The basic source of all fresh water is from precipitation, which may be in various forms such as rain, snow, hail, and dew. This from the atmosphere may either remain on surface or go underground. Our requirements of water are met from these origins. The surface and sub-surface sources of water is categorized as follows:
1) Surface Sources
a) Ponds and lakes
b) Streams and rivers
c) Storage reservoirs
2) Sub-Surface/Underground Sources
a) Wells and tube wells
In the surface sources of water supply, the water drawn from lakes and reservoirs is considered most safe, due to settlement of suspended materials usually present in the water.
However, still water standing for long periods in lakes encourages the growth of algae, weeds and other vegetation in the water. These usually contribute to the development of bad smell, taste and color to the water.
Similarly, the quality of water obtained directly from rivers is usually not useful for direct human consumption due to the presence of silt, sand and other suspended solids. Harmful bacteria and unwanted chemicals could also be present in such waters. The practice of discharging untreated sewage into the river increases such impurities in the water. Thus the water from majority of surface sources could be contaminated and cannot be used without treatment or purification.
Rivers and streams are the most important sources for public water supply schemes. The importance of water from quality viewpoint had been recognized for a long time now. That is why most of the present urban centers grew up on the banks of major rivers, which enacted continuous and regular water supply to the inhabitants.
In most of the rivers, the quality of water flow is not constant round the year but is based upon various factors including seasons. Thus to overcome this problem and to ensure a regular supply of water, a barrier in the form of a dam is constructed across the river to store the excess water that flows during the monsoon season. The pool so created upstream of a dam constitutes the storage reservoir. Smaller reservoir may also be termed as artificial lake.
The sub-surface sources of water are based on rainwater that percolates down to be stored inside the earth. The water, which is so stored inside the earth in the form of ground water reservoirs, constitutes the major sources of sub-surface water supply. Underground water is generally pure because of natural filtration during the percolation process. This type of water is expected to be free from the presence of harmful bacteria but may have higher quantities of dissolved inorganic/organic compounds depending upon the geological formations of the region.
The underground water may be brought unto the surface by some natural phenomenon like springs etc. or through some artificial means such as by constructing wells and tube wells.