Environmental Engineering Reference
In-Depth Information
Finally, municipal effluent discharged from sewage-treatment plants
is actually quite a dependable source of irrigation water. Usage of house-
hold water does not fluctuate greatly during the year, so it remains
available during drought or other periods when potable water may be
restricted. Municipal effluent used for irrigation is usually free from
pathogenic bacteria, heavy metals and other compounds posing a hazard
to humans and animals, but it may contain elevated amounts of certain
elements such as sodium and boron which can have detrimental effects
on turf. Irrigation with effluent is becoming a global issue. It will be
presented more comprehensively later in this chapter.
Water Quality
Municipal potable water usually presents no problems, but water
from other sources may contain constituents that are of minor or major
concern.
Solids such as sand, silt and clay particles may sometimes clog water-
delivery systems but are generally of little concern. On the other hand,
salts dissolved in the water are of major importance, and if their concen-
tration is too high the water may not be suitable for irrigation. High
concentration of salts affects the soil-solution osmotic potential, may
reduce water uptake by plants, may induce mineral imbalances and, if
high enough, can cause direct injury to roots. Total salt content of water is
usually measured in terms of electrical conductivity (EC
w
) and is often
reported as decisiemens per metre (dS/m). Salinity hazard is considered
low when EC
w
is below 0.75 dS/m and very high when EC
w
exceeds
3 dS/m.
Of all salts that may be present in water, those of sodium are the most
important. High sodium causes soil to disperse, usually resulting in
reduced water infiltration and reduced aeration. Sodium ions may also
be directly toxic to the roots. If other than municipal water is used, its
sodium absorption ratio
(SAR) should be determined. The same laborator-
ies that conduct soil tests usually conduct water tests. SAR values are
reported in milliequivalents per litre (meq/l) and should be below 5 if the
water is used to irrigate soils rich in clay, below 10 for use on sandy loams
and below 15 for use on predominantly sandy soils. The range is broad
because the detrimental effect of sodium is proportional to development
of soil structure. Clay soils owe their favourable properties to their
granular structure. As was described in Chapter 3, organic and inorganic
colloids glue fine soil particles together, creating larger clusters that hold
soil water, assure proper aeration, reduce physical impedance and over-
all provide favourable conditions for root growth. These clusters can
easily be damaged and fall apart if sodium ions attach themselves to
