Page - 137 - in Freshwater Microplastics - Emerging Environmental Contaminants?

measured concentrations include both primary and secondary plastics (i.e., pro- duced and fragmented/weathered particles, respectively), whereas themodel only includedprimaryplastics,whichmayhave interferedwith the reportedvalidation. Current knowledgeon theuseof nano- andmicroplastics in consumerproducts is limited, soagenericapproachwithmanyassumptionswasused in thismassflow modeling study, contrastingwith the approaches usedwithmore advancedmulti- mediamassbalancemodels, lifecycleperspectivemodels,orprobabilisticmaterial flow models. More reliable data to feed the models are needed to improve the emission estimates [90]. 4.2 GlobalRiverModels Riverpollution isaworldwideproblem.Humanactivitieson the landpollute rivers in all continents.Anumber of global river pollutionmodels exist. One of these is the Global NEWS (Nutrient Export fromWaterSheds) model [101, 102]. Global NEWS is a model that calculates river export of nutrients from land to sea as a function of human activities on the land.GlobalNEWS includesmore than 6,000 riverbasinsusinghydrologyfromthewaterbalancemodel [103]. Itcalculates river exportat the rivermouth.Themodel input ismostlyonagridof1degree longitude by 1 degree latitude. It has been used to simulate trends in river pollution for the period1970–2050, taking intoaccount change in landuse, foodproduction,urban- ization, and hydrology [103–105]. Results indicate that over time, most rivers worldwidebecomemorepolluted. Global river exportmodels for nutrients, likeGlobal NEWS, have been under developmentformorethan20years.Forotherpollutantsglobalriverexportmodels donot have such a longhistory.As a result, theGlobalNEWSapproachhas been takenasanexampleandinspirationforotherpollutants [44].Nutrients inriverscan havepointsources(e.g.,pipesdrainingintotheriver)ordiffusesources(e.g., runoff fromsoilsoratmosphericdeposition[65]).This is thecasefornutrients,butalsofor other pollutants, like plastic debris. Model structures for point sources of one pollutant can easily serve as an example for other pollutants. The same holds for diffuse sources. A river exportmodel formicroplastics, inspiredby theGlobalNEWSmodel, is currently under development (Table 1). Preliminary results for point source inputs of microplastics to European seas have been presented [91]. This plastic model calculatespoint source inputsofmicroplastics fromsewage to rivers. Inaddition, it simulates river transport of microplastics as a function of population, sewage connection, wastewater treatment, and river retention. River retention is derived from [36]. First results indicate that car tires are important point source inputs of microplastics inEuropean rivers. Modeling theFate andTransport of PlasticDebris inFreshwaters:Reviewand. . . 137