Methylmercury and Mercury in SF Bay & Wetlands

1. Methylmercury and Mercury in SF Bay & Wetlands

2. What- MeHg Worry? • Most bioaccumulative form of mercury… • Formed by sulfate reducing bacteria under anoxic conditions • But is subject to degradation- • Microbial • Abiotic (photolytic)

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

4. Puckett & Bloom 2000 Calfed study Samples frozen in field vs overnight chilled shipping No correlation in MeHg analyses of samples generate MeHg degrade MeHg Random Number Generator v1

5. Heim et al (2003) Samples collected October to December 1999 Frozen in field 0-0.5cm sediments Primarily bay margins MeHg mostly < 1ng/g dry weight SF Bay MeHg (Calfed)

6. 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)?

7. 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

8. 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

9. Hg & MeHg – Random Number Generator v2?

10. 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)

11. Dominant Factors: TN, Redox

12. Redox Does Matter

13. TOC and MeHg

14. (Total) Nitrogen  TOC?

15. 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

16. 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

17. 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 <0.2 mg/kg to see impact on MeHg

18. Schollenberger Park Water treatment plant Petaluma Marsh North Petaluma Marsh South Black John Slough Toy Marsh Carl’s Marsh Greenpoint 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)

19. Hg ~Constant Among Wetlands • Similar sources- air deposition, tidally mixed waters

20. Sediment MeHg Varies Among Wetlands • MeHg produced within wetlands- mid salinity maximum?

21. Black Rail MeHg Varies Among Wetlands • Sediment  invertebrates  birds