O level Notes: Agriculture - Water and Water Conservation

Water conservation

Water is an important natural resource that sustains plant and animal life. Agriculture heavily relies on water as a vital input. However, it is not just availability of water that matters but the quality of water is also equally important.

Fresh water supplies can disappear due to industrial and agrochemicals contamination.

Water once contaminated to high toxic levels may not sustain agricultural activities. It is important to reduce water pollution and to use water resources wisely and to conserve water as a resource.

Water pollution

Water pollution is the contamination of surface and ground water sources by pollutants of various forms that are directly or indirectly discharged into the water bodies in toxic amounts. The pollutants reach water bodies without having been treated to harmless forms to the environment.

Causes of water pollution

Farming activities

Poor farming methods: Overuse of persistent chemicals and fertilizers in agricultural activities results in some of these chemicals coming into contact with water bodies, contaminating them to a degraded state. Herbicides used in weed control and fertilizers are leached, and when they come in contact with ground water, they contaminate the aquifers. Aquifers are ground water sources that supply rivers and some deep sunk boreholes.

Improper disposal of chemical containers:

When farmers fail to dispose empty toxic chemical containers, the empty containers may end up in surface water bodies.

Most containers are plastic bottles that float in water and are therefore washed by surface water runoff to rivers and lakes.

The plastic containers also result in plastic contamination of the water.

Animal waste: Livestock production can generate a lot of organic waste that can be washed by rains and finds its way to surface water bodies.

Industrial activities

Discharge from industries:

Untreated industrial wastes in form of chemical liquids, detergents and gases contaminate both ground and surface water bodies.

Industrial chemical discharge may flow into stream, rivers and lakes. The chemical wastes build up into toxic levels that degrade the water body.

Industrial fumes contain gases that dissolve in rain water or form compounds that fall with the rains and percolate into the soil up until they reach the water table and contaminate the underground water.

Domestic or household activities

1. Burst sewage pipes: Burst sewage pipes with raw untreated sewage can flow directly to streams and rivers. Leakage of sewage pipes can also contaminate ground water sources.

2. Inorganic wastes: Wastes from households, such as plastics and synthetics, pollute surface water sources such as rivers, dams and lakes.

Types of water pollution

Ground water pollution:

This happens when toxic chemicals from industries, pesticides, herbicides and fertilizers are leached down to underground water and aquifers.

• ground water polluted by industrial and agrochemical wastes
• leaching of industrial and agrochemical wastes
• chemical wastes from industries
• fertilisers leached to the ground water source
• pesticides and herbicides
• sprays leached to the ground water source
• rainwater helps leach industrial and agrochemical wastes

Surface water pollution: Surface water pollution is when chemical pollutants and plastics are washed by ground surface runoff to streams, rivers, dams and lakes.

Nutrient pollution: Nutrient pollution occurs when sewage and fertilizers are washed into rivers, dams and lakes and causes nutrient levels build up in the water above normal. The build-up of nutrients results in over growth of microscopic algae and other water weeds to devastating levels. Lake Chivero and Manyame river in Harare have been greatly affected by nutrient pollution resulting in heavy infestation by the hyacinth or water weed.

Discharge of biodegradable matter: Oxygen

depletion, resulting from discharge of large volumes of biodegradable matter such as sewage waste, promote rapid multiplication of microorganisms which use up a lot of oxygen upon death as they decompose. This raises the biological oxygen demand (BOD). Organisms such as fish that are depended on oxygen (aerobic) will die due to lack of sufficient oxygen.

Effects of water pollution

Infestation of water bodies by over grown algae or water weeds: Nutrient pollution results in over growth of algae and water weeds which infest the water and thereby reduce volume of water and water holding capacity of the water body. The weeds also block irrigation canals and pipes.

Death of aquatic organisms: Water is a habitat (natural home) for a number of organisms such as fish, crocodiles, among others. The survival of these organisms is dependent on the status or quality of the water.

Toxic substances that end up in water bodies result in death of aquatic organisms.

Oxygen depletion, by decomposing overgrown algae, results in death of water living organisms due to lack of oxygen.

Bio-accumulation of toxic metals and pollutants in food chains: Heavy metals such as lead are consumed by smaller organisms in water that are also in turn consumed by other organisms in the food chain. Along the food chain some organisms are affected and are killed.

This results in disrupted food chains and disturbance of ecosystem balances.

Disease outbreaks: Drinking of contaminated water by humans results in outbreak of diseases such as diarrhea, cholera and typhoid.

Low quality degraded water: Water is contaminated to such an extent that it cannot be safely supplied to livestock. Water may be contaminated with salts or heavy poisonous metals such that it cannot be irrigated to crops.

Sampling local water sources to determine levels of pollution

1. Collect as many water samples as possible from various water sources in your area.

2. Label each sample for identification including details of the source of water.

3. Filter the water samples using clean white filter paper placed on correspondingly labeled funnels for identification.

4. Make observations and find out if there are any residues left on the filter paper.

5. What conclusions can you make from the

observations?

Water samples can also be taken for further tests in laboratories where resources allow.

Ways of reducing water pollution

Education: Before any action can be taken to reduce water pollution, farmers, industrialists and general citizens should be made aware of the water pollution problems, effects and the possible solutions. This can be done through schools’ curriculum, public media campaigns, radio and television jingles.

Environmental laws: Strict environmental laws inhibiting pollution of water bodies together with effective law enforcement and fines can help to reduce the levels of water pollution.

Laws also help in managing internationally

managed water sources such as the Zambezi River shared between the DRC, Zambia, Zimbabwe and

Mozambique. Laws help in cooperative management of the water sources by authorities such as the Zambezi Water Authority.

Minimum and correct use of persistent agrochemicals: Farmers should not overuse fertilizers,

persistent pesticides and herbicides. Farmers should use environmentally friendly chemicals and farming methods. Empty agro-chemical containers should be disposed off properly.

Proper disposal of sewage and domestic wastes: Organic and inorganic waste from households should be disposed off in proper designated places to avoid contamination of water bodies.

Treatment of industrial wastes: Treatment of industrial gases and chemical wastes to a less harmful state before disposal helps to reduce the negative effects of the industrial wastes. The government and relevant authorities should enforce such environmental laws.

Correct application of irrigation water: Farmers should not apply excess water to their crops as this results in leaching of herbicides, pesticides and fertilizers which