Environmental Engineering Reference
In-Depth Information
Rainfall
Water
well
Leaking
tank
Surface
flow
Disappearing
stream
Spill
Water
well
Sinkhole
Water
Table
Spring
Ground water
flow direction
Water-filled cave
Limestone aquifer
FIGURE 8.12
Contaminant migration in a karst aquifer. (From United States Geological Survey (USGS), Ground-water qual-
ity protection, Open-File Report 95-376, Nashville, Tennessee, http://www.pubs.usgs.gov/of/1995/ofr-95376
(accessed December 2009), 1995b.)
example of travel times and capture zone in groundwater beneath urban areas. Any con-
taminant reaching groundwater within the area marked capture zone has the potential to
enter the public water supply if the contaminant does not degrade before reaching a public
water supply well. Several sources of contamination listed in Section 8.1 are also shown:
septic tanks, underground storage tanks (USTs), landfills, industrial facilities, and power
plants. These contaminant sources are typical for any urban area within the United States
and pose distinct threats to contaminate a public or private water supply.
8.3.5.1  Karst Topography
In karst topographical settings, the transport of groundwater contamination may behave
similarly to surface water (Ford and Williams 2007). Some karst formations may exhibit
turbulent advective flow because they have flow rates approaching the velocities observed
in surface water flow (Heath 1983). Figure 8.12 shows an example of contaminant flow in a
karst aquifer (USGS 1996a).
8.3.6 Fate and Transport of Contaminants in the Atmosphere
Different contaminants affect different portions of the atmosphere (Chapter 7). For
instance, chlorofluorocarbons (CFCs) affect the protective ozone layer. Figure 8.13 shows
the layers of the atmosphere and the location of the ozone layer within the stratosphere.
Contaminant behavior in the atmosphere is very similar to the behavior of contaminants
observed in surface water. Advective transport, turbulent diffusion, and molecular dif-
fusion also influence contaminant migration in the atmosphere (Hemond and Fechner-
Levy 2000). Figure 8.14 shows smoke from a fire billowing up (turbulent diffusion) into the
atmosphere and the horizontal movement of the smoke by advective transport. We also
see a type of advective transport called convection; in this process, air rises due to thermal
differences in the atmosphere. Turbulent advective mixing by wind and convection of the
atmosphere is most significant within the troposphere (Schlatter 2009). These forces are
very effective at transporting contaminants in the gas phase and also do a good job of
moving solid particulate matter in the atmosphere (USEPA 2008a).
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