Geoscience Reference
In-Depth Information
18.2 Impact of Sodic Water
Sodium contaminants may reach the land surface via irrigation with saline water
and wastewater, by intrusion from seawater, and/or by dissolution of clustered salt
crystals. Within the components of saline water, sodium contaminants may
strongly affect soil and subsurface water transmission properties. Changes in
hydraulic conductivity and infiltration rate are controlled by the porosity of the
medium and by the clay content of the solid phase. Moreover, soil-clay mineralogy
was determined to be a key factor in the extent of hydraulic conductivity decrease
in soils leached with sodic water (Yaron and Thomas
1968
). Soil-subsurface clays,
with expanding layer mineralogy, may swell and disperse upon contact with sodic
water, changing the architecture of the natural host porous medium solid phase and
affecting its water transmission properties. Swelling and dispersion of clays have
been proposed to explain the decrease in hydraulic conductivity of soils. Defloc-
culation and movement of clay in conducting pores were subsequently considered
to be another major factor causing decreases in hydraulic conductivity (Frenkel
et al.
1978
; Shainberg and Letey
1984
, and references therein). Whereas a change
in soil hydraulic conductivity due to sodic water-induced swelling is a partially
reversible process, changes due to dispersion and particle redistribution in the
conducting pores of the soil-subsurface regime are irreversible. Changes occurring
to various physical properties of soil and the subsurface regime under irrigation
with sodic water were addressed in the early studies of Quirk and Schofield (
1955
),
McNeal and Coleman (
1966
), and Russo and Bressler (
1977
). The impact of
sodium
on
soil-subsurface
contaminant-induced
changes
was
revisited
more
recently (Yaron et al.
2008
,
2010
,
2012
).
18.2.1 Swelling
Swelling of soil clays occurs over a range of sodium concentrations in saline or
waste waters; it is dependent on the properties of the clay, the extent of the layer
charge, and the type of interlayer cations. Swelling of soil clays, expressed by
interlayer expansion, is attributed to the repulsive force between electrical double
layers on adjacent platelets (McBride
1992
), and based on the fact that layer
silicate clays approach a maximum hydration state beyond which they do not
extend. Farmer and Russell (
1971
) explained swelling qualitatively, as a simple
model of H-bonding when interlayer expansion is driven initially by the energy of
hydration of the exchangeable cations. Swelling was explained later by noting that
the zeta potential of Na
+
-clay is much lower than that anticipated from Na
+
ions
fully dissociated from the surface in a suspension of layered silicates in water
(Low
1981
).