Mercury Fraction and Transformation in Sediment Cores of the Eutrophic Estuary in Northern Taiwan

Abstract

The Hg fractions in three sediment cores of the eutrophic estuary in northern Taiwan were determined by the Bloom sequential extraction method, which chemically divided the sedimentary Hg into five fractions: water-soluble (F1); human stomach acid (F2); organo-chelated (F3); elemental Hg (F4), and residual (F5). The pH, redox potential, and dissolved total Hg in sediment pore waters, grain size, and total organic carbon (TOC) in sediment cores were analyzed, and the results were interpreted. The three sediment cores were in an anoxic environment. The total Hg concentrations in the sediment cores ranged between 110 and 369 ng/g, and most values exceeded the guideline value (ERL, 150 ng/g) of the EPA, U.S.A. However, the total Hg concentrations were mainly dominated by the non-labile Hg fraction (the elemental and the residual fraction), accounting for an average of 60% of the total Hg pool. The organo-chelated fraction accounted for an average of 29% of the total Hg pool. The amounts of the labile fraction (F1 + F2) of Hg in sediment cores of the middle and lower estuary were generally <2% of the total Hg pool. However, the F2 fraction in the sediment core of the upper estuary exceeded 10% of the total Hg pool. This result implied that Hg still poses a potential risk to the benthic organisms in the DRE based on the risk assessment code method. The profile variations between the labile and non-labile Hg fractions exhibited a negatively well-linear correlation, suggesting the transformation of the labile and moderately labile Hg fraction into the non-labile Hg fraction in sediment cores during the sediment burial processes. In addition, the TOC content seemed to play an important role in controlling the sediment Hg fractions in sediment cores.