Mercury dynamics in open coastal-marine system: a case study of Rosignano-Solvay (Tuscany, Italy)

Chlor-alkali plants have operated along the Mediterranean coastline since the 1940s, representing major point sources of mercury (Hg) to marine sediments. This study evaluates the regional-scale impact of the chlor-alkali plant at Rosignano (central Italy), advancing the “open-system paradigm” for legacy contaminants. Our multi-proxy investigation of 51 sediment cores, coupled with hydrodynamic observations and numerical simulations, reveals that circulation-driven transport facilitates the offshore dispersal of Hg-enriched particles, creating depocenters well beyond the original point source. Reworking processes maintain elevated Hg concentrations in surface sediments, sustaining potential exposure pathways for marine biota. Historical Hg inputs dominate present-day sediment contamination, with concentrations reaching up to 23× above background levels across 100 km 2 of seafloor. While nearshore areas show natural attenuation via burial process of contaminated layers, offshore depocenters act as bioactive zones. Surface methylmercury data indicate ongoing biogeochemical reactivation of legacy Hg via a “benthic pump” mechanism in suboxic offshore environments. These findings point to the need for integrated global monitoring frameworks under the Minamata Convention on Mercury and complementary instruments such as the Marine Strategy Framework Directive (MSFD) that account for far-field contaminant redistribution and the spatial decoupling of ecological risk in open coastal systems.