Recycled Water Irrigation in Australia - Environmental Sustainability: Role of Green Technologies - page 43

3.4 Salinity and Sodicity

mechanism remain in the soil whilst the other
free salts are leached downwards (Sumner and
Naidu 1998 ; Rengasamy 2006 ). In situations
where the other free salts remain, the soil is
known as saline-sodic and the soil structure
remains intact.
The extent to which Na + ions bind to the cat-
ion exchange sites on a clay particle is deter-
mined by the ratio of Na + ions to Ca 2+ and Mg 2+
ions in the soil solution. This can be expressed
either as the percentage of Na which occupies the
cation exchange capacity of a clay, the exchange-
able sodium percentage (ESP), or the ratio of Na +
ions to Ca 2+ and Mg 2+ ions in the soil solution, the
sodium adsorption ratio (SAR).
Sodic soils have poor physical characteristics
because the high levels of Na + interfere with the
structural integrity of clay particles when the soil
is wetted (Laurenson et al. 2010 ). As a conse-
quence, sodic soils display the typical character-
istics and problems that include (Rengasamy
2006 ) reduced porosity and permeability, reduced
infi ltration and hydraulic connectivity, surface
crust formation which impedes infi ltration and
promotes run-off and erosion, diffi cult to culti-
vate, provide an impediment to the development
of a root network and expose plant roots to anoxic
or waterlogged conditions and slow plant growth.
Reduced drainage can also lead to further
accumulation of salts through poor downward
movement of irrigation water and evaporative
concentration. Clays with low hydraulic conduc-
tivity are more prone to developing sodicity
because they have a low leaching fraction
(Rengasamy 2006 ), and these soils retain water
in their profi le which is subjected to evaporation,
leaving salts behind.

Irrigating with reclaimed water carries a risk of
inducing soil salinity and/or sodicity because
reclaimed water often contains high levels of
salts, in particular Na (Rengasamy 2006 ). Soil
salinity is seen when an elevated concentration of
soluble salts in the soil-water solution induces
osmotic stress in vegetation. Sodicity is an
increase in proportion of Na relative to the diva-
lent cations and adversely affects soil structure.
Managing soil salinity has been identifi ed as
one of the largest threats to developing a sustain-
able recycled water scheme (Sumner and Naidu
1998 ; Stevens et al. 2003 ). The salts which con-
tribute to salinity include Na + , K + , Ca 2+ , Cl − ,
Mg 2+ , SO 4 2− and HCO 3 − , but Na + and Cl − ions
exert the greatest environmental impact because
their solubility in water renders them more avail-
able for interactions in the soil (NRMMCEP and
HCAHMC 2006 ).
Reclaimed water can induce soil salinity when
salts become concentrated in the soil through
evaporation, the principle signs of which relate to
osmotic stress. Salinity reduces plant growth
because the increased osmolarity makes it diffi -
cult for plants to absorb water and nutrients. In
response to reduced water uptake, plants produce
the hormone abscisic acid which signals stomata
to close, reducing transpiration water losses.
Consequently, carbon dioxide absorption is
reduced and photosynthesis slows leading to
lower plant growth (Hartung et al. 1999 ).
To prevent salt accumulation, a leaching
fraction must be incorporated into the crops
irrigation requirements to drive salts below the
root zone. However, given climatic variations in
rainfall and evaporation throughout the year,
supplying the correct leaching volume can be
diffi cult. Maintaining soil structure so that the
leaching fraction can permeate the soil layers is
also critical and this is complicated further by
sodicity.
Reclaimed waters frequently have higher lev-
els of Na + ions compared to the other cations and
can induce sodicity or saline sodicity (Bond
1998 ). Sodicity develops when free Na + ions bind
to the cation exchange sites on clays and by this

4 Management Practices

4.1 Irrigation Methods

and Management

When reclaimed water is used as an irrigation
source, the crop irrigation requirement must be
carefully calculated to avoid the effects of
hydraulic loading which include waterlogging,