Source-specific contamination process revealed by mercury isotope analyses and hydrodynamic modeling in an estuary

While mercury (Hg) concentration and isotope analyses play pivotal roles in understanding contamination levels and Hg sources, complex hydrodynamics often obscure Hg transport pathways from source to sink. We applied hydrodynamic modeling with Hg stable isotopes to unravel source-specific contamination processes and propose effective management strategies in an estuarine system (Yeongil Bay) impacted by Hg-contaminated riverine input (Hyeongsan River) in Korea. Sediment isotope data revealed contributions of three sources: legacy Hg from the river, regional background Hg, and atmospheric Hg sources. Interestingly, atmospheric Hg was only observed at the New Port, located within the Yeongil Bay. The hydrodynamic model demonstrated that convoluted shorelines and artificial structures restricted legacy Hg transport from the river and facilitated atmospheric Hg retention. In the Old Port, Hg isotope analyses across five sediment particle size fractions and modeling revealed that legacy Hg is transported directly from the river via a recently constructed canal rather than through the wider estuarine system. This suggests that canal gate operation is critical for mitigating further Hg contamination in the Old Port. Our study illustrates that isotope analyses coupled with hydrodynamic understanding provide in-depth insights into contamination processes and risk management, which otherwise would have been neglected without physical modeling.