INCREASED use of irrigation water supplies and agro-chemicals, coupled with its irrational use, is causing significant water pollution problems.
Water pollution from agriculture occurs in three ways. One is the pollution caused by the use of pesticides. This source of pollution is very pronounced in this country. The second source is pollution by fertilisers. This is also fairly common. The third source, to which no attention has been paid as the pollution caused by sediments.
In absence of the wide-spread use of IPM (integrated pest management) in Sindh, pesticides are used indiscriminately to control pests.
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. 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. Large-scale breeding of mosquitoes leads to vector-borne diseases. In addition to the use of pesticides for controlling agricultural pests, other agro-chemicals (usually, DDT) are used to kill mosquitoes. The 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 ecological impacts of pesticide runoff on aquatic life are also governed by the ability of 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.
Fertilisers are used to improve and increase the agricultural yields. Usually, it is the nitrogen, which appears in the fertiliser runoff, as nitrate. Typically, about 50 per cent of the nitrogen application is washed away. Phosphorus is not mobile in the soil, but rains, after a recent application of fertilisers, may wash down phosphorus in the runoff. Phosphorus readily precipitates as calcium, aluminium or iron phosphate, depending upon the nature of soil. Potassium is transported through the soil by the electrostatic attraction of the negatively- charged clay colloids (base exchange). 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 chemically form of nitrogen.
The recent problems reported in Keenjhar Lake could be due to nitrates. Deaths in cattle have been reported due to fertiliser tanks being used to carry water. This has been mainly attributed to nitrate poisoning.
Water drawn from wells, at the depth of 20 metres, would reflect the conditions of fertiliser use twenty years ago. During the last twenty years, there has been significant increase in the use of fertiliser 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 fertiliser runoff requires a well-managed approach. Eutrophication of water bodies (excessive weed growth due to excessive nutrients) is an indication of fertiliser runoff. This requires regular qualitative assessment of the trophic state of water bodies.
Much of the sediment supply to rivers and other water bodies is attributed to the agriculture, as the result of erosion. The loss of top soil by sheet erosion and gully erosion 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 find 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 as water pollutants. The silt and clay fraction (less than 63 micrometer, in size) carries chemicals, heavy metals, chlorinated pesticides and phosphorus, which are “adsorbed” to the sediments, and are transported to the aquatic systems.
Measures, which can control erosion, include vegetative cover, contour farming, controlling down slope runoff by diversions, grassed waterways, strip cropping and terracing, among others.