Anthropogenic U-236 and U-233 in the Baltic Sea: Sources, Distributions, and Tracer Applications

The Baltic Sea is one of the world’s most polluted seas suffering serious contamination, eutrophication, and hypoxia expansion. Large amounts of hazardous substances and nutrients continuously enter the Baltic Sea and accumulate for decades due to its limited water renewal. An in-depth understanding of the oceanic dynamics in the Baltic Sea is of particular importance for the management of the marine environment.

Being source-specific and highly soluble, anthropogenic uranium isotopes (236U and 233U) have been demonstrated to be promising tracers for investigating oceanic dynamics. In this project, we investigated 236U and 233U in the Baltic Sea encompassing their sources, distributions, transport, and budgets through ocean observations and long-term hindcast simulations.

Radionuclides (236U, 233U, 129I, and 99Tc) released from the European nuclear reprocessing plants were exploited as oceanic tracers to study the long-term hydrodynamic processes determining pollutant/nutrient dynamics in the Baltic Sea and quantify the water sources in the entire transition zone between the North Sea and the Baltic Sea.

This thesis contains studies in the following aspects: •The radiochemical procedure and quality control measures for 236U and 233U analyses were optimized, which significantly improved the laboratory backgrounds of 236U and 233U, allowed the determination of ultra-trace level 236U and 233U in Baltic Sea water and sediment samples by accelerator mass spectrometry, and assured the analytical results with desirable precisions and satisfactory reproducibility ( Paper I ). •Through systemic seawater and sediment sampling in the Baltic Sea, the spatiotemporal distributions of 236U and 233U were investigated.

The global fallout of atmospheric nuclear weapon testing and the discharges from the European nuclear reprocessing plants have comparable contributions to the 236U in the Baltic Sea, whereas the 236U from the region fallout of the Chernobyl accident and the discharges from local nuclear facilities were estimated to be marginal ( Paper II and III ). •Combining ocean observations and hindcast simulations, the budget calculations were performed for global-fallout-derived and nuclear-reactor-derived 236U in the Baltic Sea water taking the major fluxes into account, including the atmospheric deposition, the oceanic exchange between the North Sea and the Baltic Sea, and the scavenging to the sediments ( Paper II and III ). •The long-term hindcast simulations suggested that a limited portion of radiotracers was transported from the European nuclear reprocessing plants to the Baltic Sea due to the limited water exchange between the North Sea and the Baltic Sea.

The Baltic Sea has a strong memory effect arising from its slow water renewal: any existing pollutants/nutrients in the Baltic Sea have up to ~20 years of ecological half-life and are continuously exported to the ambient areas for decades ( Paper III and IV ). •Using a multi-tracer approach (salinity-127I-236U) and an end-member mixing algorithm, the major water masses in the transition zone between the North Sea and the Baltic Sea were distinguished and quantified.

The transition zone shows a highly stratified structure characterized by a relatively high proportion of freshwater in surface waters, English Channel water above the intermediate layer, and Atlantic water in bottom waters. Significant seasonality is observed with more Atlantic water in summer and English Channel water in winter ( Paper V ).