TP7 Fugacity Models: New and Different Uses
Room 18C/D, Level 4
2:10 PM - 5:30 PM, Tuesday, 11 November 2003
Chair: Diamond, Miriam ,
(302) A mercury regional mass balance model applied to the San Francisco Bay area.
MacLeod, M1, McKone, T1, 2, Mackay, D3, 1 Lawrence Berkeley National Laboratory, Berkeley, CA, USA2 University of California School of Public Health, Berkeley, CA, USA3 Trent University, Peterborough, ON, Canada
ABSTRACT- We present a regional contaminant fate model for mercury based on the fugacity concept. The model accounts for the transport and transformation of three species of mercury (elemental, divalent and methyl mercury) in a unit-world made up of five compartments; the atmosphere, soils, vegetation, water and sediments. Mass balance equations for the three inter-converting mercury species are formulated by constructing a conventional fugacity model for elemental mercury and expressing all transfer coefficients in terms of the corresponding transfer coefficient for the elemental species. This method is only applicable if the ratios of mercury species concentrations in each environmental medium is fixed. Deposition of divalent mercury from the atmosphere to soils, vegetation and water is calculated using a rate-constant approach since the fugacity of involatile species in air is undefined. We evaluate the model by developing a mass balance for mercury in the San Francisco Bay area, and comparing our results to monitoring data gathered for the Regional Monitoring Program by the San Francisco Estuary Institute. Tributary loading and re-suspension of sediments contaminated during historical mining activities have been identified as the dominant sources of mercury to the estuary. We were able to rationalize mercury concentrations observed in the system with identified sources implying there are no large and unknown source categories. The response time to changes in mercury loading is discussed using results from a dynamic version of the model.
Key words: mercury, mass balance, San Francisco Bay, model evaluation