Environmental Engineering Reference
In-Depth Information
Even more complex than the presence of multiple contaminants is the pres-
ence of liquids with a different density in the groundwater. Light Non-aqueous
Phase Liquids ( LNAPLs ), for example, are lighter than water and may often float
on the water table. Dense Non-aqueous Phase Liquids ( DNAPLs ), on the con-
trary, have a higher weight than water. Density differences may lead to so-called
stable or unstable displacement. The instability of the flow pattern will increase,
and show a more chaotic behaviour, when the density differences are bigger. Also
fluid density variations caused by changes in the contaminant or colloidal concentra-
tion, temperature, or pressure of the groundwater, may drive contaminant transport
(e.g., Simmons et al. 2001 ). Except for transport of pure liquids, the LNAPLs and
DNAPLs are a source for contaminants which may migrate from the pure liquid into
the adjoining (clean) water bodies. An example of a transport model that calculates
the transport of contaminants originating from the dissolution of DNAPL pools in
aquifers is given in Tatalovich et al. ( 2000 ).
The following processes control contaminant transport in a more or less homo-
geneous soil:
convection (advection): migration of a mass of groundwater (and soil gas in a
water-unsaturated upper soil layers), driven by pressure differences;
(molecular) diffusion: migration of contaminants within the groundwater (and,
when present, soil gas), driven by differences in contaminant concentrations;
(hydrodynamic) dispersion: migration of contaminants within the groundwater,
driven by irregularities of the soil pores and of the flow velocity pattern within a
pore;
adsorption and desorption: exchange of contaminants between the solid phase of
the soil and the pore water or groundwater;
degradation: the biological or chemical reduction of a contaminant to one or more
less complex compounds;
water uptake by roots (upper unsaturated layer, only).
As an alternative for adsorption and desorption processes, the chemical part
of the models may be described on the basis of thermodynamics as, for example,
shown in Meinrath and May ( 2002 ) with regard to contaminant leaching from mine
waste.
17.4.3 Reliability of Model Calculations
17.4.3.1 Uncertainties
For many decades, there has been a debate on the reliability of models that predict
contaminant transport on the basis of the processes mentioned in Section 17.4.2.2 .
In fact, this debate is still ongoing. The reliability of groundwater flow and contam-
inant transport models is a complex issue, even in homogeneous porous media. An
abstraction of the reality must be done to be able to use models.
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