17.7 Control of water pollution
The control of pollution should ideally take place at the point of generation, or, in other words, it should be prevented at source. As you have learned from the sanitary survey, you should look out for possible sources of pollutants in your locality.
The control of excess nutrients is an important issue both from a public health perspective and to keep natural waters free from eutrophication. An increasing proportion of water pollution originates from diffuse (non-point) sources, such as agricultural use of fertilisers. Farmers may need guidance on good agricultural practices that will help reduce water pollution from agriculture. For example, the amount of fertiliser used and the timing of its application can make a significant difference.
Imagine you are a farmer thinking about the best time to apply fertiliser to your field. Would it be better to spread the fertiliser before or after heavy rain?
It would be better after the rain because if the fertiliser was spread beforehand then much of it would probably be washed away. This would not only pollute the nearest river but would, of course, also reduce the effectiveness on the crop.
Pollution prevention is best achieved by ensuring that each potential point source is properly sited, designed, constructed and managed; the aim being to contain the pollutants and prevent their uncontrolled release to the environment. Sources of pollution should be sited as far from watercourses as possible (at least 15 m away) and below any water sources on the site. Appropriate use of excreta disposal, solid waste disposal and animal waste disposal will help prevent contamination of both surface and groundwater. (You will learn more about this in the waste management study sessions that follow.)
Springs usually become contaminated when latrines, animal yards, sewers, septic tanks, cesspools or other sources of pollution are located on higher land nearby. In areas with limestone rocks, contaminated material can enter the water-bearing channels in the rock and descend through cracks and holes or other large openings and may be carried along with groundwater for long distances. Other rock types can have a similar effect so it is important to have knowledge of the local geology to assess the probability of groundwater contamination.
What are the key preventive measures that will help to ensure that spring water is of a consistently high quality?
The key measures are:
- Dig a diversion ditch above the spring that will take surface water away from it.
- Build a fence to keep animals away from the spring.
- Design and build a protection box for the spring that will prevent contamination.
- Monitor the condition of the spring and the quality of the water regularly.
Monitoring of the quality of spring water and other sources would be done by you and environmental health experts.
For rainwater harvesting, pollution control means proper maintenance of the roof and gutters and careful cleaning at the beginning of every wet season. Some form of mesh should be placed between the guttering and the pipe that leads to the storage tank to prevent the entry of coarse debris; it then becomes important to clean the screen regularly to prevent blockage. The worst fouling of roofs occurs when they are situated under trees in which birds roost. A rainwater storage tank should be completely covered and well maintained.
The catchment area of the water source is the total area of surrounding land that slopes towards the source. Water can become polluted from sources in the catchment even though they may be some distance away. Ideally, the whole catchment area should be protected to avoid pollution and erosion. Preserving the vegetation in the surrounding area can help protect the spring from pollution and from siltation caused by soil erosion.