Mercury and methylmercury flux estimation and sediment distribution in an industrialized urban bay

•Stream water discharge was the prime input source of Hg in Masan Bay.•Groundwater discharge and sediment diffusion were the major input sources of MeHg.•Organic matter either promoted or constrained %MeHg of urban/industrialized estuaries.•%MeHg averaged 0.52 despite large variation of Hg in urban/industrialized estuaries.

Our objectives of this study were to estimate mercury (Hg) flux to Masan Bay, an industrialized urban bay in Korea, and to examine important factors influencing the Hg distribution and transformation in Masan Bay sediment. As the first attempt to estimate Hg flux for Masan Bay, we found out that stream water discharge was the prime source of Hg, contributing 76% of the total input. Estimating the methyl Hg (MeHg) flux showed that groundwater discharge is the major source, contributing 53% of the total input. The large MeHg input through groundwater discharge emphasizes the importance of the in-situ sediment production of MeHg. The fraction of MeHg over total Hg (%MeHg) in the bay sediment, ranging from 0.10% to 1.5%, showed a strong negative correlation with the sediment organic matter content (2.8–14% as a loss on ignition). However, when different urban and industrialized estuarine sediments were compared, sediment organic matter either promoted or constrained %MeHg in sediment, suggesting that the role of sediment organic matter for the net Hg methylation varies, relying on the biogeochemical conditions of estuarine sediments. Despite the large variations in Hg and MeHg concentrations among different urban and industrialized estuarine sediments, relatively small variations were determined for the %MeHg, i.e., 0.52% on average.