Extremely elevated methyl mercury levels in water, sediment and organisms in a Romanian reservoir affected by release of mercury from a chlor-alkali plant

•Fish from Olt River reservoirs are among the most Hg-contaminated worldwide.•Mercury is methylated in the water column and in sediment near the sediment–water interface.•Fluxes of Total-Hg and methyl-Hg (MMHg) from sediments show strong seasonal variation.•Hg released from the chlor-alkali plant appears highly reactive and thus available for methylation by microorganisms.•High sulphate levels, yet low free sulphide levels, likely promoted Hg methylation by sulphate-reducing bacteria.

We examined mercury (Hg) biogeochemistry and biomagnification in the Babeni Reservoir, a system strongly affected by the release of Hg from a chlor-alkali plant. Total mercury (THg) concentrations in river water reached 88 ng L−1 but decreased rapidly in the reservoir (to 9 ng L−1). In contrast, monomethylmercury (MMHg) concentrations increased from the upstream part of the reservoir to the central part (0.7 ng L−1), suggesting high methylation within the reservoir. Moreover, vertical water column profiles of THg and MMHg indicated that Hg methylation mainly occurred deep in the water column and at the sediment–water interface. The discharge of Hg from a chlor-alkali plant in Valcea region caused the highest MMHg concentrations ever found in non-piscivorous fish worldwide. MMHg concentrations and bioconcentration factors (BCF) of plankton and macrophytes revealed that the highest biomagnification of MMHg takes place in primary producers.

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