Let's think about the first two steps as refining the SCEM. You might start this as soon as you know more about the situation, perhaps after the PA/SI. A SCEM describes chemical sources, release mechanisms, exposure media and pathways, uptake routes and receptor populations. Let's go through these quickly as generalities, then we will consider their meaning with some examples.

Step 1. Characterize the Exposure Setting

Physical Setting

• Geology Consider topography: land forms, slope, distance to water bodies
• Hydrogeology Number and use of aquifers, groundwater flow direction
• Climate Temperature and precipitation, relation of seasons to land use
• Meteorology Prevailing wind direction, storms
• Vegetation Soil cover, potential for dust, food chain exposure
• Soil type Chemical binding and leaching potential

Identification of potential receptors

• Residents
• Workers
• Trespassers
• Recreational Users
• Special: children, elderly
• Activity patterns: season activity, portion of time spend in exposed locations.
• Present population, future population

Step 2. Identify Exposure Pathways and Potential Intake Routes.

Chemical Sources and release mechanisms.

• Types of Chemicals chemical properties: solid, liquid, persistence
• Releases past, present, future
• Source leaking drums, contaminated soil

Exposure Points

• On site
• Off site

Exposure routes through which intake of the contaminant might occur

• Ingestion of Water deliberate drinking from ground- or surface water sources. Incidental ingestions of surface water during swimming and recreation.
• Dermal contact with Water: recreations, showering
• Ingestion of soil or sediments. if soil is exposed, consider amounts of vegetation and paving. Soil is ingested by hand to mouth transfer. Sediment ingestion is less frequently an issue, unless water bodies dry up or during recreation.
• Dermal contact with soil and sediments. 
• Inhalation exposure. both vapor phase, particle phase
• Exposure to biota. plants and animals (which includes an evaluation of how it got into the biota.)

For both steps 1 and 2, you must: 

When identifying the pathways and potential intake routes, you must consider both the present and the future.

• Changes in land use. From industrial to residential, from uninhabited to inhabited.
• Changes in the concentration of contaminant.
• Transport of the contaminant. Migration over time to distant sites.
• Transport of the contaminant to different media. Soil washed into a river.
• Accumulation in plants and animals over time.
• Uses of the media. Soil for crops, water for irrigation or drinking.

Land use. Here are some common land uses:

• Agricultural
• Residential
• Commercial
• Industrial
• Recreational
• Mining
• Mixed: Often the case. Must focus on the use that will result in the highest most exposure
• Unused.

A completed exposure pathway must have:

• A chemical source and mechanism of release
• An Intermedia transport mechanism, if the exposure point differs from the source
• A point of potential human contact (called an exposure point)
• An exposure route through which chemical uptake by a receptor can occur.

In general we must "address" all exposure pathways, but then only consider the completed pathways in our detailed analysis. For example, if there was no groundwater table at a contaminated site, you would not do any detailed analysis of exposure via groundwater. You would need to mention in your final risk characterization report that there was no groundwater on the site.

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