Sources from which water is available for water supply schemes can conveniently be classified into the following two categories according to their proximity to the ground surface.
1. Surface Sources
2. Sub-surface Sources or Underground Sources
Surface Water
Water that gets collected on the surface of the ground or top layer of a body of water is called surface water. In this type of source, the surface runoff is available for water supply schemes. Usual forms of surface sources are as follows.
1) Lakes and Streams
A natural lake represents a large body of water within land with impervious bed. Hence, it may be used as source of water supply scheme for nearby localities. The quantity of runoff that goes to the lake should be accurately determined and it should be seen that it is at least equal to the expected demand of locality. Similar is the case with streams which are formed by the surface runoff. It is found that the flow of water in streams is quite ample in rainy season. But it becomes less and less in hot season and sometimes the stream may even become absolutely dry.
The catchment area of lakes and streams is very small and hence, the quantity of water available from them is also very low. Hence, lakes and streams are not considered as principal sources of water supply schemes for the large cities. But they can be adopted as sources of water supply schemes for hilly areas and small towns. The water which is available from lakes and streams is generally free from undesirable impurities and can therefore be safely used for drinking purposes.
2) Ponds
A natural large sized depression formed within the surface of the earth, when gets filled with water is known as pond. A pond is a man-made body of standing water smaller than a lake. Thus ponds are formed due to excessive digging of ground for the construction of roads, houses, etc. and they are filled up with water in rainy season. The quantity of water in pond is very small and it contains many impurities. A pond cannot be adopted as a source of water supply and its water can only be used for washing of clothes or animals only.
3) Rivers
Since the dawn of civilization, the ancient man settled on the banks of river, drank river water and ate fish caught from river water and sailed down rivers to find out unknown lands. Large rivers constitute the principal source of water supply schemes for many cities. Some rivers are perennial while others are non-perennial. The former rivers are snow fed and hence, water flows in such rivers for all the seasons. The latter type of rivers dries in summer either wholly or partly and in monsoon, heavy flood visits them. For such types of rivers, it is desirable to store the excess water of flood in monsoons by constructing dams across such rivers. This stored water may then be used in summer.
In order to ascertain the quantity of water available from the river, the discharges at various periods of the year are taken and recorded. The observations over a number of years serve as a good guide for estimating the quantity of water available from the river in any particular period of the year. Generally, the quantity of water available from non-perennial rivers is variable throughout the year and it is likely to fall down in hot season when demand of water is maximum. It becomes therefore essential to augment such source of water supply by some other sources so as to make the water supply scheme successful.
The quality of surface water obtained from rivers is not reliable. It contains silt and suspended impurities. When completely or partly treated sewage is being discharged into the river at some upstream point, the river water is to be suspected for high contamination. The river water requires to be properly analyzed as regards to the contents of disease bacteria, harmful impurities, etc. The presence of all such undesirable elements in river water requires an exhaustive treatment of water before it can be make fit for drinking purposes. It should however be noted that the quality of river water is subject to the widest variations because it depends on various uncertain factors such as character of the catchment area, the discharges of sewage and industrial wastes, climatic conditions, season of the year, etc. The character of the water differs not only with each individual river, but also at many points along the course of the same river. It is usually found that the quality of river water at its head is good, but it goes on deteriorating as the river proceeds along its course. The chief use points to be considered in investigating a river supply of water are as follows.
- Adequacy of storage of purified water so as not to disturb the distribution system during periods of fold when the river water is turbid
- Efficiency of the subsequent stages of purification system adopted
- General nature of river, the rate of flow and the distance between the sources of pollution and the intake of the water
- Relative proportions of the polluting matter and the flow of river when at its minimum.
4) Storage Reservoirs
An artificial lake formed by the construction of dam across a valley is termed as a storage reservoir. It essentially consists of the following three parts
- A dam to hold water
- A spillway to allow the excess water to flow and
- A gate chamber containing necessary valves for regulating the flow of water
At present, this is rather the chief source of water supply schemes for very big cities. The multi-purpose reservoirs also make provisions for other uses in addition to water supply such as irrigation and power generation.
5) Oceans
Normally, it is not used as water supply source.
Underground Sources
In this type of source, the water that has percolated into the ground is brought on the surface. The entrance of rain water or melted snow into the ground is referred to as infiltration. The movement of water after entrance is called percolation. It is observed that the surface of earth consists of alternate courses of pervious and impervious strata. The pervious layers are those through which water can easily pass while it is not possible for water to go through an impervious layer. The pervious layers are known as aquifers or water-bearing strata. If aquifer consists of sand and gravel strata, it gives good supply of drinking water. The aquifer of limestone strata can supply good amount of drinking water, provided there is presence of cracks or fissures in it.
Forms of Underground Sources
Following are the four forms in which underground sources are found.
1) Infiltration Galleries
An infiltration gallery is a horizontal or nearly horizontal tunnel which is constructed through water bearing strata. It is sometimes referred to as horizontal well. Horizontal tunnels constructed at shallow depth (3-5m). The gallery is usually constructed of brick walls with slab roof. The gallery obtains its water from water bearing strata by various porous drain pipes. These pipes are covered with gravel, pebble, etc. so as to prevent the entry of very fine material into the pipe.
The gallery is laid at a slope and the water collected in the gallery is led to a sump from where it is pumped and supplied to consumers after proper treatment. The manholes are provided along the infiltration gallery for the purposes of cleaning and inspection. The infiltration galleries are useful as sources of water supply when ground water is available in sufficient quantity just below ground level or so. The galleries are usually constructed at depth of about 5 to 10 metres from the ground level.
Fig. 1 Infiltration Gallery
2) Infiltration Wells
In order to collect large quantities of water, infiltration wells are sunk in series in the banks of river. The wells are closed at top and open at bottom. They are constructed of brick masonry with open joints. Various infiltration wells are connected by porous pipes to a sum well, called jack well.
3) Springs
Natural outflow of ground water at the earth’s surface forms a spring. A pervious layer sandwiched between two impervious layers form a spring. Springs are capable of supplying very small amount of water and is not recommended for water supplies. Good developed springs can sometimes use for water supply source for small towns, especially in hilly areas. The two types of springs are
a) Gravity Springs – when the ground water raises high and water overflows through the sides of a natural valley or depression.
b) Surface Springs – sometimes an impervious obstruction (or) stratum supporting the underground storages become inclined causing water table to go up and get exposed to the ground surfaces.
c) Artesian Springs – when water comes out of pressure it’s called artesian springs. Since water comes out of pressure; they are able to provide higher yields and may be considered as source of water supply.
Fig. 2 Spring
4) Open Well
Well is a vertical structure dug in ground for the purpose of bringing ground water to the earth’s surface. Open wells have comparatively large diameters and lower discharges. Usually they have discharge of 20 m³/hr but if constructed by efficient planning it gives discharge of 200-300 m³/hr. They are constructed of diameter of about 1-10 m and have depth of about 2-20m. They are constructed by digging therefore they are also known as dug wells. It can put 8 to 10 cm diameter bore hole in the center of the well, to extract additional water. Types of open wells are given below.
a) Shallow Open Well - These are the wells resting on the water bearing strata and gets their supplies from the surrounding materials.
b) Deep Open Well - These are the wells resting on the impervious layer known as mota layer beneath which lies water bearing pervious layer and gets their supply from this layer.
c) Kachha wells - These type of wells is only constructed when water table is high as these type of wells sometimes collapses.
d) Driven Well - This is a shallow well, constructed by driving a casting pipe of 2.5cm – 15cm in diameter. The bottom end of casting pipe is pointed known as well point or drive point. The discharge of these wells is very small and these are suitable for domestic purpose only.
5) Tube Well
A tube well is a long pipe sunk in ground intercepting one or more water bearing strata. As compared to open well their diameter is less about 80-600 mm. It is deep as 70 to 300m and tap more than one aquifer. Types of tube wells are given below.
a) Shallow Tube Well - These are the tube which has depth limited to 30 meters and maximum have discharge of 20 m³/hr.
b) Deep Tube Well - These are the tube wells which have maximum depth of about 600 m and may give discharge more than 800 m³/hr.
c) Strainer type Tube Well - These is most commonly used tube such that in general a tube well means strainer tube well. In this type of well, a strainer which a wire mesh with small openings is wrapped around the main pipe which also has large openings such that area of opening in strainer and main pipe remains same. Annual space is left between two strainers so that the open area of pipe perforations is not reduced. The type of flow is radial.
Fig. 3 Strainer type Tube Well
d) Cavity type Tube Well - A cavity type tube well consists of a pipe sunk in ground up to the hard clay layer. It draws water from the bottom of well. In initial stages fine sand is also pumped with water and in such manner a cavity is formed at the bottom so the water enters from the aquifer into the well through this cavity.
Fig. 4 Cavity type Tube Well e) Slotted type Tube Well - If the geological formation of the earth strata is such that; sufficient number of water bearing stratum are not available for the construction of strainer tube well even up to the depth of 100m, slotted type tube wells are constructed.
Fig. 5 Slotted type Tube Well
f) Perforated type Tube Well - When the water table is very near to the ground or the tube wells are required for obtaining water for short duration only, these types of tube wells are used. In these tube wells the pipes are made perforated by drilling holes in them.
Fig. 6 Perforated type Tube Well
