Environmental Engineering Reference
In-Depth Information
Mercury
2008
2007
2006
2005
2004
2003
PCBs
Chlordane
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
Dioxins
DDT
Other
0
2,000,000 4,000,000 6,000,000
Lake acres under advisory
8,000,000 10,000,000 12,000,000 14,000,000 16,000,000 18,000,000
FIGURE 8.7
Advisories by contaminant from 1993 to 2008. (From United States Environmental Protection Agency (USEPA),
Biennial National Listing of Fish Advisories for 2008 , EPA-823-F-09-007, Washington, DC, 2009b.)
and diffusion is the action of spreading of molecules from areas of high concentration
to areas of low concentration at the molecular level. Groundwater low lacks turbu-
lent diffusion because velocities are typically much slower. In some instances, how-
ever, groundwater does display turbulent dispersion, especially in karst topography
(Chapters 2 and 3), where water flowing beneath the surface flows and behaves much
like a stream at the surface.
Figure 8.8 shows a spill from an underground storage tank (USGS 2006a). Here, advec-
tive transport of contaminants in groundwater is occurring at the water table boundary.
Diffusion, biodegradation, volatilization, and recharge from surface precipitation affect-
ing the contaminant migration are also shown. The effects of dispersion and diffusion
are represented by the spreading of the contaminant plume as it migrates from a hole or
ruptures at the bottom of the tank (USGS 1998, 2006a).
The representation of dispersion in Figure 8.8 is overly simplistic, because the geology
of unconsolidated sediments is very complex and typically displays a high degree of het-
erogeneity and anisotropic distribution patterns. As a result, contaminants migrating in
unconsolidated deposits do not migrate uniformly but migrate within the physical param-
eters of advection dictated by the particular subsurface geology.
Search WWH ::




Custom Search