Contrasting high-resolution porewater heavy metal distributions and sediment-water interface fluxes in the Pearl River Estuary and Daya Bay.

Estuaries and semi-enclosed bays are important land-sea transition zones where sediment porewater regulates dissolved heavy metal (HM) exchange across the sediment-water interface (SWI). However, comparative studies on dissolved HM behavior across these coastal environments remain limited. In this study, a high-resolution dialysis sampler (HR-Peeper) was employed to characterize porewater distributions and diffusive fluxes of nine dissolved HMs (Cd, Cr, Fe, Mn, Mo, Pb, Sb, V, and Zn) in the Pearl River Estuary (PRE) and Daya Bay (DYB). Sampling was conducted during a single late-dry-season campaign, with one sediment core collected at each of the four stations. The two PRE stations showed clear differences in dissolved HMs, with mean porewater Zn and Fe concentrations declining by 64.9% and 59.5%, respectively. In contrast, differences between the two DYB stations were generally weaker, while mean porewater Mn concentrations were approximately 3.1 times those at the PRE stations. Estimated diffusive fluxes were predominantly upward at the upstream estuary and inner-bay stations, with Mn fluxes of 554.81 and 1024.63 μg/m2/d, respectively, whereas downward fluxes were more common at the downstream estuary and outer-bay stations. These distribution and flux patterns may be influenced by freshwater-seawater mixing and salinity-related processes at the PRE stations, and by water exchange, local sediment conditions, and element-specific geochemistry at the DYB stations. Element associations were consistent with potential terrestrial and anthropogenic influences at the PRE stations, whereas those at the DYB stations may reflect diffuse regional inputs and local processes. This study provides insights into dissolved HM distribution and mobility across contrasting coastal settings and suggests that monitoring should consider hydrogeochemical differences and element-specific behavior near the SWI.