1 / 19

Mercury Methylation in Contaminated Sediments

Mercury Methylation in Contaminated Sediments. Nate Johnson Dr. Reible Research Group Meeting 12/02/2005. Outline. Mercury background Where does mercury come from? What is the primary exposure pathway? Mercury behavior in sediments Biogeochemical conditions Methylation Sulfur speciation

belinda
Download Presentation

Mercury Methylation in Contaminated Sediments

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Mercury Methylation in Contaminated Sediments Nate Johnson Dr. Reible Research Group Meeting 12/02/2005

  2. Outline • Mercury background • Where does mercury come from? • What is the primary exposure pathway? • Mercury behavior in sediments • Biogeochemical conditions • Methylation • Sulfur speciation • Experimental design • Cap simulation • Microprobes

  3. Mercury background • Sources of Mercury • Almost all mercury in aquatic environments is from anthropogenic sources • Atmospheric sources, wet/dry deposition(Hg0) • Widespread b/c of long residence time in atmosphere) • Industrial (legacy) sources (Hg2+) • Higherconcentrations of mercury • Complications with other contaminants

  4. Mercury background • Exposure Pathways • Fish tissue is primary exposure pathway • Methylmercury (CH3Hg) readily bioaccumulates in fish tissue • Variations of mercury in fish tissue cannot be accounted for by variations in bulk aqueous mercury concentrations • Un-impacted lake has high methylmercury in fish

  5. Mercury background Bioaccumulation >105 x higher concentrations in fish than in water!

  6. Mercury background • Methylmercury • Processes producing and eliminating methylmercury (methylation processes) control human exposure • Most of these processes occur in sediments at the bottom of a water body • Many processes shown (in lab) to affect mercury methylation • Primary process observed in field is tied to Sulfate Reducing Bacteria

  7. Mercury background Mercury Methylation in Sediments

  8. Mercury behavior in sediments • Nutrient Cycling • Sulfate Reducing Bacteria are most important for methylation • Only occurs in specific geochemical conditions • Location of SRB activity is defined by nutrient cycling • Biogeochemical conditions defined by bulk nutrient concentrations

  9. Mercury behavior in sediments Nutrient Cycling Fossing et. al 2004

  10. Mercury behavior in sediments • Mercury-Sulfur Speciation • Mercury-Sulfur speciation is key to understanding mercury methylation • Affects bioavailability • Affects mercury solubility • Much evidence is only circumstantial • Increased methylation w/sulfide, sulfate, HgS0… • Extremely difficult to measure different mercury-sulfur species

  11. Mercury behavior in sediments Mercury – Sulfur Speciation (continued…) Sulfate Reducing Bacteria Hg(CH3)2 Hg0 SO4 Demethylation? HS- S0 HgS0 HgS22- Sulfate Reducing Bacteria (fermentation…?) Hg(HS)20 HgS2H- Hg(Sn)SH20 Uncharged (bioavailable) Charged (not bioavailable) CH3Hg+ my as yet incomplete picture… HgS(s)(cinnabar) Solid precipitate

  12. Mercury behavior in sediments • Modeling Mercury • Many models try to trace large scale mercury transport • MCM – Mercury Cycling Model, Florida Everglades • WARMF – TMDL calculations • Focus on transport from watershed, to biota, up food chain • Simplistic representationsof methylation processes

  13. Mercury behavior in sediments • Modeling Mercury (continued…) • Old models do not include sulfur complexes • Only recent research has shown importance of sulfur speciation Schnoor 1997, no explicit treatment of sulfur

  14. Mercury behavior in sediments • Modeling Mercury (continued…) • Modeling Questions • Can mercury methylation processes be simply ‘piggy-backed’ on a bulk nutrient parameter, i.e. sulfide? • Is a more detailed treatment of sulfur speciation necessary to understand methylation? • Can we accurately measure/model detailed sulfur speciation in sediment porewaters?

  15. Experimental Questions / Design • Research Questions • How will placing a cap affect mercury methylation? • Change redox conditions • Shift microbial activity • Physically separate from water column • Has not been detailed analysis of how acap affects microbial populations in sediments • So far have had bigger problems to worry about • May be important now

  16. Experimental Questions / Design • Preliminary Experimental Design • T-Cell with contaminated sediment from Anacostia • Initially, flow pond water over sediment and observe steady state nutrient (biota) profiles and methylmercury production • Next, cut off oxygen, organic carbon, nutrient fluxes to simulate conditions after cap placement • Monitor transition to new steady state and observe changes in nutrient (biota) profiles and methylmercury production

  17. Experimental Questions / Design • Microprobes • Oxygen • They’ve been a pain in the neck • Extremely temperature sensitive • Response time, stabilization, drift, ice… • Redox • Haven’t tried it yet • Sulfide • Don’t have all the equipment for yet

  18. Experimental Questions / Design • Design Questions • Will data from probes alonebe sufficient to locate mercury methylation areas in sediment? • Will we have to take cores? • Can we Measure detailed speciation of Mercury-Sulfur complexes? • Can we Model detailed speciation of Mercury-Sulfur complexes?

More Related