Underground Sources of Water: 4 Sources | Water Engineering

The underground (or subsurface) sources of water are of the following four forms: 1. Infiltration Galleries 2. Infiltration Wells 3. Collector Wells or Radial Wells 4. Springs.

From each of the first three forms relatively small quantity of groundwater is obtained and hence these may be considered as the minor forms of the underground sources of water. On the other hand most of the groundwater is extracted from the last form viz., wells, and hence it is a major form of underground source of water.

Source # 1. Infiltration Galleries:

An infiltration gallery is a horizontal or nearly horizontal tunnel usually rectangular in cross-section and having permeable boundaries so that groundwater can infiltrate into the same, and hence it is also sometimes known as horizontal well. It is generally provided in highly permeable aquifers with high water table so that adequate head is available for gravity flow of groundwater into the gallery. It is frequently located near a perennial recharge source and hence it is usually placed along the bank, or under the bed of a river. The usual depth at which the gallery is placed ranges from 3 to 10 m below the ground surface.

The gallery is generally constructed by the cut and cover method. It is usually made of brick masonry walls and R.C.C slab roof or an arch roof. Sometimes instead of bricks, porous concrete blocks are used for the construction of the gallery walls. In the walls of the gallery number of openings are provided to permit the entry of water into the gallery.

The openings are covered with graded gravel to prevent entry of fine sand particles into the gallery. In some cases the infiltration galleries are made of a series of open-jointed masonry barrels, parabolic in cross-section and the joints between the adjacent masonry barrels being covered with graded gravel to prevent entry of fine sand particles into the gallery. Along the infiltration gallery manholes are provided for the purposes of cleaning and inspection.

The gallery is laid at a slope and the water collected in the gallery is led to a sump from where it is pumped out and supplied to consumers after necessary treatment. Infiltration rates of 1500 to 7000 m³/day per 100 m length of gallery are commonly obtained.

The infiltration galleries are useful as source of water supply when groundwater is available in sufficient quantity just below the ground level or so.

Sometimes a horizontal pipe having perforations all around its surface is laid in place of rectangular tunnel or parabolic masonry barrel. The perforations are covered with gravel to prevent entry of fine sand particles into the pipe. Such construction is known as infiltration pipes and it is adopted when the groundwater is available in small quantity.

Source # 2. Infiltration Wells:

Infiltration wells are the shallow wells constructed in series along the banks of a river to collect the water seeping through the banks of the river. The wells are closed at top and open at bottom.

These wells are constructed of brick masonry with open joints. For the purpose of inspection, manhole is provided in the top cover of the well. The water infiltrates through the bottom of these wells and as it has to pass through sand bed it gets purified to some extent.

The various infiltration wells are connected by porous pipes to a collecting sump well-known as jack well. The water collected in the infiltration wells flows by gravity into the jack well. The water from the jack well is pumped to treatment plant and supplied to the consumers.

Source # 3. Collector Wells or Radial Wells:

A new technique which has been recently developed is that of constructing collector wells or radial wells. The collector wells or radial wells are used when large discharges are required but a thin highly permeable aquifer is available.

A collector well consists of a reinforced concrete caisson or well about 3 to 6 m in diameter from which horizontal lateral screened pipes are projected radically near the bottom. The caisson is sunk into the water-bearing stratum by excavating the earth from the inside and its bottom is sealed by a concrete plug. The horizontal screened pipes are heavy steel pipes 150 to 200 mm in diameter; 30 to 150 m in length depending upon the geologic conditions and the discharge required; and are perforated with longitudinal slots.

These pipes are jacked hydraulically into the aquifer through precast portholes in the caisson to form a radial pattern of horizontal pipes. In some cases a casing pipe is first introduced through the porthole after which the screened pipe is inserted and the casing pipes pulled out. Each of these radial collector pipes is then developed either by through the porthole after which the screened pipe is inserted and the casting pipe is pulled out.

Each of these radial collector pipes is then developed either by compressed air or by pumping so that the finer materials of the aquifer are washed out and natural gravel packs are formed around these pipes. About 10 such radial collector pipes can be installed at one level. Similar set or sets of radial collector pipes may be installed at different levels, if possible, so as to increase the yield.

The inner end of each collector pipe is fitted with a sluice valve which can be operated from pump house above. The inflow of water into the well is thus controlled. As the water obtained by this method is generally clean and free from bacterial contamination, it can be pumped directly from the well into the mains for supplying to the consumers.

The initial cost of a collector well exceeds that of an ordinary well, but the collector wells have larger yields, reduced pumping heads and low maintenance costs. Yields of 27000 m³/day are common for collector wells located near streams and those away from streams give an average yield of about 15000 m³/day.

This type of well construction is very common in France, and hence it is sometimes referred to as French system of tapping groundwater. A particular collector or radial well which has been patented is Ranney well which consists of an R.C.C. caisson about 4 m (13 feet) in diameter and 0.45 m (1, 5 feet) thick.

Source # 4. Springs:

A spring is natural outflow of groundwater which appears at the ground surface as a current or stream of flowing water.

Springs may be classified into:

(i) Those resulting from gravitational forces, and

(ii) Those resulting from non-gravitational forces.

(i) Gravity springs results from water flowing under hydrostatic pressure.

The following are the different types of gravity springs:

(a) Depression Springs:

These springs are formed due to overflowing of the water table, where the ground surface intersects the water table. The flow from such a spring is variable with the rise or fall of water table and hence in order to meet with such fluctuations a deep trench may be constructed near such a spring. The deeper is the trench, the greater is the certainty of continuous flow because the saturated ground above the elevation of the trench bottom will act as a storage reservoir to compensate for the fluctuations of the water table.

(b) Contact Springs or Surface Springs:

These springs are created by a permeable water bearing formation over lying a less permeable or impermeable formation that intersects the ground surface. However, in such springs, because of the relatively small amount of underground storage available above the elevation of the overflow crest, the flow from them is uncertain and is likely to cease after a drought. As such these springs can also be developed by the construction of a cutoff trench or a cutoff wall.

(c) Artesian Springs:

These springs result from release of water under pressure from confined aquifers either at an outcrop of the aquifer or through an opening in the confining bed. The amount of water available in an artesian spring may be large if the catchment area is large. The flow may be slightly increased by the removal of obstructions from the month of the spring.

(ii) Non-gravity springs include volcanic springs and fissure springs. The volcanic springs are associated with volcanic rocks and the fissure springs result from fractures extending to great depths in the earth’s crust. These are usually thermal springs. Thermal springs discharge water having a temperature in excess of the normal local groundwater. These are also designated as warm springs and hot springs. Waters of thermal springs are usually highly mineralized and often contain sulphur.

In general springs are capable of supplying small quantity of water and hence these may serve as sources of water supply only for small towns, especially near hills or bases of hills. Further the hot springs cannot be used to supply water for domestic purposes. However, the water obtained from some of the hot springs is found to be useful for the cure of certain skin diseases.