Environmental Engineering Reference
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reduction in contaminant mass within the groundwater, hence they do not contribute
to the overall rate of output from groundwater. But in the broader sense, when defin-
ing a contaminant plume as the volume of groundwater that contains contaminants
above a certain limit, e.g., the detection limit of the contaminant, the plume's out-
line shown by this concentration isoline can become stationary just due to dispersion
and diffusion processes. An analytical solution to calculate the time that is needed to
achieve steady-state conditions for a certain concentration isoline only considering
dispersion and diffusion can be found in (Domenico
1987
).
Sorption, volatilisation or uptake by plants entail not only a contaminant con-
centration reduction, but also a contaminant mass reduction in the groundwater by
shifting the contaminants into another compartment (i.e., into sediment, soil gas or
plants). The sole destructive processes that reduce contaminant mass, not only by
shifting them into other compartments, but by degrading them into - hopefully -
harmless end products, are biodegradation and chemical transformation.
The way Natural Attenuation processes act on the development of a contami-
nant plume in groundwater is schematically depicted in Fig.
22.3
. A mathematical
description of the transport processes and explanation of important parameters
influencing these processes is found in Rolle et al. (
Chapter 19
of this topic).
Fig. 22.3
Schematic illustration of the impact of transport processes on contaminant plume
development in groundwater at different time steps (Teutsch et al.
1997
)
22.2.2 Proving Natural Attenuation and Implementing Monitored
Natural Attenuation
Considering Natural Attenuation as a sole or additional remediation option, two key
questions concerning the source zone and the evolving contaminant plume have to
be addressed:
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