Sub-module 8B, page 3
Learning Goals
Become familiar with some common terms
If you look at the three persons in the figure on page two , you realize that the concentrations of contaminant they are exposed to are quite different. The worker at the site is exposed to a higher or lower concentration than the trespasser . In general, the concentrations decrease farther from the source, and often the concentrations at all points decrease with time as well. But how much do they decrease? The answer to that question for any particular site requires the expert input of engineers, geologists, hydrogeologists, chemists, and often biologists and statisticians. Frequently their work is tied together with mathematical models. Later in the course we will use some models and experiment with how different parameters, say ground water flow rate, effect the exposure point concentrations.
For now I'd like to introduce you to environmental media :
AirWater
Fresh water may be divided into surface water,
lakes and streams, and groundwater, water under
the ground surface. When modeling water, we often ignore the dissolved salts
and minerals, and use the chemical and physical properties of pure water. Particulate
matter in surface water is often very important and seldom neglected
in modeling. Particulates may be divided in mineral matter, such as clays, and organic matter. A percentage, perhaps 50% or 60% of the
organic matter is carbon and this is sometimes called organic
carbon. Non-organic carbon may also be present from bicarbonate and
carbon dioxide. Organic matter may range from bass and pike down to humic acids
that are essentially in solution and not particles. In general, fish and other
members of the "aquatic biota" are separated
from non-living organic matter. The particles do settle out, however many are
also resuspended by the storms and currents. Certain chemicals contaminants
are bio-concentrated or otherwise partition in
to the organic carbon and aquatic biota.
Bottoms Sediments.
Bottom sediments are often considered a separate
media from both the water and terrestrial soils. Bottom sediments often trap
and store contaminants from the water and its particulates. Sometimes the bottom
sediments remain as a source of contamination, long after it has dissipated
from the terrestrial and aquatic environment. Note there is a difference between "sediment pollution" which means the smothering of the bottom life by micro-particles versus "contamination of the sediments" which is the partitioning of chemicals contaminants into organic material in the sediment.
Soils (terrestrial)
Soil is a mixture of air, water, mineral mater, and organic
matter. It's chemical composition and structure vary considerably
with depth and often laterally as well. Contaminants often sorb
to organic matter, but some mineral matter also
"holds" contaminants, especially some clays. Often there
are complex phase transfers between the soil water, air and solids. Surface
soil is moved by rainwater runoff and blown away
as dust.
Terrestrial biota and plants
Certain plants absorb contaminants, and animals both absorb and ingest
contaminants.