 3 were accounted for.Althoughnot yet formally validated for plastic particles because of lacking monitoring data, earlier model simulations for nano-CeO2 showed good agreement with measured nano-CeO2 submicron particles in the same river [96].Themodelcanbe implementedforothercatchmentsusingDUFLOWModel- ing Studio [110] and allows for the inclusion of tributaries and diffuse aswell as point sources (e.g.,WWTPs) [96]. To simulate the transport of plastic debris, parameter valueswere set based on literature data.Data for the attachment efficiency for heteroaggregation are scarce and therefore were also determined experimentally. A 40 km stretch of the river Dommel(theNetherlands)wasmodeledwithaspatial resolutionof477sectionsof an average 87.7m length andwith sectionwidths ranging from 8 to 228m. The effectofallprocesseswascalculatedpersectionand theresultwaspassedonto the next. An upstream point source with known mass concentration was used as a boundary condition at time zero, based on an average order of magnitude of published concentrations of microplastics in freshwaters. Scenario studies aimed at identifying how plastic debris of all sizes and densities would be distributed along the river. Realistic flowdatawere used. Impacts of long-termvariability in Modeling theFate andTransport of PlasticDebris inFreshwaters:Reviewand. . . 139