Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 813

18.3.2.2 Linear and Non-Linear Sorption

The advection-dispersion equation given by Eq. ( 18.34 ) contains three unknown
concentrations (those for the soil phase, liquid phase, and soil gas), while Eq. ( 18.35 )
contains two unknowns. To be able to solve these equations, additional information
is needed that relates solid phase and liquid phase concentrations to each other.
A common approach is to assume instantaneous sorption and to use adsorption
isotherms to relate the liquid and adsorbed concentrations. The simplest form of
the adsorption isotherm is a linear isotherm, sometimes very appropriate at low
contaminant concentrations, given by:

s = K d c (18.38)

where K d is the distribution or partitioning coefficient [L 3 M − 1 ]. Equation ( 18.38 )
assumes reversible sorption (adsorption equals desorption). Substitution of this
equation into Eq. ( 18.36 ) leads to a constant value for the retardation factor, i.e.,

+ ρ

K d
θ

R = 1 (18.39)

Whereas the use of a linear isotherm greatly simplifies the mathematical descrip-
tion of contaminant transport, sorption and exchange are generally non-linear and
most often depend also on the presence of competing species in the liquid phase.
Unlike linear adsorption, the contaminant retardation factor for non-linear adsorp-
tion is not constant, but changes as a function of concentration. Many models have
been used in the past to describe non-linear sorption. Two widely used non-linear
sorption models are those by Freundlich ( 1909 ) and Langmuir ( 1918 ) given by:

K f c β s = (18.40)

K d c

s = (18.41)

+ η

1 c

respectively, where K f [L 3 β M − β ] and

β [ − ] are coefficients in the Freundlich

[L 3 M − 1 ] is a coefficient in the Langmuir isotherm. Examples of
linear, Freundlich and Langmuir adsorption isotherms are given in Fig. 18.13 .

isotherm, and η

18.3.2.3 Volatilization

Volatilization is increasingly recognized as an important process affecting the fate
of many organic contaminants, including pesticides, fumigants, and explosives in
soils (Jury et al. 1983a , 1984 ; Glotfelty and Schomburg 1989 ). While many organic
contaminants dissipate by means of chemical and microbiological degradation,
volatilization may be equally important for volatile substances, such as certain pes-
ticides. The volatility of pesticides is influenced by many factors, including the