River-sea interaction-regulated spatial distribution and source apportionment of mercury in the soil of the Yellow River Estuary coastal wetland

Coastal estuarine wetlands are vulnerable to river overflows and tidal inputs, yet it is still unknown how they contribute to the cycle and source inputs of mercury (Hg). This study focuses on characterizing the spatial heterogeneity of total mercury (THg) and methylmercury (MeHg) shaped by the river-sea tidal interaction in a typical coastal wetland, Yellow River Estuary coastal wetland (YREW) in China, aiming to elucidate the Hg distribution and transport in coastal wetland and apportioning contamination sources. The results revealed a clear increasing gradient in sediment THg and MeHg concentrations from the riverine area toward the coast, indicating that offshore tidal action represents a significant source of Hg. Higher MeHg/THg ratios were observed in near-sea sediments than in inland areas, which implies that tidal inputs of salts such as sulfate may stimulate microbial methylation of Hg in coastal sediments. Hg isotopic source apportionment further showed that soil-derived Hg contributed 34.1% to Hg accumulation in the wetland, so as industrial (31.9%) and atmospheric (31.8%) sources. These findings underscore the central role of river-sea dynamics in regulating Hg fate in coastal wetlands and establishing a scientific foundation for assessing future MeHg exposure risks under sea-level rise.