Free essay on Ground-water resources

Ground water is a part of the hydrologic cycle that lies beneath the surface, but is tied to surface supplies. Ground-water, of course, is the largest potential source of supply of fresh water in the hydrologic cycle-larger than all surface lakes and streams combined.

According to an estimate of scientists about 22 per cent of the earth’s fresh water is found between the earth’s land surface and a depth of 4 km worldwide.

Despite its volume and importance groundwater is being widely abused. It is said that about half of the U.S. population gets a portion of its fresh water from groundwater sources.

Movement of groundwater:

Rocks through which water can pass without any obstruction is said to be permeable. Permeability is the capacity of a rock to allow water to pass through it. On the contrary, impermeable or impervious rocks are those through which water cannot pass freely.

A permeable rock may be porous like sand or sandstone; or may be non-porous like granite, but it allows water to flow through it because of the presence of joints, cracks, and cleavages.

Remember that porosity refers to the percentage of free space or voids in the total volume of the material concerned. The percentage of porosity shows wide variation in rocks according to the degree of compaction and the amount of cementing material.

Groundwater movement depends on the slope of the water table which, to a certain extent, follows the contours of the land surface.

When there is an alternate arrangement of permeable and impermeable rock strata, especially when folded, faulted and jointed, it forms underground reservoirs, and natural water storage of various kinds. Water wells must reach the water table to make their potential flow maximum.

Where the water table intersects the surface, it creates springs, the seepage water feeds lakes, and the river beds also receive a continuous supply of groundwater.

It is, of course, the water table due to which the rivers are able to sustain their flow even during the dry season.

Other characteristics of water table have already been pointed out in this chapter. It is evident from the foregoing discussion that the supply of underground water is an important factor in keeping streams flow continuous.

Besides, groundwater plays the most significant role in conserving the rainwater, though only a part of it, and turning it over to the streams during dries periods.

The water table provides the evidence of its existence and significance in the form of wells, springs, artesian wells, geysers, mineral springs, and hot springs. Each of these landscape features has been described in brief in the following paragraphs.

Wells:

Wells are holes dug or drilled in the land surface below the water table. Thus, the wells are artificial openings in the land surface. It may be pointed out that the holes dug into the ground must reach the water-bearing permeable rocks or fissured rocks.

In these wells, water is extracted manually or with the help of electrically operated or oil-fuelled pumps. The deeper wells provide more reliable sources of supply and are not affected by the dry spells.

On the contrary, shallow wells dry up during the dry periods when the water table falls below the reach of such wells.

Since groundwater is filtered, because it passes through many layers of rock before it reaches an aquifer the result is that groundwater is free of sediment, but it carries various kinds of chemicals. Ground water is said to be hard, whereas the rain­water is soft.

At present, increase in population, urbanization, and rapidly growing industria­lization have resulted in the pollution of our groundwater supplies.

Artesian Wells:

Artesian wells are named after the deep flowing wells at Artois, France. Artesian wells or artesian springs are those in which water rises to the surface and flows out under hydrostatic pressure.

Such wells are produced by boring down into an aquifer enclosed by impermeable strata. The aquifer comprises a basin of sedi­mentary rocks which produces a constant supply of water that rises to the ground surface.

There are the following necessary conditions for an artesian well: (i) There must be a porous aquifer, such as a sandstone, exposed at the surface in an area of heavy rainfall or infiltration. (ii) The aquifer must absorb rainwater at the surface, (iii) It must incline downwards to a much greater depth of hundreds or thousands of meters below the earth’s surface, and must be overlain and underlain by impermeable rocks so that water is prevented from upward escape.

Under such conditions, an aquifer acts as a pipe that conducts water through the subsurface. The water in the aquifer (compared with a pipe) has no exit, so is under pressure from the water above.

Under this overhead pressure, water will move upward towards any available outlet. If that outlet is a well drilled though the impervious layer and into the aquifer, the water will rise in the well automatically and will gush out at the surface.

Water from the artesian well is very clean and pure. Being very deep, the artesian well water is free from harmful chemicals, because this water is well-filtered. The depth of these wells varies from about 60 meters to several hundred meters.

Artesian wells are of great importance in large semi-arid basins, which are bordered by a series of hills which are virtually the catchments areas. Australia is said to have the largest artesian basins.

There the aquifers underlie a very large area (more than half a million square miles). The aquifers receive huge amount of water from the rains occurring in the Eastern Highlands.

There are about 9000 artesian wells in that area. Some of these wells are very deep (about 1500 m or so). Some of the wells yield saline water unfit for human consumption as well as for irrigation.

Hot Springs :

Hot springs commonly occur where groundwater sinks to very great depths, remains there for a long period of time to be heated, then flows out of the ground in a continuous and non-explosive way, contrary to the flow of a geyser.

Hot springs can occur in non-volcanic areas. Generally its occurrence is more common in areas of current or recently active volcanoes. Heated groundwater flows out through a natural opening so rapidly that it hardly finds time to cool off.

They may also occur where magmatic waters, liberated due to the cooling of subsurface igneous rocks, find their way up to the ground. The depth to which the water must percolate in order to be heated is dependent upon the underground thermal conditions.

Hot springs may contain a lot of minerals in solution, since hot water is able to dissolve most of the minerals than cold water.

When hot spring water comes up on the land surface, some of the dissolved minerals in it may be precipitated through the reduction of its temperature and loss of gases present in water that tended to keep the minerals in solution.

Besides, algae and other organisms living in water also secrete mineral matter. In Yellow Stone National Park alone there are more than 4,000 hot springs.

At the mouths of some of the hot springs, there are huge deposits of travertine. In addition, silica, alum and sulphur are also deposited at their mouths.

It may be pointed out that the Gardiner River receiving its water from the hot springs transports about 100 tons of calcium carbonate in solution form every day.

Some of the hot springs show at their openings different beautiful colours such as, indigo, opal, emerald, and other colours like red, yellow or green.

Geysers and their deposits:

Geysers are intermittent hot springs that from time to time eject steam and hot water from their openings. The ejection of steam and superheated water from an underground source through a natural hole in the ground is violent and explosive. Geyser is an Icelandic term ‘geysir’ meaning roarer or gusher.

The underground structure of a geyser consists of a number of water-filled chambers interconnected with a central pipe. The whole system is heated by the tremendous pressure exerted by the height of the column of water.

Due to high pressures the water is converted into gaseous (steam) form. Sometimes the temperatures are extremely high with the result that the geyser shoots violently into the air to heights of 60 m. Old Faithful Yellowstone National Park geyser, U.S.A. shows a regular periodicity of ejection.

It may be noted that all known geysers are situated in regions of present or recent volcanic activity. Geysers differ from hot springs. Geysers are characterized by their intermittent action.

Geysers are so violent that they throw steam and water very high into the air. Some of the Yellowstone Park geysers throw steam and water up to a height of about 100 meters.

As an exceptional case, Waimangu, a New Zealand geyser, is reported to have thrown steam etc. to a height of more than 400 meters. See figure 17.

Geyser Deposits:

Geyserites are deposited from the boiling waters of a geyser, both inside and outside of its openings (vents). Deposits of geyserites present a common sight in Yellowstone Park. Remember that geyserites are another name for silica deposits around the vents of geysers.

Relatively small deposits of geyserites have accumulated, and spread irregularly over the ground in the form of low mounds. Some of these deposits have accumulated in irregular columns and small castle-like forms.

Spring:

Spring is defined as a natural flow of water from the ground at the point where the water-table intersects the surface. Where permeable layer lies above an impermeable layer, a spring generally appears at the point where water-table intersects the surface.

It may be stated that if the saturated layer is quite large, a permanent spring may result. Obviously, the origin of a spring is closely related to the geological structures. The distribution of springs is controlled by the nature and relationship of the rocks of an area.

Besides, a spring is also related with the profile of the surface relief,. If there is a line of springs, it is called ‘spring- line’. The spring-line is dotted with a row of villages which depend on the springs for their water- supply.

Depending on their locations, the springs are classified as scarp-foot and dip-slope springs, fault-and joint springs and intermittent springs. There is another type of spring; called vaulusion springs which occur in a limestone country.

The underground conditions that favour the occurrence of springs are the following:

“Any opening, usually a fault or more or less cylindrical fissure which connects the surface of the earth with the ground-water table may give rise to a spring.

Since the amount of water in the ground fluctuates seasonally or through a period of years, the springs are likely to go dry or diminish their flow after periods of drought, or they may flow more freely than normal when the ground water table was raised, due to a series of years of excessive precipitation.” (Worcester).

Seepages:

A part of ground water appears on the surface in the form of seepages. Seepages ooze out slowly, especially along a fault line.

In India springs are found in three regions: The foothills of the Western Ghat, the low granitic hills and uplands of Chhota-nagpur, Bihar, and the Kumaun Himalaya in Uttar Pradesh.

In the upper part of mountainous regions, where there are deposits of heterogeneous mixture of boulders and clays, springs rise wherever the contact zone inters- sects the hill slop.

A step like pattern of springs occurs on the valley slope where the dip of the bed coincides with the gradient of the slope, and impermeable beds alternate with permeable beds. It is in the contact zone of permeable and impermeable beds touching the valley slope that the springs occur.