The Long Term Fate of PCBs in San Francisco Bay

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The Long Term Fate of PCBs in San Francisco
Bay Jay A. Davis San Francisco Estuary Institute
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Regional Monitoring Program

• Regional Monitoring Program for Trace Substances
  in the San Francisco Estuary
• Innovative partnership between government,
  dischargers, and scientists
• 3 million/year of stable funding
• Began in 1993
• World class monitoring
• www.sfei.org

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PCB Concentrations in San Francisco Bay Fish
Fillets, 2000
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PCBs in mussels 1981-2000 ng/g lipid 50
decline
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Questions

• Why is the Bay responding so slowly?
• How long will it take for fish to be safe to eat?
• What would the response be with reduced inputs?
• How large are the inputs?
• What studies are needed to better understand PCB
  fate in the Bay?

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A PCB Mass Budget

• A first step toward a PCB mass budget for the Bay
• Followed approach described by
• Gobas et al. 1995 (EST 29 2038-2046)
• Mackay et al. 1994 (JGLR 20 625-642)
• One-box model for the whole Bay
• A water and sediment model
• Individual congeners
• PCB 118 used as typical PCB
• Report benefited from extensive peer review

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Input Data

• Approximately 30 input parameters
• Physical data for the Bay
• Flow
• Sediment budget
• RMP concentration data
• Chemical properties
• Sensitivity analysis conducted on all parameters

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PCB Fate in Bay Water and Sediment
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Predicted Long Term Trends in PCB Mass with
Varying Loads
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Predicted Trends in PCB Mass for Different PCB
Congeners

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Predicted Long Term Trends in PCB Mass with
Varying Depth of the Active Sediment Layer

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Conclusions

• At this stage, the value of the model is in
  showing the response to ranges of input values,
  not in the precision of estimates
• The most influential parameters included
  degradation half-life in sediment, Kow, outflow,
  average PCB concentration in sediment, and depth
  of the active sediment layer
• Sediment dynamics are very important, including
  mixing and erosion/burial

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Conclusions (continued)

• The model suggests that annual loads from 1982 to
  2000 were in the 0 to 20 kg range
• Annual inputs of 10 to 20 kg could significantly
  delay declines in PCBs
• Different PCB congeners are predicted to have
  very different response times
• For more information or a copy of the report
  jay_at_sfei.org

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Next Steps for PCB Fate Modeling in San
Francisco Bay

• Food web model coming soon (Frank Gobas and John
  Wilcockson)

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Modeling PCB Trophic Transfer
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Mass Budget for a White Croaker
Growth Dilution
Gill Elimination
Metabolism
Gill Uptake
Dietary Uptake
Fecal Egestion
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Next Steps for PCB Fate Modeling in San
Francisco Bay

• Quantify uncertainty of estimates
• Go multibox

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The Multibox Model

• Collaboration with Dave Schoellhamer (USGS,
  Sacramento)
• Builds on existing model calibrated for salinity
• RMP really interested in 5 boxes

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Next Steps for PCB Modeling in San Francisco Bay

• Better characterization of sediment dynamics
• Also need better estimates of
• Degradation rates
• Outflow
• Average concentrations
• Historic long term trends

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Next Steps for Management

• Look for manageable PCB loads
• Watch out for PCBs of the future (e.g., PBDEs)