RI/FS work plans for human health and ecological receptors (e.g., birds, fish, and plants) serve as the guiding documents for studies to be conducted over the next nine to twelve years. To read workplans, see page 15 for EPA website and depository locations.

At the heart of the work plan is a conceptual site model. The model begins by identifying chemicals of interest, e.g.--metals (including mercury) and organics (e.g.--PCBs - polychlorinated biphenyls).

The conceptual site model shows the various pathways that chemicals of interest can become present in the ecological or human environment. Each pathway begins with a source, e.g. a smelter operation. A pathway can then separate into more pathways as scientists consider different mechanisms of discharge into the environment. For instance, a smelter could discharge metals in the form of slag or wastewater, or as air particles through a smoke stack.

For each discharge mechanism, different physical and chemical interactions can occur. Called transport and fate, scientists study how, where, and in what form chemicals of interest settle in the environment. Granular slag, for instance, can end up buried at the bottom of the lake, or be suspended in the water column. Particles can also change during transport as they attach to larger particles, split into smaller particles or dissolve.

These complex transport and fate mechanisms can distribute chemicals of interest throughout the Lake Roosevelt environment. Called exposure media, chemicals may end up in the air, surface water, groundwater, porewater (water found between particles of sediment), sediment, soil, and biota (plants and animals).

Based on how chemicals are dispersed, scientists must then determine if there are ecological receptors that transfer these chemicals into the aquatic and riparian food web. Microscopic plants, clams, worms, and insects are examples of potential ecological receptors that can be found at the base of the food web. For chemicals migrating into the food web, bioaccumulation becomes a critical concern.

For instance, the ecology of an area can be compromised if chemicals build up in the tissues of worms, insect larvae, and other organisms affect reproduction or growth. And as one organism consumes another, the concentration of organic contaminants (like PCBs) and mercury can increase (called biomagnification).

The conceptual site model provides the framework for assessing these very complex interactions. Through rigorous study, both human health and ecological risks can be assessed. The difference between the ecological and human health work plans is determining the effect and risk of chemicals that may accumulate in our bodies vs. the environment.

Possible Migration of Contaminates Up the Food Chain

chart to enlarge and/or print.