Tyre and road wear particles - A calculation of generation, transport and release to water and soil with special regard to German roads

Tyre and road wear is one of the main emission sources of particulate plastics (microplastics). In this study, the emissions of tyre wear particles (TWP) which are annually generated on the German road network were calculated. Emissions are calculated by applying two different data sets of emission factors and the annual mileage for distinct vehicles and road types (urban roads, rural roads, highways). Environmental entry paths of tyre and road wear particles (TRWP) were considered including releases to ambient air, soils and surface waters. Road runoff treatment was taken into account differentiated into sewage systems in urban areas and at non-urban roads. This study identifies the soundest data available concerning emission factors, data on traffic and road systems as well as comprehensive information on road runoff treatment in Germany. Applying this data and assuming that 5% of the total emissions are 'fine' air-borne particles, the emissions of coarse 'non-airborne' particles are 75,200-98,400 t/a; . the transport to road banks and soils near roads is 57,300-65,400 t/a (66-76%) including runoff and drift; 8700-19,800 t/a (12-20%) are released to surface waters. Due to lack of data, degradation in soils and surface waters was not considered. Besides soils, urban wastewater treatment plants are expected to be an important sink of TRWP assuming that most of the particles are incorporated in the sewage sludge. Due to the application of sewage sludge as a fertilizer, 1400-2800 t/a TWP are currently deposited on agricultural areas. No reliable data was available to estimate the masses of TWP which are transported from freshwaters into the marine environment. Existing mitigation measures should be improved according to the principle of precaution by installation of road runoff treatment systems and retrofitting of existing plants regarding optimum fine particle retention.