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Pollution control for
agriculture
Of the many dangers that face
our agrarian economy, water pollution poses a real threat to
the fields of Pakistan.
Pakistan is an agrarian economy. Nearly 70 per cent of the
country's rural population is dependent on agriculture, while
50 per cent of the labour force is engaged by it. However, the
country's growing population has put strain on the
agricultural capacity, and though more land is now cultivated
and higher yield derived expansion in agriculture has also
meant increased irrigation water supplies as well as more use
of fertilizers and pesticides. As expected, this has brought
about new problems. One of them is the water pollution.
Problems related to water pollution from agriculture are so
many that today these cannot be controlled. The result is that
the agriculture sector continues to pollute the surface and
groundwater resources.
FUTURE WATER REQUIREMENTS:
It is estimated that the volume of water used for
irrigation in the 2000 was 95 billion cubic meters per year.
In the coming twenty-three years, this figure is expected to
rise up to 132 billion cubic meters per year. Other than this
fact, the quantity of water used for irrigation, in future,
will be dependent on many factors. These factors include,
irrigation efficiency, seepage losses, water conservation
measures in place, water management, type of crops and water
requirements of crops. However, all these will become
nullified in the face of water pollution.
WATER POLLUTION:
This type of pollution occurs in three ways. One is the
pollution caused by the use of pesticides whose source of
pollution is very pronounced in this country. The second is
pollution by fertilizers which is also fairly common. The
third source, to which no attention has been paid as yet, as
it not known to many, is the pollution caused by sediments.
The "cide" in the word "pesticide" means "to kill." Hence,
pesticides kill insects, pests and, in case of Sindh province,
human beings too, as many cases have been documented of
pesticides killing men, during pesticide's spray, use and
handling. In agriculture, pesticide includes insecticides
(insects), herbicides (weeds), fungicides (fungi),
rodenticides (vertebrate poisons) and nernatocides
(nematodes).
In absence of the wide-spread use of IPM (integrated pest
management) in Sindh, the use of pesticides is indiscriminate
in the province. During the course of irrigation and during
wet season, the pesticides are washed down to surface water
bodies, and in some cases, the groundwater resources as well.
Agricultural use of pesticides in Sindh is the single most
important factor of water pollution.
In addition to the toxic active ingredients of the pesticides,
most pesticides contain inert substances to improve
absorption. These inert substances, quite often, are not
included in the contents shown on the product's label.
Chloroform and carbon tetrachloride are sometimes used as
inert substances. They are toxic to liver and central nervous
system.
Irrigation of lands in Sindh usually creates a habitat, which
encourages mosquito breeding. To control this menace,
agrochemicals (usually, DDT) are used to kill mosquitoes. Use
of DDT is banned in developed countries, because of its major
public health impact, pollution of water bodies and disruption
of marine and terrestrial eco-systems.
The toxicity of pesticides in aquatic environment is governed
by certain important influencing factors. These include
degradation (pesticide, after degradation may form components,
which may be more toxic than the original pesticide);
persistence (some pesticides are relatively more persistent in
the environment, e.g., DDT); toxic response (the response may
be acute, chronic, carcinogenic, teratogenic, inhibited
reproduction, etc); environmental fate (affinity of pesticide
for solid, liquid, gas phases and biota).
The ecological impacts of pesticide runo ff on aquatic life
are also governed by the ability of the pesticide to
bioconcentrate and biomagnify at the organism level and, at
various trophic levels along the food chain.
In Sindh, the drinking-water treatment plants (rapid-sand
filters and slow-sand filters) are not designed to remove
pesticides in the drinking-water. So, if the raw water
contains pesticides (which most likely it does), they will
pass on to the finished drinking-water and, on to the human
beings. Since, the cost of removal of pesticides from raw
waters is prohibitive, it is prudent to prevent generation of
pesticide runoff. This will also prevent environmental
pollution.
FERTILIZERS:
These are used to improve and increase the agricultural
yields. Nitrogen, phosphorus and potassium are the main
ingredients of fertilizers. Usually, it is the nitrogen, which
appears in the fertilizer runoff, as nitrate. Typically, about
50 per cent of the nitrogen application is washed away.
Phosphorus is not mobile in the soil, but rains may wash down
phosphorus in the runoff. Phosphorus readily precipitates as
calcium, aluminium or iron phosphate, depending upon the
nature of soil. Also, potassium has a restricted mobility.
The build-up of nitrate in the groundwater is a matter of deep
public concern. Nitrate, in itself, is relatively non-toxic to
humans. Health problems arises after nitrate enters the body
and, is converted to nitrite, which is the chemical form of
nitrogen. In infants, under six months, nitrite causes a
condition called methemoglobinemia (blue-baby syndrome), under
which the oxygen-carrying capacity of blood is reduced.
Haemoglobin carries oxygen in the blood; methemoglobin do not.
High pH of an infant's stomach creates a favourable
environment for conversion of nitrate to nitrite. Infant
methemoglobinemia is the basis for nitrate-nitrogen drinking
water standard. In addition, recent research suggests that,
nitrite assist in the formation nitrogen-nitrosamine
compounds, which may cause stomach cancer.
The US EPA has set the standard of less than 10mg/l
(milligrams per litre) of nitrate-nitrogen in drinking-water.
Who, also has the guideline value of nitrate-nitrogen in
drinking-water as 10mg/l. However, the 1996 version of Who,
drinking-water guidelines gives the nitrate value of 50mg/l
and of nitrite as 3mg/l, but the sum of the ratios of the
concentrations of each to its respective guideline value
should not exceed one.
A significant feature of nitrate problem in the lag time,
between the fertilizers' application and, their arrival in
groundwater. Nitrate move through the soil at an approximate
rate of one metre per year. Water drawn from wells, at the
depth of 20 metres, would reflect the conditions of fertilizer
used twenty years ago. During the last twenty years, there has
been a significant increase in the use of fertilizer
application.
When high levels of nitrate are present in raw water, other
contaminants (coliform bacteria, chemical pollutants) may also
be present. Removal of nitrate from water entails costly
treatment units. Boiling the water will not remove the
nitrate. On the other hand, boiling will increase the
concentration of nitrate in the water.
Controlling water pollution from fertilizer runoff requires a
well-managed approach. Eutrophication of water bodies
(excessive weed growth due to excessive nutrients) is an
indication of fertilizer runoff. This requires regular
qualitative assessment of the trophic state of water bodies.
Other controlling measures include reducing leaching of
nutrients, rational nitrogen application (nitrogen
requirements should be calculated on the basis of crop
nitrogen balance); good irrigation and agricultural practices;
planting of green manure crops; delaying ploughing of straw
and leaves into the soil; and keeping a vegetative cover on
soil.
SEDIMENTS:
Much of the sediment supply to rivers and other water bodies,
is attributed to agriculture, as a result of erosion. The loss
of top soil by sheet erosion and gully erosi on, generates
sediment, which moves down to rivers and lakes. Soil erosion
is usually the result of poor agricultural and irrigation
practices. It is known that, soil is rendered as a wasteland
due to the practices, which are not conducive to proper use of
soil. The resulting unproductive soil is subjected to erosion,
due to weathering processes, causing sediment load, which
finds its way to water bodies. In addition to high turbidity
that is caused in water bodies, due to sediments, ecological
impacts are caused, as a result of deposition in river beds.
These ecological impacts are more pronounced in downstream
areas.
There is also a chemical dimension to the sediments. The silt
and clay fraction (less than 63 micrometer, in size) carries
chemicals, heavy metals, chlorinated pesticides and phosphorus
that are 'adsorbed' to the sediments, and are transported to
the aquatic systems.
Measures, that can control erosion, includes vegetative cover,
contour farming, controlling downslope runoff by diversions,
grassed waterways, strip cropping and terracing, among
others.
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