Effect of Electron Donor to Sulfate Ratio on Mercury Methylation in Floodplain Sediments under Saturated Flow Conditions

A column transport experiment was conducted to examine the release and methylation of Hg using Hg contaminated sediment from the floodplain of the South River near Waynesboro, Virginia. Three input solutions were sequentially introduced into the column. Input 1 was unamended South River water, Input 2 was river water amended with 100 mg L⁻¹ SO₄ and 3600 mg L⁻¹ lactate, and Input 3 was river water amended with 500 mg L⁻¹ SO₄ and 340 mg L⁻¹ lactate. During the first stage of the experiment (Input 1) the effluent Hg concentration was initially 4 µg L⁻¹ and peaked at 21 µg L⁻¹ and after 21 pore volumes stabilized at 13 µg L⁻¹. During the second stage, at high lactate to SO₄ ratios, elevated concentrations of acetic and propionic acids were detected, indicating that fermentative bacteria were dominant. During the third stage, at high SO₄ to lactate ratios, a decrease in SO₄ and an increase in H₂S concentrations were detected in the column effluent indicating that SO₄ reduction was occurring. Concentrations of methyl Hg (MeHg) in the effluent were variable over the duration of the experiment. During the first phase, concentrations of MeHg remained <3.3 ng L⁻¹. During the fermentative stage, concentrations of MeHg increased to a maximum value of 32 ng L⁻¹, and during the sulfate-reducing stage to a maximum value of 266 ng L⁻¹. When the column was deconstructed both molecular and cultural techniques indicated that sulfate reducing bacteria were most dominant near the influent port. These results indicate that the formation of MeHg in the sediment is not limited by the availability of Hg and that the bacterial community that contributes to mercury methylation can respond quickly to changes in the abundances of electron donors and acceptors.