MERCURY METHYLATION AND DEMETHYLATION IN SOILS NEAR ABANDONED MERCURY MINES IN ALASKA, USA

We measured methylmercury formation and degradation along with total mercury (THg), methylmercury (MeHg), and mercury (II) (Hg(II) concentrations in soil samples collected near two abandoned mercury (Hg) mines in Alaska. Total organic carbon (TOC) was also measured in these samples. At several of the sample sites we collected soil samples in spring and in summer to determine seasonal differences. As expected, consistently high levels of all Hg species (up to 3,200 mg/kg THg, up to 41 µg/kg MeHg, and up to 318 mg/kg Hg(II)) were found in samples near the mine or in mine tailings while lower levels (up to 36 mg/kg THg, 28 µg/kg MeHg, and 3.5 mg/kg Hg(II)) were observed in samples upstream from the mines or at background sites. However, an inverse relationship appears to exist between the THg concentration and the proportion of MeHg found. That is, when low concentrations of THg are found, the proportion of MeHg is high and visa-versa. MeHg concentrations show a fairly close relationship to TOC content in the soil samples; generally MeHg levels increase as TOC increases. Methylmercury formation and degradation rates were determined by adding ²⁰³HgCl or ¹⁴C-MeHg radiotracers to soil sample slurries and measuring the radioactive end products formed after incubation. Overall demethylation rates were higher in the spring (up to 4.1%/day) than in the summer (<1.9%/day) samples. Both methane (¹⁴CH₄) and carbon dioxide (¹⁴CO₂) are produced in the spring soil samples suggesting that the mer system and the oxidative degradation pathways are active in the spring when soils are saturated with water and cold. Only ¹⁴CH₄ was observed in the summer samples suggesting that the mer system is the dominant degradation pathway in the summer when soils are drier and warmer. Mercury methylation rates determined by measuring the amount of ²⁰³Hg-MeHg produced are low overall (< 0.3%/day) but are higher in the spring samples than in the summer samples. Methylmercury production and degradation appear to show little relationship to any of the Hg species concentrations measured indicating that biogeochemical conditions in the soil might be important factors in determining Hg methylating and demethylating activities.