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Abstract
Urban sewage water pathways seem most important for microplastics emissions to the Baltic Sea. We use microplastics emission data for the entire Baltic Sea region, calculate emissions for three sewage water related urban pathways and develop emission scenarios for the majority of microplastics particles. All plastics are divided into potentially floating (density 0.8–1.0 g/cmł) and sinking (1.1–1.5 g/cmł) polymers and we address the size class of 20–500 μm. 6.7 × 1013 microplastics particles enter the Baltic Sea annually from urban pathways. 62% result from stormwater runoff including sewer overflow, 25% from wastewater treatment plants (WWTPs) and 13% from untreated wastewater. The emission scenarios serve as input for 3D-model simulations, which allow estimating transport, behaviour and deposition in the Baltic Sea environment. Our model approach suggests average annual microplastics concentrations in the water body of the central Baltic Sea of 1–4 particles/m2 sea surface and 1 particle/m3 in the upper 2 m sea surface layer. The majority of the particles is accumulated in upper sea surface layers. The model suggests that only between 6% (Arcona Basin) and 21% (Gotland Basin) of the particles are below a depth of 25 m. In coastal waters, the concentrations can exceed 10 particles/m3 in the upper 2 m surface water layer (e.g., Gulf of Riga, Gulf of Gdańsk) and 1 particle/m2 on the sediment surface. Usually within weeks, emitted microplastics are washed ashore causing annual coastal accumulations of up to 109 particles/m coastline within a few kilometres distance to emission sources. On average, above 106 particles/m are annually accumulated and trapped at coasts around the Baltic Sea. The reduction of the annual sewer overflow from presently 1.5% of the annual wastewater loads to 0.3% would reduce the total emissions to the Baltic Sea by 50%. If all sewage water would be connected to WWTPs and undergo a tertiary treatment, a reduction of 14.5% of the total emissions could be achieved. The effect of retention in rivers seems limited in the Baltic Sea region, because near coast emissions contribute around 50% of the total microplastics emissions.
Highlights
Microplastic covers the size class below 5 mm
Shorter rivers such as the Umeälven (470 km length) and/or rivers where the wastewater treatment plants (WWTPs) are mainly located near the coast, such as the Neva, do not show a strong impact of increasing retention factors on the microplastic loads
In eastern European river basins and countries, the connection of all wastewater to WWTPs and the implementation of a tertiary treatment can reduce the total microplastics by 15% and is recommendable
Summary
The overwhelming majority of microplastics losses (98%) are generated during landbased activities (Boucher and Friot, 2017). River discharge plays a major role for marine plastic pollution (Bergmann et al, 2015; Schmidt et al, 2020). The top 20 polluting rivers, mostly located in Asia, account for 67% of the global total emissions (Boucher and Friot, 2017; Lebreton et al, 2017). The mean annual riverine runoff to the Baltic Sea is 14,425 mł/s (HELCOM, 2018c) and comparable to rivers such as the Mississippi, the Mekong or the Ganges. It can be expected that the Baltic is an emission hotspot for plastics, as well, and that emissions with rivers play the dominating role
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