Seite - 142 - in Freshwater Microplastics - Emerging Environmental Contaminants?

5 Recommendations andGuidance for theDevelopment ofFateModels forPlasticDebris fromaRiskAssessment Perspective 5.1 DataandKnowledgeGapswithRespect toFurtherModel Development Quality Criteria for Analysis andDetection To date, few studies havemeasured concentrations and characteristics of plastic debris in the freshwater environment, which implies that more and also better data are of utmost importance. Quality assurancecriteria are common inanalytical chemistryor ecotoxicology [113, 114] butare less self-evident formonitoringofplasticdebriswhich isa relativelyyoung field of science [115]. There also is anurgentneed to standardize theunits used toquantifyabundanceof plastic debris [81]. For instance, for freshwater systems, concentrations of plastics inwater and sediment haveuntil nowbeen reported inmass per unit of volumeof water [13],masspermassof sediment [15], particlespervolumeofwater [13,19], particles per surface area of water [18, 19, 83, 116], and particles per mass of sediment [15]. Utility of data formodelingwould improve enormously if studies would at leastmention bothmass and particle count data and,when takingwater samples,mention the sampling depth and sampling net dimensions, whichwould enable a surface-to-volume conversion or vice versa. This conversion only holds under the assumption that particles are evenly distributed over the sampled depth, which is also often assumed inmodels. Depending on the aims of the modeling, measured plastic abundances should meet specific requirements.Tovalidatemassflowanalysis, anestimateof the total mass of plastic per unit of volume of the modeled media would be required. Multimedia models like SB4N [72] model the free,<0.45 μm aggregated, and >0.45 μmaggregated species, and validation ideallywould requiremass concen- trations for these size classes. Because the lattermodels start with emission data, i.e., fromproductionfigures, themodelingwill usually relate to a specificpolymer type. For deterministic spatiotemporally explicit modeling, sufficient detail with respect to actual size and polymer density distributions is required because such approaches aim to simulate the reality as closely as possible. This implies that analysis and characterization of plastic in environmental samples would need to include (a) sufficient detail in the particle size and density distributions and (b) sufficient detail in the classification of shape, i.e., like fibers, fragments, and spherules [81].What is to be considered as “sufficient” in this respect depends on themorespecificaimof themodelingandisbeyondthescopeof this review.Given that particle interactions as well as potential ecological effects across different species traits are size dependent, standardization ofmethods, including those for nano- andmicrometer-sizedplastic particles, is consideredvery important. 142 M.Kooi et al.