Surface water, main source of water supply in many areas, originates from rain water. Examples of surface water include rivers, tanks, lakes, wadis (water sources which are dry in seasons other than rainy), man-made reservoirs and sea water. Surface water is prone to contamination from human and animal sources. As such it is never safe for human consumption unless subjected to sanitary protection and purification before use.
The vast majority of Indian cities and towns depend upon surface water source, which includes
- Impounding Reservoirs
- Rivers and Streams
- Tanks, Ponds and Lakes.
In general, surface water supplies possess a high probability of organic, bacterial and viral contamination.
- Impounding reservoirs:
These are artificial lakes constructed usually of earthwork or masonry in which large quantities of surface water is stored. Dams built across rivers and mountain streams also provide large reserves of surface water. The area draining into the reservoir is called "Catchment Area". One disadvantage of storing water for long periods in reservoirs is the growth of algae and other microscopic organisms, which impart bad tastes and odour to water.
- ranks next to rain water in purity
- Free of pathogenic organisms.
The impurities are derived from -
- catchment area
- human habitations
- animal keeping or grazing
Keep the catchment area free from human or animal intrusion
Rivers and Streams:
Many rivers furnish a dependable supply of water. Gross pollution is the major disadvantage of river water and is many times unfit for drinking without treatment. Characteristics:
- Turbid during rainy season, may be clear in other seasons.
- Contains dissolved and suspended impurities of all kinds
- The bacterial count may be very high.
- Organisms of alimentary canal of the people.
- Surface washings
- Sewage and sullage water
- Industrial and trade wastes
- Drainage from agricultural areas.
- Customs and habits of the people
- Animal washing
- Disposal of the dead
SelfPurification: Certain amount of self-purification does occur in river water by natural forces of purification; dilution, sedimentation, aeration, oxidation, sunlight, p!ant and animal life.
These agencies are not sufficient to render the water potable. River water needs purification before it can be used for drinking purposes.
Tanks, Ponds and Lakes:
Tanks, an important source of water supply in Indian villages, are large excavations in which surface water is stored.
- Receive all types of contamination.
- Full of silt and colloidal matter
- Contain aquatic vegetation (older tanks)
- Used for washing clothes, cattle, humans, cooking pots,
- Children use it for swimming
- Regular defecation around the edges which will be washed into the tank at the next rains.
Tanks are thus subjected to unlimited possibilities of contamination and are highly dangerous, as a source of drinking water, even at the best of times. Unfortunately, the tank water is used without being boiled, disinfected or treatment of any kind. This leads to an array of diseases, sickness and death especially of children.
Improvement of Tanks:
A certain amount of natural purification takes place in tank water because of storage, oxidation and other agencies but these are not sufficient to render the water safe. The sanitary quality of tank water may be improved by the following actions:
- Raising the bunds of the tank to prevent the entry of surface washings.
- Fixing a fence around the tank to prevent access to animals.
- Denying permission to enter into the tank directly.
- Elevated platform to draw water.
- Periodical removal of aquatic weeds
- Cleaning the tank at the end of dry season.
In spite of these precautions from a practical point of view it is not possible to prevent pollution of tanks as the people who consume the tank water are often among the poorest in the country and do not have sanitary concepts. It is believed that the simplest solution consists of subjecting the tank water to sand-filtration. The addition of chlorine would undoubtedly add to the value of sand filtration.
Though this source is plentiful, it has great many limitations. Sea water contains 3.5 per cent salts. Offshore waters of the oceans and seas have a salt concentration of 30,000 to 36,000 mg/litre (30-36g/litre) of dissolved solids including 19,000 mg/litre of chloride, 10,600 mg/litre of sodium and 1270 mg/litre of magnesium. Desalting and demineralization process involves heavy expenditure. It is adopted in places where sea water is the sole source available.