ABSTRACT
We now know what pollution is, how and where it originates, and what it can do to our bodies if we are exposed to most forms of pollution. We know that pollution only presents a risk to human health and environment if there is a completed exposure pathway. So, where do the different types of pollution end up when released into the environment and how do humans and the environment become exposed to pollution?
The answer to these questions is obtained through an understanding of the process called contaminant fate and transport, defined as the sequence of anthropogenic and natural events involving a pollution source, its mobilization or transport, and its ultimate fate or resting place, termed a sink. Sources of pollution include many human activities performed primarily at the surface resulting in the release of toxic substances into the environment. These toxic substances may be transported over time, or remain relatively close to their source before they are degraded, transformed, or destroyed. Because: (1) pollutants released into the environment are often mixtures; (2) each contaminant is unique chemically; and (3) the geologic environment in which the pollutants are released is also unique, understanding the physical chemistry of each, the microbiologic environment, and the geologic environment into which they are released becomes necessary for characterizing their fate and transport.
During their transport and before reaching their final sink, certain pollutants may reside at multiple intermediate sinks for different periods of time. Intermediate sinks include surface water, groundwater, and the atmosphere, because the contaminants held within these water-containing sinks will flow and ultimately reach the oceans. Aquifers with a very low hydraulic conductivity are for practical purposes considered final sinks, as are inland bogs and some wetlands. Sediment and soil can function as intermediate or as final sinks, since erosion may move both of these unconsolidated materials. The oceans are almost always a final sink of pollution, but wave action occasionally brings pollutants onshore. To a large extent, the level of human health and/or environmental risk is a function of two fundamental concepts introduced in this chapter: mobility and persistence. Mobility is a measure of a substance’s potential to migrate. Persistence is a measure of a substance’s ability to remain in the environment before being degraded, transformed, or destroyed. Substances with higher toxicity pose greater health risks, but the risk to humans and the environment grows exponentially if the chemical is both mobile and persistent. For example, a highly toxic but immobile chemical may affect a few people in a warehouse through inhalation, whereas a mobile and persistent chemical of moderate toxicity can contaminate a public water supply, or migrate to a different sink where the potential for widespread human exposures and ecosystem damages is much higher.
There are tens of thousands of pollutants existing everywhere. After being released into the environment, they migrate in air, soil, and water. Some are persistent, while others are not. Some dissolve in water and some do not. Some are transported around the globe in the atmosphere while others do not. Along with the chemistry of pollutants, geography, geology, hydrogeology, and atmospheric conditions also play a significant role in affecting the fate and transport of pollutants and determines their final disposition.
