| One of the biggest challenges
faced by irrigated agriculture in Australia is to resolve the conflict
over water allocations. The use of water must become more efficient
and sustainable to prevent the depletion of environmental flows and
recharge.
According to the Australian Bureau of Statistics, there has been a 35
per cent increase in water use for cotton in Australia since 1993–94
without a significant increase in the available water. The recycling
and reuse of water is therefore becoming more important to maximise
economic and environmental benefits.
The majority of cotton farms in Australia use furrow irrigation. In
this process, relatively clean river or underground water picks up plant
material, sediments, microorganisms and chemicals present on the soil
surface. The runoff is returned to large storage dams without treatment
— so water quality may decline on cotton farms throughout the
season.
Cotton storage dams and irrigation channels have become surrogate habitats
for native flora and fauna. Unfortunately, the quality of water in these
structures can sometimes be detrimental rather than beneficial to them.
Reduced water quality could also present a greater risk of illness to
livestock and farm workers, increase the potential spread of cotton
diseases around the farm, and importantly, put limitations on the possible
end use for that water. Cleaner water means more flexibility in decision-making
and associated economic benefits.
The aim of this article is to identify the factors that comprise water
quality, examine how they affect cotton production and suggest simple
methods of designing irrigation and storage systems to enhance water
quality.
The Australian Cotton CRC with National Heritage Trust funding has been
conducting feasibility trials to determine local benefits of constructed
wetlands, so improved water quality can be achieved economically on
cotton farms as part of the farming operation.
What are the indicators of water quality?
Indicators of water quality can be physical, biological, microbiological
or chemical. Farmers will often have an idea of the quality of water
on their property simply by observing any changes in physical characteristics
of the water, such as colour, odour and turbidity, and the biology (type
and livelihood) of any organisms using that water.
For example, the development of ‘scum’ on the water surface
coupled with a rotting odour and the presence of dead fish gives a clear
indication that the water is not fit for drinking!
Unfortunately, not all cases are this clear cut — the degree of
the water quality and cause of any fouling is usually more difficult
to determine. In these instances, microbiological and/or chemical testing
is necessary. In fact, the biological status of water — which
is what we are most concerned with — is nearly always dependent
on these other two factors.
But the contaminants of concern depend on the desired end-use for the
water. For example, although certain water may not be fit for human
consumption, it may still be suitable for use in the irrigation of cotton.
So we need to address potential contaminants individually.
Animal pathogens
Contamination of farm water by animal pathogens can be important on
cotton farms as they are typically run in conjunction with cattle production.
If stock are watered from troughs or other above ground structures,
contamination is rare. But if their water source is surface water, such
as in dams or channels, pathogens may be present from faecal contamination
by livestock or native animals.
Plant pathogens
Cotton is susceptible to a number of plant pathogens. The most significant
are the soil-borne fungi (Table 1) that infect cotton plant roots, often
at different stages and under different conditions. One of the main
mechanisms by which they move around within farms is via irrigation
tailwater, either in sediment or on plant trash.
Guidelines from the Australian Cotton CRC suggest that run-off from
affected fields should be kept out of distribution systems to prevent
infection of clean fields, but treatment by sub-surface flow wetlands
is a possible solution.
Nutrients
In an undisturbed state, Australian soils and waters are generally low
in nutrients. Native flora and fauna have adapted accordingly, so an
excess of nutrients can unbalance the natural ecosystem. One outcome
of nutrient enrichment is eutrophication by algae, with the dominance
of a few fast-reacting species and a reduction in biodiversity.
Cyanobacteria (blue-green algae) in particular are quick to proliferate.
Some of these photosynthetic bacteria produce secondary substances that
are toxic to animals. Another outcome of eutrophication is a reduction
in dissolved oxygen levels due to the increased respiration by aquatic
microorganisms. This can cause the suffocation and death of fish and
other aquatic organisms. So it is desirable to remove excess nutrients
from farm runoff water.
Pesticide residues
It is unusual to find tailwater totally free of pesticide residues.
This does not mean that all tailwater is acutely toxic to animals and
plants using that water, but it can increase the risk of toxicity —
both acute and chronic.
Furthermore, even though residues may not be toxic, they can be detrimental
to the marketability of a number of other products that may be grown
in conjunction with cotton, such as cattle (recall endosulfan or Helix)
or grain. It is in the best interests of growers not only to decrease
the usage of pesticides, but also to remove any residues remaining in
tailwater following irrigation.
Designing irrigation systems to improve water
quality
Flow-through filters
Filtration is an effective method of removing pollutants. Water can
be filtered by pushing it through porous material, either passively
under gravity or actively by pumping. For the low-level pollution and
high volumes of water found on cotton farms, gravity-driven flow is
usually sufficient and a lot more cost effective.
Depending on the level of clean up required, a number of different materials
can be used. Coarse particles, including cotton trash and larger sediments,
can be removed simply by allowing a body of water to slowly flow through
a stand of aquatic plants.
This reduces the velocity of the water, allowing the larger particles
to sediment out under gravity and floating trash to beach itself on
shore. Aquatic plants may also bind some chemical contaminants, but
the extent of this depends on the water speed and plant density.
To remove finer particulates, a method known as sub-surface flow can
be used. This involves filtering water through a bed of porous material
such as gravel, wood chips, or sand.
Generally the finer the composition of the porous material, the finer
the sediment removed. Sub-surface flow also increases the contact of
the water with the solid surface, and reduces pollution by physical
interactions — that is, binding or sorption. Because fine sediments
also hold the highest percentage of nutrient and pesticide residues
on a weight basis, sub-surface flow also helps remove these chemical
contaminants.
The length of flow-through filter required depends heavily on the volume
of water and load of contaminants that needs treatment, and the level
of treatment that is desired. A pilot-scale system on Auscott, Narrabri,
has been established to develop these guidelines
Shallow, vegetated ponds
Aquatic plants can assist in the clean up of water in a number of ways.
But apart from some floating and submerged species, most require shallow
(maximum 0.5–1.0 metre deep) water for growth. Examples of species
that have been successfully established in pilot-scale wetlands in cotton
tailwater are shown in Table 2.
Lab trials have identified some of these species that can directly reduce
herbicide concentrations by direct uptake. Residues removed in this
way are usually either irreversibly bound or metabolised into non-toxic
products.
All of the above species, and indeed other suitable plants, also enhance
pollutant removal via a number of non-specific mechanisms.
First, because they require nutrients for growth, aquatic plants can
remove excess nitrogen and phosphorus from tailwater. The rate at which
this occurs depends on the species.
Second, they contribute decaying organic matter to the water and sediments,
which binds nutrients, pesticides, detergents and oils. Dissolved organic
matter, such as humic acid, can also speed up the breakdown of some
pesticides in a process known as photo-sensitisation. Plant material
also acts as a food source for beneficial microorganisms that speed
up pesticide breakdown and enhance nutrient cycling.
Third, many aquatic plants transport oxygen into otherwise anaerobic
sediments. Again, this increases the rate of pesticide breakdown and
nutrient cycling.
A pilot-scale ponded wetland has been established on ‘Mollee’,
a cotton farm near Narrabri, to determine whether aquatic plants can
enhance pesticide residue degradation under field conditions. Results
over two seasons indicate that pesticide removal is 20 per cent higher
than non-vegetated ponds under normal conditions, but open water can
be as effective in some circumstances. Overall, the Cotton CRC study
has shown that over 50 per cent of pesticides in runoff can be removed
by such constructed wetlands in 10 days.
Deep, open ponds
Depth is a good way to regulate water velocity and naturally assists
in sedimentation of heavy particles. Although it is possible to grow
floating plants in deep water, it is often better to leave deep ponds
free of vegetation to allow penetration of light.
Pesticide breakdown is increased in open water relative to shaded water
by exposure to UV light on cloudless days, in a process known as photolysis.
Sunlight also promotes algal growth that can assist in pesticide degradation.
But under most circumstances, open water bodies should follow on from
an initial treatment to reduce nutrient concentrations — such
as filtering or vegetated ponds — to prevent the growth of unwanted
blue-green algae (Figure 1). This pretreatment also decreases turbidity
and thus allows better light penetration.
A further benefit of having sections of open water is the destruction
of animal pathogens. Most animal pathogens are relatively fastidious,
requiring conditions similar to those found in animal guts for survival
and proliferation. So in aquatic environments, pathogens will usually
die out over time without special treatment, due to unfavourable climatic
conditions, exposure to sunlight (ultraviolet light) and competition
and predation by other indigenous microorganisms. Treatment wetlands
for this purpose should be designed with a retention time of greater
than a week and at least 50 per cent open water.
Aeration
As previously mentioned, the majority of pesticides degrade faster to
non-toxic products under aerobic condition. Similarly, the cycling of
some nutrients requires oxygen. So processes that increase the oxygen
content of water can help remove pollutants
Oxygen is best introduced into water physically. The easiest method
of achieving this is by increasing water movement that incorporates
air, rather than the more expensive method of bubbling aeration like
in a fish tank.
One example would be to design return channels that incorporate baffles
(a fancy word for underwater hills). Another inexpensive method of providing
aeration is to position existing structures such as pumps or other moving
machinery near deep water to improve mixing.
Wetting and drying
A number of the processes above involve the removal of pollutants on
sediments or filters. Because these usually remain underwater for long
periods, pollutant breakdown can be slow. Again, aeration is a solution.
Good aeration can be achieved by completely drying out storage dams
and channels.
Excavation
The accumulation of sediment can fill water storage areas and channels
rapidly, even over one season of irrigation. It is important to monitor
these areas to identify when such sedimentation is limiting not only
to water storage and movement, but also the capacity for pollutant removal.
As commonly occurs on most cotton farms during the non-growing (winter)
season, channels and storages should be excavated to alleviate sedimentation.
This sediment is comprised of fine, usually nutrient-rich clays and
organic material and is usually relatively fertile.
If possible it would be beneficial to return and incorporate any excavated
sediment back onto fields. In doing so, any remaining pollutants are
again aerated and dispersed to reduce levels further.
Potential benefits?
• Greater end-uses for recycled tailwater. Depending on the degree
of treatment, tailwater may be re-used for purposes other than the irrigation
of cotton. This may include aquaculture or the irrigation of other crops
that are susceptible to cotton herbicides found in untreated tailwater.
• Reduced spread of water-borne cotton pathogens.
• A decrease in the chance of illness in livestock and native
fauna resulting from ingestion of water contaminated with pathogenic
bacteria and viruses.
• An immediate benefit to the farm ecosystem and the surrounding
environment. In particular, native fauna that passively use recycled
water, such as water birds and amphibians, are at a lower risk of suffering
illness from pesticide residues, animal pathogens or algal toxins.
Clean water on farms also promotes an increase in biodiversity, thus
fostering those benefits discussed earlier. Furthermore, in the event
that tailwater does escape off farm, the risk of toxicity to the surrounding
environment will be lowered.
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