In Situ Remediation Technologies - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 951

21.2 In Situ Remediation Technologies

21.2.1 Principles

21.2.1.1 Equilibrium Relations of Organic Contaminants in Soil

At contaminated sites the contaminants are most often present for a long time before
In Situ remediation will be started. Under such conditions, it is often assumed that
the contaminants in the pore water are in equilibrium with the soil matrix. Under
these conditions specific equilibrium relations are valid.
In many cases, especially when the density of the contaminants is below the
density of water, the source of the soil contamination is present in the unsaturated
zone (i.e., the zone above the groundwater table). If downward transport occurs,
low density contaminants are often present in the water-saturated zone. Especially
in the source area of soil contamination, pure product can be present in a pure
organic liquid phase (NAPL Non Aqueous Phase Liquid). In case the density

is below the density of water, such as for oils or Total Petroleum Hydrocarbons
(TPH), high concentrations of pure product can especially be found floating at the
groundwater as LNAPL (Light Non Aqueous Phase Liquid). When the density
of the pure product is above the density of water, as in chlorinated solvents, a
complex pattern of pure product can be found at large depths upon impermeable
soil layers or clay lenses, known as DNAPL (Dense Non Aqueous Phase Liquid)
contamination. The NAPL contamination may function as a secondary source of
contamination (Norris et al. 1994 ).
Volatile organic contaminants can be present in the soil in four different phases,
namely adsorbed to mineral soil particles and organic material, in the dissolved
phase (dissolved in the soil moisture or in the groundwater), and as vapour in the
unsaturated zone in the soil gas. Each phase is related to the other phases via the
equilibrium equations.
The non-aqueous liquid phase is only present if the water solubility (S (mol/m 3 ))
of the contaminant and the vapour pressure (P (Pa)) are exceeded. As water solubil-
ity and vapour pressure are dependent on temperature, the presence of this phase is
not only dependent on concentration but also on temperature.
The equilibrium relation between adsorption of contaminants to soil and the con-
taminant dissolved in the water can be expressed by different equations, such as the
Freundlich isotherm:

=

C l q s = K ∗ n ≤ 1 (21.1)

with:

q s =

amount of adsorbed contaminant (mg/kg dw )
C l =

concentration dissolved contaminant (mg/m 3 )

K = constant

n = constant

where the value of n influences the dimension of K . The Freundlich isotherm has
no adsorption maximum, which is in contrast to reality. Nevertheless, this relative