Geoscience Reference
In-Depth Information
gas is about 10,000 times faster than in fluid and therefore the diffusion speed is
mainly determined by the saturation degree. After strong and long rain periods the
diffusion will be significantly less. In the air, the sum of oxygen and carbon
dioxide is about 21%, both in soil and atmosphere; the nitrogen concentration in
soil is about equal to the atmosphere. The oxygen concentration in soil rarely drops
under 15%, and CO
2
content rarely rises above 6%.
B
PHYSICAL
-
CHEMICAL PROCESSES IN SOIL
Transport in soil
Transport in soil is mainly caused by groundwater movement. Permeability and
pressure differences control the groundwater flow. Three aspects of transport by
groundwater flow are considered: convection, diffusion and dispersion. The largest
amount of transportation in the soil is by groundwater convection of dissolved or
suspended matter. It is expressed as
qc
, where
q
is the groundwater discharge
velocity and
c
the concentration of the dissolved or suspended matter.
(a) velocity inside a pore (b) variation of pore diameter (c) variation of direction
Figure 14.2 Dispersion by groundwater flow at micro level
Diffusion and dispersion
In the transport process diverse scales are used. Microscopically, grains and
pores exist and the concept of porosity does not apply. In case the concentration is
not uniform, molecules will move differentially, driven by the concentration
gradient. This mode of transport is called diffusion and described by Fick's law:
wc
=
D
d
c
, where
w
is the local fluid velocity in a pore, and
D
d
the molecular
diffusion coefficient. The value of
D
d
in pure water is in the range of 10
-8
to 10
-11
m
2
/s. The transport is also affected by convection, characterised by microscopic
velocity profiles. Suspended particles in 'favoured' flow lines will move faster than
in more stagnant flow lines (Fig 14.2). Because of these microscopic differences in
velocity in the pore structure extra spread occurs, referred to by dispersion (Bear,
De Josselin de Jong, Strack, Hassanizadeh).
At a larger scale, at macroscopic level (multiple of pores), the transport is
described by a law similar to Fick's law with a dispersion coefficient
D
, related to
the average fluid velocity
v
=
q
/
n
(porosity and Darcy velocity
q
exist). It contains
a longitudinal term
D
L
= D
d
+
L
v
and a transversal term
D
T =
T
v
, where
L
and
T
are dispersivity parameters with dimension of length.
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