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
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.
NEXT
ENVE 651 Home Module 5 Index. Back