Page - 144 - in Freshwater Microplastics - Emerging Environmental Contaminants?

GlobalNEWSlinkmassflowmodels for rivercatchments,which thusaccounts for spatial variation among catchments on a global scale, but not within catchments [91,117].Asforapplications, suchmodelscanrankcatchments, regions,countries, orcontinentswithrespect toemissionintensitytothemarineenvironment[92].The multimedia model SB4N can also accommodate various spatial scales, like regional, continental, and global, but always calculates one average concentration for soil, sediment, air, lake, river, and seawater. It is possible, however, to run models like SB4N for a certain grid, within an overarchingmodel that provides input on a scale of, for instance, 200 200km[118]. For more accurate local estimates of concentrations of plastic debris, system- specificzero-Dmassbalanceapproachescanbeusedforsmallersystems, like lakes [63].However, tobetteraccount forvariability, spatiotemporallyexplicitmodels in 1, 2, or 3 dimensions can be used. As far as we know, the 1-DNanoDUFLOW model discussed above is themost elaboratedmodel available. By defining small segments in a river, full hydrologycanbe taken into account.This is important for answeringquestionswith respect to“hot spot” locations, quantifyingwhichplastic typesandsizes canbeexpectedwhere (includingnanosizedplastic), calculationof retention versus flow-through to sea, and prospective assessments of fate and exposure on a detailed local scale. It has been argued recently that suchmodels may be able to predict biologically relevant nanoparticle aggregate species as a function of time and space, which in turn can be linked to exposure by biota inhabiting thewater system in question [24].We propose that a similar approach also ispossible forplasticdebris, althoughfurthervalidationof fatemodelsaswell as further assessment of what has to be considered bioavailable and ecologically relevant is required. These last steps are particularly important whenmodels are used in the frameworkof a formal risk assessment. 5.3 Fate andExposureModels in theContext ofERA forPlastic inFreshwaterSystems Todate,noERAframeworkhasbeendefinedorapplied toplasticdebris.Here,we postulate that for plastic debris the samebasic components ofERAcanbeusedas for traditional chemicals and engineered nanomaterials: problem definition stage, an exposure assessment, an effect assessment, and a risk characterization step [119, 120]. For plastic debris, exposure presently is difficult tomeasure, so there is a relativelyhighneed formodeling tools.Acrucial aspectof exposuremodeling and effect assessment in the context of ERA is what is to be considered the “ecotoxicologically relevant metric” (ERM) [120]. The ERM is the “common currency” used in the exposure and the effect assessment, which links these two, such that theycan lead to aconsistent risk characterization.For soluble chemicals, the ERM always is concentration, which is why ERA for chemicals uses the 144 M.Kooi et al.