Risks
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A plume occurs when chemicals are carried
in moving fluid. Laminar flow occurs when a fluid moves in
"straight lines" through an area. A plume under laminar flow
conditions can be visualized by thinking of a smokestack on a day when
the wind is blowing steadily. The smoke plume moves with the fluid and
spreads out as a cone turned on its side. Chemical plumes are a
"worse-case" scenario. Plumes of toxins are not observed at the
Stauffer Site. Unlike most sites the Stauffer site is tidally
influenced and the groundwater flow can be reversed near the Anclote
river. This sets up a complex hydrology that is atypical since
underground mixing can occur. This
may be beneficial, since toxins are more readily dispersed. It also
may result in reduced stability of in
situ stabilized structures. |
Decisions
arrived at during the course of a Superfund Cleanup are made using mathematical
formulas to calculate the amount of risk associated with a course of action. The
Remedial Investigation phase of the cleanup is meant to provide the data (raw
numbers) used in risk calculations. During the Feasibility Study Phase the raw
data is applied in risk "algorithms" to project the effect a cleanup
option has on the risk to humans and the environment. Risk calculations consider
exposure pathways, amount of
exposure under both commercial and residential scenarios, toxic nature of the
chemicals, and "conservative" factoring. These elements are discussed
below.
Exposure
Pathways
An
exposure pathway is a route by which a chemical moves in the environment to
threaten humans, animals, or plants. For example, a chemical that dissolves in
water can move downward from rainwater action and laterally from groundwater
flowing underground. The chemical may move toward a well from pumping activity.
Therefore, the risk for groundwater is calculated based on the chemical's
mobility (solubility in water and the nature of the soil), the amount of
rainfall, the groundwater flowrate, and the presence of the active wells.
Besides groundwater chemicals can move with rainwater erosion laterally across a
site, can move as windborne particles, or can be biologically transported by
absorption/locomotion in plants and animals.
Amount
of exposure under both commercial and residential scenarios
The
future use of the land effects the risk calculation. Exposure to a chemical can
be 24 hours per day in a residence. Children and the elderly are often more
susceptible to chemical injury and are not likely to be present at a commercial
site. Commercial exposure is
usually limited to the 40-hour work week. Also,
the type of commercial activity is important. Skin contact with the ground is
unlikely in an office, but very common in agricultural activities. Generally the
EPA uses risk calculation that are "orders of magnitude" (powers of
ten) different for commercial and residential. Residential exposure tends to
involve more pathways as well. Chemicals can become attached to clothing from
drying outside on a clothesline, for example, whereas in a commercial scenario
the exposure would be limited to only direct contact.
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