Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 793

4

Fig. 18.4 Example of
retention curves for sand,
loam and clay textural classes
(based on Carsel and Parish
( 1988 ) soil hydraulic
parameters, Table 18.1 )

Sand
Loam
Clay

2

0

-2

0 0.1 0.2 0.3 0.4 0.5

Water content [-]

Table 18.1 Soil hydraulic parameters for the analytical functions of Van Genuchten ( 1980a )for
the twelve textural classes of the USDA soil textural triangle (Carsel and Parish 1988)

r [L 3 L − 3 ] s [L 3 L − 3 ] [cm − 1 ] K s [cm d − 1 ] Textural class θ θ α n [ − ]

Sand 0.045 0.430 0.145 2.68 712.8

Loamy Sand 0.057 0.410 0.124 2.28 350.2

Sandy Loam 0.065 0.410 0.075 1.89 106.1

Loam 0.078 0.430 0.036 1.56 24.96

Silt 0.034 0.460 0.016 1.37 6.00

Silty Loam 0.067 0.450 0.020 1.41 10.80

Sandy Clay Loam 0.100 0.390 0.059 1.48 31.44

Clay Loam 0.095 0.410 0.019 1.31 6.24

Silty Clay Loam 0.089 0.430 0.010 1.23 1.68

Sandy Clay 0.100 0.380 0.027 1.23 2.88

Silty Clay 0.070 0.360 0.005 1.09 0.48

Clay 0.068 0.380 0.008 1.09 4.80

θ r is the residual water content [L 3 L − 3 ],

where θ s is the saturated water content

[L 3 L − 3 ], and [L − 1 ], n [ 1/ n ) [-] are shape parameters. The

dependency of the Van Genuchten model on the parameters

α − ] and m ( = 1 −

and n is shown in
Fig. 18.5 . The curves in this figure are plotted in terms of effective saturation given
by S e =

α

θ s − θ r ).
As a first approximation and on intuitive grounds,

( θ - θ r )/(

θ s = η (total porosity) and

θ r = 0. In reality, however, the saturated water content θ s of soils is often smaller

than the total porosity η because of entrapped and dissolved air. The residual water

content θ r is generally larger than zero, because of the presence of adsorbed water.