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Recommendation forModelValidation Validationwould require sufficientdata to verify the credibility of themodelwith statistical rigor. Formassflowormultime- diamodels, limiteddataper systemyet forahighnumberofaquatic systemswould bepreferred.Forspatiotemporally resolvedmodels,however, itwouldbepreferred to sampleonecatchment indetail. Suchacase studycatchmentcould thenbeused to calibrate andvalidatemodels,whichcould later beapplied toother catchments. As for process parameters, little is known yet especially about the time scales of aggregation, fragmentation, and biofouling. Thismeans that experimentalwork is needed, afterwhich the parameter values obtained from these experiments can be applied inmodels.Thedevelopmentof fatemodels for freshwatermayalsobenefit fromexperimentalandmodelstudiesonmarineplasticaggregation, fragmentation, andbiofouling. 5.2 Comparing theModels:WhatModel forWhich Question? Intheprevioussections,wedescribeddifferentcategoriesofmodels indetail.Here, webrieflydiscusswhat categoryofmodel is needed forwhich typeof questionor application. In essence, this categorization does not differ from that for soluble chemicals or engineerednanomaterials. For emission-based regional estimates of environmental concentrations of plas- ticdebris,massbalance,massflow,andespeciallymechanisticmultimediamodels are adequate. Recently, for nanoparticles suchmodels have been developed, like the SB4N model [63, 72, 73] and the MendNano model . It is highly recommended that suchmechanisticmultimediamodels are adapted for plastic as well.Beingneither temporalnor spatiallyexplicit, suchmodels are screening level models that can be used to assess relative concentrations among classes of nano- and microplastics or among plastic emission scenarios. Hence, such models are useful tocalculate regionalaverageorbackgroundconcentrations (PECs,predicted environmental concentrations) for different plastic types, for different regions, or fordifferent futureemissionscenarios.Multimediaparticlemodelscanalsobeused todetect theparameters towhich themodel output ismost sensitiveor toquantify uncertainty in PECs, which than can be applied in probabilistic risk assessments (discussedbelow). Comparedtotheoutputprovidedbymultimediamodels,morerealisticestimates of local environmental fate and concentrations can be obtainedwith spatiotempo- rally explicit models. However, fate models that are spatially explicit only yield betterestimates ifdataonspatialvariability inemission intensitiesareavailable.At present, there is only limited information on such spatial variation. Furthermore, estimating regional average concentrations still requires definition of what is defined as “a region.”Differentmodels use different scales,whichmeans that the research question defineswhatmodel ismost adequate. Global rivermodels like Modeling theFate andTransport of PlasticDebris inFreshwaters:Reviewand. . . 143
Freshwater Microplastics Emerging Environmental Contaminants?