Hazardous Materials in the Environment

The movement of contaminants within a medium such as air, groundwater, or soil is known as transport.

Hazardous materials can enter the environment either from a specific source that can be pinpointed, known as a point source, or from sources that are more spread out, known as area sources. A factory smokestack and the flow of toxic waste from a pipe to a stream are point sources, while the liquid runoff from a field in which pesticides were used is considered an area source.

Contaminants behave differently in the environment depending on their physical state. A solid may stick to surfaces, scatter, or form a dust cloud; a liquid may seep into the ground, flow along the ground, or vaporize and become a gas; a gas will expand and be carried by the wind. Some chemicals are volatile, meaning that they evaporate easily. Such a chemical may enter a stream as a liquid but rapidly become an air pollution problem.

A non-volatile chemical entering the same stream at the same point may behave quite differently. A soluble chemical is one that will dissolve readily in water, and would be carried by the stream. Soluble chemicals tend to be mobile, meaning that they will move rapidly in the ground because they can be easily dissolved in groundwater. Another chemical might be more likely to adsorb to soil particles, becoming attached to particle surfaces. Such a chemical would attach to particles in the stream and eventually settle at the bottom. If the chemical were a persistent one, which resists breakdown in the environment, it might remain there for some time in the same form, while bacteria might break down a less persistent chemical. This breakdown is called biodegradation, and is an important risk management concept. Sometimes it is possible to increase biodegradation so that materials lose their harmful properties more readily.

The process of chemical breakdown, or biodegradation, can cause materials to lose their harmful properties and, in effect, “disappear.”

Certain chemicals tend to become more highly concentrated as they move through the food chain. This process is known as biomagnification.

Contaminants enter any of the various media—air, groundwater, surface water, or soil—and move as a mass along with the general flow of that medium. This movement of contaminants within a medium is called transport. Substances in transport also tend to spread out as they move, becoming diluted to a varying extent by the medium. This generally reduces the concentration, and therefore lowers the level of hazard.

Once a toxic substance is released into a medium, a number of different processes can occur:

The substance moves in a pathway determined by its own characteristics and those of the medium that is carrying it.
The substance spreads out or disperses, reducing the level of hazard. This means of reducing risk is not always reliable or consistent, however. For example, there may be periods of low flow in streams when the volume of water is reduced and less dilution occurs.
The material may change chemically or break down into other elements or compounds. Sometimes a contaminant will combine with another substance to become a more dangerous chemical; at other times it will be rendered less harmful by the encounter. Some chemicals have a synergistic effect. (A synergistic effect is the effect of two chemicals acting together causing a greater effect than the simple sum of their effects when acting alone.)
A chemical may also potentiate. (Potentiation is the ability of one or more chemicals to increase the toxicity of another chemical to cause greater harm than the total effects of the two expected reactions.) An example is an alcohol. When mixed it will potentiate the effects of many chlorinated hydrocarbons.
The reverse is also possible regarding a chemical. It may present an antagonistic effect. Or the results of two or more chemicals may lessen the total effects of their combined exposure in the body. An example might be one taking aspirin to lessen the effects of an alcohol (wine or beer) exposure from the night before.
A toxic substance may move from one medium to another (for example, evaporating from water into air)
Toxic substances can build up in the food chain. Organisms can absorb contaminants such as pesticides in a process known as bioaccumulation. These contaminants are later released into another organism that eats that animal or plant. Certain chemicals also tend to become more concentrated as they move up the food chain. (For example, toxic concentrations may be higher in a bird that ate insects containing poison than in the insects themselves.) This is known as biomagnification. Often, an important part of understanding a chemical’s risk to humans is being aware of how a particular contaminant will move through a food chain and how each animal or plant in the chain may be affected.

The way a pollutant is transformed by chemical reactions and transported through the environment is called its fate. As we have seen, the fate of chemicals released at the same site may be extremely different.