Geoscience Reference
In-Depth Information
18.2.3 Water Transmission Properties
Water transmission properties, expressed in terms of hydraulic conductivity (HC)
and infiltration rate (IR), and solute movement in the soil and subsurface regions
can be changed irreversibly as a result of sodic water disposal on the land surface.
The hydraulic conductivity is a property that describes the ease with which water
can move through pore spaces and/or fractures and depends on the intrinsic per-
meability of the material. The infiltration rate is the speed at which water enters the
soil-subsurface domain. The extent of changes in transmission properties is con-
trolled largely by the clay content and clay type present in the solid phase, and by
the sodium concentration of the incoming water.
18.2.3.1 Hydraulic Conductivity
Swelling and dispersion are the two processes affecting the soil-subsurface
hydraulic conductivity. The Na
+
cations from incoming contaminated water are
adsorbed on clay minerals, inducing their swelling and/or dispersion. The extent of
this process is governed by the concentration of the Na
+
cations in the incoming
water. In their review on sodium and electrolyte concentration effects on soil
hydraulic conductivity, Yaron and Shainberg (
1973
) noted that swelling is not the
main cause for the decrease in HC of clayey soils. In contrast, dispersion of clay due
to Na
+
is the main cause of irreversible reduction in HC of clays, because it results
in rearrangements and blocking of the pores through which flow takes place.
It is clear that clay dispersion is a major factor that irreversibly affects the HC
of soils irrigated with sodic water (Shainberg and Letey
1984
, and references
therein). Oster et al. (
1980
) found that clay dispersion is highly sensitive to low
levels of exchangeable Na and increases sharply with very small increases in
exchangeable sodium percentage (ESP). The movement of water through clay
systems when HC was affected only by swelling and when the water movement
was affected by both swelling and dispersion of particles is shown in Fig.
18.24
.
Soil HC at equilibrium with salt-contaminated water is determined by the
concentration of electrolytes, the ionic composition of the water, and the
exchangeable cations in the soil complex (Quirk and Schofield
1955
). In a mixed
system, the ESP is the most important characteristic influencing HC. This was
noted by Quirk (
1957
), who showed that soil permeability decreases with
increasing percentage of exchangeable sodium. A quantitative relationship
between solution concentration and ESP was expressed as a ''threshold concen-
tration.'' The threshold concept was proposed initially by Quirk and Schofield
(
1955
) and is defined as the salt concentration needed to maintain the hydraulic
conductivity in a Na-saturated system 20 % below that existing in a calcium-
saturated system.
The relationship between soil hydraulic conductivity and electrolyte type, and
concentration
of
the
infiltrating
water
is
affected
by
the
soil-subsurface
