Tracing the connection between contaminants in the environment and potential human and ecological risks is very complex. The Conceptual Site Model serves as a schematic for investigators to collect data from multiple sources, and then trace these sources to the point where an environmental or human health risk may be present. On a system as large as the Upper Columbia and Lake Roosevelt, this requires collecting and analyzing thousands of data points.
The model begins by identifying chemicals of interest (e.g.--metals, including mercury) and organics (e.g.--PCBs, polychlorinated biphenyls). For each chemical there are potential sources, e.g. -- a smelter operation that discharged metals in the form of slag, wastewater, or air particles.

Scientists (as the site model graphic shows) identify the exposure media, the way in which chemicals of potential concern can enter our air, water, soil, etc. From here, exposure pathways are identified to assess whether a human health or ecological risk is present. The Pathways section shows how this can occur for humans.

Different physical and chemical interactions can occur in each of the pathways. The interactions, collectively called transport and fate, are studied by scientists and evaluated to better understand how, where, and in what form chemicals of interest may be present in the environment. Granular slag, for instance, can end up in riverine sediments in the reservoir, 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 upper Columbia and Lake Roosevelt environment.

RI/FS activities measure the chemical concentrations within exposure media, then consider risks to people, wildlife and plants exposed. For example chemicals may end up in the air, surface water, sediment, soil, groundwater, pore water (water found between particles of sediment), and biota (plants and animals). The concentrations of these chemicals are needed to complete the human health and ecological risk assessments.

The graphic shows the exposure media and their pathways. It also shows what data has, will, or may be collected for each. The potential that a receptor can come into contact with a contaminant is illustrated by the solid circles within the CSM (e.g., a complete pathway). For this reason, concentrations in certain exposure media (such as surface water) can provide information for both the human health and ecological risks assessment.

As data are collected and analyzed, EPA (in consultation with tribes and participating parties) must determine if data gaps still exist and additional sampling needed. The time line of RI/FS of activities (see page 10) shows this feedback loop. Decisions about when sufficient data has been collected are the most important variable in how long RI/FS activities will continue.