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

2. 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

3. PCB Concentrations in
San Francisco Bay Fish Fillets, 2000

4. PCBs in mussels
ng/g lipid
~ 50% decline

5. 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?

6. A PCB Mass Budget A first step toward a PCB mass budget for the Bay
Followed approach described by:
Gobas et al (EST 29: )
Mackay et al (JGLR 20: )
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

7. 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

8. PCB Fate in Bay Water and Sediment

9. Predicted Long Term Trends in PCB Mass with Varying Loads

10. Predicted Trends in PCB Mass for Different PCB Congeners

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

12. 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 A B C

13. 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: A B C

14. Next Steps for PCB Fate Modeling in San Francisco Bay
Food web model coming soon (Frank Gobas and John Wilcockson) A B C

15. Modeling PCB Trophic Transfer

16. Mass Budget for a White Croaker
Growth Dilution
Gill Elimination
Metabolism
Gill Uptake
Dietary Uptake
Fecal Egestion

17. Next Steps for PCB Fate Modeling in San Francisco Bay
Quantify uncertainty of estimates
Go multibox A B C

18. The Multibox Model
Collaboration with Dave Schoellhamer (USGS, Sacramento)
Builds on existing model calibrated for salinity
RMP really interested in 5 boxes

19. 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 A B C

20. Next Steps for Management
Look for manageable PCB loads
Watch out for “PCBs” of the future (e.g., PBDEs) A B C