Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 790

and dissolved contaminants through macropores (e.g., between soil aggregates, or
created by earthworms or decayed root channels, see e.g. Mallants et al. ( 1996a )) or
rock fractures, with much of the water bypassing (short-circuiting) the soil or rock
matrix. However, many other causes of preferential flow exist, such as flow instabil-
ities caused by soil textural changes or water repellency (Hendrickx and Flury 2001 ;
Ritsema and Dekker 2000 ;Šimunek et al. 2003 ), and lateral funneling of water due
to inclined or other textural boundaries (e.g., Kung 1990 ). Alternative ways of mod-
eling preferential flow are discussed in a later section. Here we first focus on the
traditional approach for uniform flow as described with the Richards equation.

18.2.1 Water Retention and Hydraulic Conductivity

18.2.1.1 Water Retention

Above the groundwater table, a zone of a few to several tens of meters occurs where
part of the pore space is occupied by the air phase. This is the unsaturated or vadose
zone, of which the upper part typically contains a soil profile. When the soil becomes
drier owing to internal drainage and/or evapotranspiration, air replaces water first in
the coarse parts of the pore space and at successively lower (negative) values of the
water potential (see further) also in the finer pores. In the unsaturated zone, water is
held in the soil pores by so-called surface-tension forces. In other words, capillary
forces (and to a lesser extent also adsorption) bind water to solids. This leads to the
existence of a negative pressure, also referred to as the suction, tension, or matric
head (by definition pressures less than atmospheric are considered negative).
Capillary forces are the result of a complex set of interactions between the solid
(particles) and liquid (pore water) phases involving the surface tension of the liq-
uid phase, the contact angle between the solid phase and the liquid phase, and the
diameter of pores. As a consequence of these forces, water will rise to a height H
[L] when a capillary tube of radius R is placed into a water reservoir open to the
atmosphere (Fig. 18.2 ). This capillary rise is given by the Laplace equation :

2 σ cos γ

H = (18.1)

ρ w gR

is the surface tension [MT − 2 ] (7.27 10 − 2 kg/s 2 at 20 ◦ C),

where σ × γ is the contact

ρ w is the density of the liquid phase [ML − 3 ] (998 kg/m 3 at 20 ◦ C), and g is
the gravitational acceleration [LT − 2 ] (9.81 m/s 2 ). For water at 20 ◦ C with

angle,

γ = 0, Eq.

( 18.1 ) can be simplified to H = 10 − 5 / R (with both H and R in m).

Since a soil can be viewed as a complex system containing pores of various diam-
eters, water in those pores will rise to different heights (Fig. 18.3 ), and hence will
be held with different potential energies. Because each soil has a different distribu-
tion of pore sizes, the distribution of water above the water reservoir will also be
different. This simple conceptual model assumes that soil pores can be represented

1.5 ×