PARENT SESSION
SA5 - Selenium: Toxicity Investigations of Ecological Risks
Chair: Harb, John¹, ¹ nharb@do.usbr.gov
8:00 AM to 12:00 PM - Sunday, 17 November 2002
Room Ballroom D

(049) Using ecosystem-scale modeling to forecast implications of different selenium guidelines.

Luoma, Samuel^*,1, Presser, Theresa¹, ¹ US Geological Survey, Menlo Park, CA, USA

ABSTRACT- The state of knowledge about at least some contaminants is sufficient that most of the steps between loadings and effects can be considered in evaluations, from risk assessments to determination of environmental guidelines. In this paper we use an ecosystem-scale modeling approach to forecast the environmental implications of different choices for selenium guidelines. The "ecosystem-scale model" is a stepwise approach that combines existing knowledge (both generic and site-specific) about selenium loadings, concentrations, speciation, transformations, bioaccumulation and trophic transfer to forecast the fate and effects of Se under different environmental conditions. Forecasts from the model are then compared to proposed guidelines for water, sediment, invertebrate tissues and predator tissues to provide context. An advantage of the stepwise approach is that it confronts knowledge uncertainty more directly than traditional methods. In this case, the model is used to show what range of loadings to San Francisco Bay might be possible for dissolved guidelines in incoming rivers or in the Bay of 1, 2, 5 and 75 g/L. The particulate concentrations of selenium that would result at the different dissolved guidelines are forecast using transformation constants typical of rivers, wetlands and estuaries. Uptake by invertebrates from two different food webs are estimated from DYMBAM models for animals of the San Francisco Bay-Delta, and tissue concentrations are compared to tissue guidelines. Tissue concentrations in different predators are forecast from existing field data. The results show that protection provided by a guideline (except the acute criteria) is highly dependent upon environmental conditions. Transformation constants and food web-type are critical in determining what animals are threatened and when. The method could provide a framework for site-specific analysis of the implications of selenium contamination.

Key words: selenium, guidelines, bioaccumulation, San Francisco Bay