Long-term environmental risks of the Baltic Sea's memory effect revealed by ocean modeling and observation of reprocessing-derived radiotracers

Although previous research indicated that the Baltic Sea has a strong memory effect for trapping pollutants/ nutrients, the associated environmental risks are not well understood due to the knowledge gaps in the long-term hydrodynamics-driven exchange of pollutants/nutrients between the North Sea and the Baltic Sea. In this work, we exploited Tc-99 and I-129 released from the two European nuclear reprocessing plants as oceanic tracers and pollutant proxies, and performed a five-decade hindcast simulation to quantitatively estimate the fluxes and timescales of marine transport of pollutants/nutrients in the North-Baltic Sea. Modeling results underline two potential environmental risks of the Baltic Sea's memory effect: (1) similar to 26 years of environmental half-life for any existing water-soluble pollutants/nutrients in the Baltic Sea driven by its hydrodynamics; (2) the Baltic Sea as a pollutant reservoir continuously exporting 3 % of contaminations per year to the downstream areas after any pollution event. Our findings provide fundamental knowledge for understanding the long-term hydrodynamicsdriven pollutant/nutrient transport in the North-Baltic Sea, facilitating the future regional management of the marine environment.

Automated summary

This study examines the long-term memory effect of the Baltic Sea on pollutants and nutrients using simulation and tracer analysis. The results highlight the continuous export of pollutants and nutrients from the Baltic Sea to downstream areas, emphasizing the importance of understanding and managing marine environment in the North-Baltic Sea region.