Microplastic transport in European river networks

Abstract

Microplastics (MP) enter the aquatic environment through both diffuse and point sources, and are transported through the river networks into the seas and oceans. MP threatens the aquatic ecosystems and are present in water, sediment and biota. One of the main entry paths of MP pollution are wastewater treatment plant (WWTP) effluents as well as untreated surface runoff and combined sewer overflows (CSO). In this study, we aimed to estimate the average annual load of MP to the Seas and Oceans for 125 European catchments of different sizes. We coupled a mass balance model modified adapted from (Bollmann et al. 2019) and a transport model representing the river network as graph theory network (GTN). The GTN is based on the HydroShed network (Lehner et al. 2008) with WWTPs inserted as additional nodes. The partitioning of MP was calculated for three sinks (sewage sludge, river sediments, load to the sea) relying on literature-derived MP concentrations from untreated surface runoff, combined sewer overflow, and WWTPs effluents. Concentrations for average discharge conditions were calculated for all stream segments using steady-state discharge data from the HydroShed database. Based on 125 European catchments containing approximately 75% of the European WWTPs with population equivalents > 2000, we found that 77% of MP entering the river network originates from WWTP effluents, the remaining 23% is sourced from untreated surface runoff and combined sewer overflow. Of the MP that has entered the river systems, 24% are transported to seas and ocean while 76% accumulate in the river sediment. The most sensitive parameters in the model related to the loads to seas and oceans are sedimentation rates. In a next step, the model will be updated with improved hydrological parameters. Furthermore we will apply it to future scenarios of hydro-climatic and socioeconomic conditions. As the HydroShed database is globally available, the model can be applied to other regions of the world.