Contaminant Fate and Transport - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 273

8
Contamin antFateandTransport
8.1 Introduction
We now know what contaminants are, where they come from, and what they can do to
our bodies if we are exposed to them. We know that a contaminant only presents a risk to
human health and environment if there is a completed exposure pathway. So, where do
contaminants end up when they are released into the environment, and how do humans
and the environment become exposed to contaminants?
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 contaminant source, its mobilization or transport, and its ulti-
mate fate or resting place, termed a sink (Rogers et al. 2007). Sources of contamination
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) contaminants released into the environment are often mixtures,
(2) each contaminant is unique chemically, and (3) the geologic environment in which
the contaminants are released is also unique. It is, therefore, necessary to understand the
physical chemistry of each contaminant, the microbiologic environment, and the geologic
environment into which they are released when characterizing their fate and transport
(Rogers et al. 2007).
During their transport and before reaching their final sink, certain contaminants may
reside at multiple intermediate sinks for different periods of time. Intermediate sinks
include surface water, groundwater, and the atmosphere, and 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 contamination, although wave action and ocean currents
may occasionally bring contamination 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
(Rogers et al. 2007). In Chapter 7, it was noted that substances with higher toxicity pose
greater potential for adverse 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. In these locations, the potential for widespread