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Abstract Title: Mercury Methylation of Coagulant-Treated Soils Upon Rewetting: a Controlled Laboratory Incubation
Presenter Name: Naivy Dennise Rodal Morales
Company/Institution: University of California Merced, Graduate Division, School of Engineering
Session: Mercury in Freshwater Ecosystems
Co-Authors: Naivy Dennise Rodal Morales

Abstract Information :

Cache Creek Settling Basin (CCSB) traps contaminated sediments from upstream historical mining activities and limits mercury (Hg) loading to the downstream Yolo By-Pass and Sacramento-San Joaquin Delta, California. Although the basin reduces net export of Hg, anaerobic conditions promote methylmercury (MeHg) production. We are assessing the potential for in-situ addition of commercially available coagulants, including ChitoVan? (shell-based), FERRALYTE? (iron-based), and ULTRION? (aluminum-based), to enhance Hg capture. Laboratory and field tests show enhanced removal of Hg via coagulant addition. However, coagulant-treated soil could be a subsequent source of MeHg. To test this, coagulant-treated soils were rewetted and incubated under anaerobic conditions. Treatments included the three coagulants and a non-amended-control soil, each from three field sites. Additionally, we performed a ?stress-test? with addition of organic matter (OM), spirulina powder, on select soils. After rewetting, MeHg concentrations in water (~0.1 ng/L) and soil (~1.2 æg/kg dry weight (dw)) changed little during incubation and were similar in all treatments. Water-quality measurements indicated that the rewetted soils were at redox potentials higher than those favorable for iron or sulfate reduction and not conducive to Hg methylation. The addition of OM stimulated microbial activity and led to redox potentials associated with sulfate reduction and MeHg production. MeHg concentrations peaked at ~30-40 ng/L in water and 6-9 æg/kg dw in soil by day 2-4, then decreased to initial concentrations by day 8. Sulfate concentrations declined through day 8, indicating that sulfate-reducing bacteria were still active after the MeHg peak. Results suggest that enhanced demethylation and/or low Hg(II) bioavailability accounted for the drop in net MeHg production after days 2-4. The study demonstrated that all three coagulants, which could enhance Hg removal in CCSB, do not pose a risk of enhanced or sustained MeHg production. Moreover, controlling OM and sulfate inputs to the basin could minimize MeHg production.

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