Seite - 126 - in Freshwater Microplastics - Emerging Environmental Contaminants?

models developed earlier for other types of (low density) particles apply also to plastic debris. A unique feature of plastic debris, however, is its combination of high persistence, low density, and extremelywide size distribution, ranging from thenanometer to the>cmscale.Thiscausesthesystembehaviorofplasticdebris to show a far wider variety than most other materials or chemicals. We provide recommendations for further development of these models and implications and guidance for how fate and transport models can be used in a framework for the tiered risk assessment of plastic debris. Keywords Fate, Freshwater,Microplastics,Modeling,Nanoplastics 1 Introduction Contaminationof the environmentwithplastic debris has received increasingatten- tionfromthepublic,environmentalists, scientists, andpolicymakerssince the1970s [1,2].Modelpredictionssuggest thatcurrentlyover5trillionplasticparticlesfloaton theoceansurface[3]andthat in2010alonebetween4.8and12.7millionmetric tons ofplastic entered theocean [4].Plasticsoccur inawide rangeof sizes, andparticles can thereforebe ingestedbyavarietyof terrestrial [5]andaquaticspecies [6]. Inges- tionofmicroplastics, particles<5mmin length [1], cannegatively affect hatching, growth rates, and food ingestion [7, 8]. Besides the potential effect of ingestion, plasticparticlescanactasvectors fororganicpollutants[9]orfunctionasfloatersfor (invasive)raftingspecies[10].Theoccurrenceanddistributionofplasticdebris inthe marineenvironmenthasbeenstudiedeveninthemostremoteareas,suchasthearctic [11]andtheoceanfloor[12].However,eventhoughriversare recognizedasamajor sourceofmarinelitter [13–15], theoccurrenceofplasticdebris infreshwatersystems just started to receive attention [16, 17]. Microplastics have been found in freshwater systems around the world, as summarized in a recent review by Eerkes-Medrano et al. [17]. Occurrence of microplastics in freshwater systems ranges from remote lakes [18] to industrial rivers such as the Rhine [15, 19] or St. Lawrence River [20]. Sources of plastic debris in freshwater systems have not been studied extensively but likely include effluents from wastewater treatment plants (WWTP), sewage sludge, shipping activities, atmospheric fallout, direct disposal from thepublic, beach littering, and runoff from agricultural, recreational, industrial, and urban areas [16, 21]. High loads are estimated to enter themarine environment: for example, an average of 1,533 tplasticperyearwasestimated toenter theBlackSea fromtheDanube [13], and an average of 208 t plastic per yearwas estimated to enter theMediterranean from theRhone [22].However, river loads exhibit a high degree of variation. For example, rain events were shown to increase the plastic concentration up to 150 times in an urban part of the Rhone catchment [22]. Also, total loads in the Danube varied between 10.9 43.6 and 2.2 3.0 g (mean SD) per 1,000m3 from2010to2012[13], indicatingboththeuncertaintyintheloadestimatesandthe temporal change of plastic loads. Transport of plastic near the bottomof the river 126 M.Kooi et al.