Methylmercury and Mercury in SF Bay

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Methylmercury and Mercury in SF Bay Wetlands
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What- MeHg Worry?

• Most bioaccumulative form of mercury
• Formed by sulfate reducing bacteria under anoxic
  conditions
• But is subject to degradation-
• Microbial
• Abiotic (photolytic)

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More MeHg Worries?

• Horvat et al., 2003- MeHg higher collected under
  N2- why?
• Less degradation? Or more production? (4 hrs
  field to lab)

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Random Number Generator v1

• Puckett Bloom 2000 Calfed study
• Samples frozen in field vs overnight chilled
  shipping
• No correlation in MeHg analyses of samples
• generate MeHg
• degrade MeHg

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SF Bay MeHg (Calfed)

• Heim et al (2003)
• Samples collected October to December 1999
• Frozen in field
• 0-0.5cm sediments
• Primarily bay margins
• MeHg mostly lt 1ng/g dry weight

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What (Not) to Do?

• Yes
• Freeze, ASAP
• Minimal thawing before analysis
• No
• repeated freeze-thaw (Horvat et al 1992)
• Separate small samples as replicates
• processing in air (but maybe not an issue if
  frozen quickly)?

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RMP Methodology ( Mistakes?)

• RMP 50 stratified random sites
• van Veen grab, top 5 cm
• Composite of 2-3 grabs
• Homogenized on board (In air)
• 40 minutes, 1st grab to last jar in freezer
• Analyzed months to years later

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MeHg for RMP

• July August 2002- 2004 (2002-3 shown here)
• 0-5cm sediments
• Mix of shallow deep sites
• MeHg range 0-2ng/g dry weight
• Hg, TOC, TN, grainsize,
• Redox starting 2003

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Hg MeHg Random Number Generator v2?
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Hg Necessary, Not Sufficient

• Literature correlations over many orders of
  magnitude
• 95 of RMP Hg within 10x (0.04-0.35 mg/kg)
• MeHg production degradation -uptake
  transport
• Need reducing conditions for methylating bacteria
• Other important parameters?
• TOC, TN, grainsize, Eh (redox potential)

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Dominant Factors TN, Redox
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Redox Does Matter
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TOC and MeHg
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(Total) Nitrogen ? TOC?
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Why TN and ORP?

• TN biologically active organic matter (TOC
  includes refractory material)
• Low ORP anoxia, measure of net status
• Metal sulfides (black) visually apparent
• Field meter measurement

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Conclusions

• Surface (0-5cm) sediment MeHg similar range to
  previously found 0-0.5cm concentrations
• MeHg poorly/un-correlated to total Hg, TOC
• MeHg better correlated to TN, Redox
• Consistent w/ indicators of active microbes
• Relationships match expectations despite possibly
  compromised handling

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What Can Be Done?

• Reduce reducing environments?
• Prevent eutrophication anoxia
• May run counter to other ecosystem goals (Bay
  wetland productivity habitat)
• Decrease sediment Hg?
• May need total Hg lt0.2 mg/kg to see impact on MeHg

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Future Directions

• RMP Sampling
• Unmixed site replicates
• Rapidly homogenized splits
• Sediment Hg speciation
• Petaluma River Wetlands CBDA
• MeHg processes in fresh/brackish to saline tidal
  marshes
• Food web studies in channel/low marsh, and high
  marsh plain
• SFEI, Avocet Assoc., USGS (MP, Wi, BRD)

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Hg Constant Among Wetlands

• Similar sources- air deposition, tidally mixed
  waters

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Sediment MeHg Varies Among Wetlands

• MeHg produced within wetlands- mid salinity
  maximum?

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Black Rail MeHg Varies Among Wetlands

• Sediment ? invertebrates ? birds