Sub-module 8B, page 3

Learning Goals
Become familiar with some common terms

 

Environmental Media

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 :

Air
Air moves laterally by wind. Contaminants are both advected by the wind and diffuse though the air. Air often contains aerosols. Some common aerosols of interest are fog or cloud droplets, dust particles from soil, and smoke and soot from combustion. Many contaminants travel through air by clinging to these aerosols. The contaminant may be absorbed into the particle, or adsorbed onto the surface of the particle. We sometimes use the word "sorbed" to include either process. Later the particles may be deposited on the land or into surface water. This may be dry deposition, as in the settling of dust particles, or particles that are washed out of the atmosphere by rain. The aerosols may be considered as a separate media from the air that is transporting them.

Water
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.

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