The most prominent feature of the landscape of that garden is a large river that runs through it southwards. A part of its course is however hidden under the mountains and when updrawn with kindly thirst through the veins of porous earth it issues forth as a fresh fountain.

Rivers flow like God's blessings and the beauty and the fecundity not only of the garden of Eden but also of the entire earth are their gifts. As a feature of the surface of the earth a river is so prominent and imposing that there is no possibility of its being mistaken.

It is a surface flow in full view and therefore very familiar to us. It is always in our sight and can never be out of our mind. But there is another kind of rivers, so the early man thought. They flow underground out of our sight. Without them he found it impossible to account for the sources of water in wells, springs and fountains. These 'ingulfed' flows find a place in many ancient myths and legends. In real life also early man knew how to use naturally occurring springs and water holes as sources of fresh water. In fact the history of such use of groundwater goes to prehistoric times. During droughts and times of water scarcity early man must have deepened them. The ancient Iranians developed water-collecting galleries and tunnels, called 'kanats', which were several miles long and yielded considerable quantities of water. In course of time the people of Egypt and Afghanistan also learnt this primitive technology. The ancient Chinese perfected the art of drilling deep wells. With crude and primitive materials they built drilling machineries capable of drilling to the depths of 5000 ft, a feat not duplicated until modern times.

Though man developed the skill to exploit groundwater quite early he was yet to understand its nature. The ancient Greeks and Romans, for example, speculated about it. They thought that the limestone caverns common in their countries were passages which penetrated the entire earth to form underground rivers that were fed by the oceans whose salt water was freshened by a process of earth metabolism. As their countries were located in regions of relatively low precipitation, not the rains alone but also this water could only account for the water in their wells and springs. The ancient Indians went a step further. They believed that the natural regime of a river extended far beyond the visible surface of the earth. The Ganges, their most sacred river, not only watered their land but also the skies and the underground world where she was respectively called the Akash Ganga and the Patal Ganga. They had a myth also to explain the disappearance underground of another sacred river, Saraswati, in the dry sands of Rajputana and Sindh. It would not perhaps be too much to interpret these Indian myths as an intuition or intelligent guess about the existence of what is now called the hydrological cycle.

Sir Jagadish Chandra Bose, a modern Indian scientist, told an allegorical story in his Bhagirathir Utsa Sandhane (in search of the source of the Bhagirathi), which uses these myths and is in fact the story of this cycle. Till the beginning of the 18th century it was not recognized by educated people that rivers and groundwater were only two links in the vast and complex hydrological cycle. The credit for the final acceptance of this idea in modern times goes to two Frenchmen. Pierre Perrault (1608-80) showed that the total discharge of the Seine river from 1668 to 1670 was only one-sixth of the total volume of rainfall. Edme Mariotte (1620-84) verified Perrault's work by measuring the discharge of the Seine. He also measured the infiltration of rainwater into the subsurface grounds. Another noted French engineer, Henri Darcy (1803-58), was the first man to clearly state the mathematical law governing the flow of groundwater.

The time when these Frenchmen were trying to understand and explore groundwater in good earnest is very significant. By their time the industrial revolution was about to make its appearance in the western European countries. Till that time the world population was small, predominantly agricultural and technologically backward and stagnant. Life was simple, its demands on the physical resources of the earth was not much. Everything was in plenty and seemed inexhaustible. This was particularly the case with fresh water. People were settled mainly near rivers and other surface water bodies which were easily accessible and were more than sufficient to meet their limited fresh water needs. This gave birth to an attitude of carelessness in their use of water. But now the population began to expand and settle in areas away from surface water sources.

Growing population, industrialization, urbanization, revolutionary changes in men's ways of life and their consumption pattern are now making ever increasing demands on all the physical resources of the earth. Most phenomenal has however been the growth of the demand on fresh water. Earlier it was needed mainly for agricultural and domestic purposes. Now it is needed for industries also. The traditional surface sources began to be exploited more extensively and intensively. The sources which so long seemed inexhaustible were found to be insufficient to meet the growing needs and had to be supplemented by supplies from other sources.

This is how intensive exploration and exploitation of groundwater began. Now it has become an essential part of fresh water supply, so much so that in many areas it is the only source of fresh water. Though stress on fresh water supply is being acutely felt in many areas of the world there has not been any significant change in man's attitude of carelessness. Greater human interference in the natural regimes of the surface water bodies has brought about their degradation. Similar has been the case with groundwater. Surface water is visible but groundwater is not. While we have been careless in our use of the visible resources it is no wonder that we should not be less careless in our use of this invisible resource. As it is out of our sight it stands very much in danger of being out of our mind. The degeneration of a river is apparent to everyone-- the expert and the layman alike. But the threat to groundwater is not so. It needs special efforts to understand how precious is this hidden treasure and how criminally we have been squandering that treasure.

The importance of groundwater can hardly be overestimated. 97.5% of the total quantity of water contained in the hydrosphere is in the oceans and cannot be readily used because it is saline. The remaining 2.5% is fresh water and is essential for life. About 70% of this fresh water is in a frozen state in the Arctic, the Antarctic and the mountains, about 30% is under the ground and less than 1% is in lakes, reservoirs and rivers and streams. Thus in volume the position of groundwater is second only after the permanently frozen water and is about 30 times larger than all the water of rivers, lakes and other surface water bodies taken together. In arid and semi-arid countries it is the main, if not the only, source of fresh water, while in many water-rich countries it accounts for as much as 50% of the total supplies.

According to a European Environment Agency report, Groundwater Quality and Quantity in Europe, the share of groundwater needed by different countries of the continent to meet their total demand for fresh water ranges from 9% to 100%. About 80% of the rural water supplies in the USA and Canada comes from this source. In India, among other things, the intensive use of groundwater made the 'green revolution' possible. In highly industrialized countries about 50% of the water used for industrial purposes comes from groundwater sources. In some countries groundwater has begun to be used for unconventional purposes. In Canada, for example, it is being used as a source of energy. In that country the City of Moose Jaw has developed a geothermal heating system for a public swimming pool and recreational facility, the Carleton University of Ottawa uses groundwater to heat and cool its buildings and the health centre complex in Sussex, New Brunswick, has been utilizing an aquifer for thermal energy storage since 1995. More importantly, groundwater supplies about 30% of the water in rivers and streams. In developed countries about 70% to 80% of the population live in cities which depend mostly on groundwater for their water supplies. The same trend has overtaken the rest of the world. Today we have become so much dependent on groundwater that without it our survival is impossible. Therefore it has become very imperative for us to understand this precious treasure which nature has kept hidden underground.

What is this groundwater?

It is water that is found underground. But is it in some rivers or lakes as primitive man thought? Or is it, as Milton imagined, a river which passes a part of its course 'underneath ingulfed' in 'porous' earth and issues forth on the surface of the earth in the form of wells and springs 'with kindly thirst updrwan' i.e., by capillary action? Many such springs and wells have in fact given birth to rivers and streams, as it did in the garden of Eden. In certain respects Milton seems to anticipate what the scientists have found out, but his basic concept that it is a river is not correct.

According to the scientists it is neither a river nor is it collected in some underground lakes. It is not confined to only a few channels or depressions in the same way that surface water is concentrated in streams and lakes. Rather, it exists almost everywhere underground in the spaces between particles of rocks, gravels, sand and soil, or in crevices and cracks in rocks. Most of it comes from precipitation which infiltrates below the ground into the subsurface soil. Precipitation that falls on the surface of the earth is distributed in four main ways: some is returned to the atmosphere by evaporation, some is intercepted by vegetation and then evaporated from the surface of leaves, some percolates into the soil by infiltration and the rest flows on the surface of the earth as runoff giving rise to rivers, streams and lakes. Thus groundwater is an essential part of the hydrological cycle that involves the continuous circulation of water in the earth-atmosphere system.

In the upper layers of the ground this percolated water exists along with air and is called the aerated or unsaturated zone where the interstices or empty spaces are occupied partially by water and partially by air. The water found in this zone is called soil moisture. Below this zone occurs the saturated zone where all the interstices are filled with water. The topmost level of this zone is called the water table which rises or falls according as the quantity of water in the saturated zone increases or decreases as a result of heavy precipitation or drought or over-abstraction. The water table may be deep or shallow, it may be only a foot below the ground's surface or it may be hundreds of feet down. Some parts of the saturated zone contain more water than others because of their peculiar geological formation of permeable rocks or porous and loose materials like gravels, sand, limestone etc. These are called aquifers. They are like sponges which can absorb liquids that ooze out under pressure. They come in all sizes. They may be small, only a few hectares in area, or very large, underlying thousands of square kilometres of the earth's surface and extending across political boundaries. They may be only a few metres thick, or they may measure hundreds of metres from top to bottom.

Groundwater is not static but mobile, it is always in motion. But the rate of its movement varies according to the variable geological formation of the aquifers. As permeable or porous and loose or unconsolidated materials contain interconnected cracks or spaces that are numerous enough and large enough to allow water to move freely, in aquifers formed of such materials groundwater may move several metres in a day, whereas in aquifers formed of relatively impermeable or consolidated materials the rate of movement may be very slow, only a few centimetres in a century.

There are materials which are very porous yet impermeable. Clay, for instance, has many spaces between its grains, but the spaces are not large enough to permit free movement of water. Groundwater continues to descend beyond the saturated zone until at some depth it merges into a zone of dense rocks. Water is contained in the pores of these rocks, but the pores are not connected and water cannot migrate. Such water may be thousands of years old. Some aquifers are called fractured aquifers because they are rocks in which groundwater moves through cracks, joints or fractures in otherwise solid rocks. Examples of such aquifers include granite and basalt.

Limestones are often fractured aquifers, but here the cracks and fractures may be enlarged by solution, forming large channels or even caverns. Limestone terrain where solution has been very active is called karst. Sandstone is a porous media, but by solution it may become so highly cemented or re-crystallized that all its original spaces are filled and as a result it no longer remains porous. If however it contains cracks it can still act as a fractured aquifer. Aquifers may again be either unconfined ' those that are bounded by the water table ' or confined ' those that lie beneath layers of impermeable materials. The aquifers of the latter kind are sometimes called artesian wells. The water in these wells rises higher than the top of the aquifer because of confining pressure. If the water level rises above the ground surface a flowing artesian well will occur. Groundwater usually flows downhill with the slope of the water table. Like surface water it flows toward, and eventually drains into streams, rivers, lakes and the oceans. In aquifers underlying surface drainage basins this flow does not always mirror the surface flow. Groundwater may move below the ground in directions different from those of the water flowing overland.

Groundwater moves mainly because of the operation of the hydrological cycle. The aquifers receive their supplies from the percolation of water from precipitation. This is called their charging. Eventually this water reappears above the ground by flowing into rivers, streams, lakes and the oceans. This is called discharge. The extent of charging of the aquifers depends on precipitation, the slope, soil type and the vegetable cover of the surface ground above them and the geological formation of the aquifers themselves and their consequent capability to absorb water. This also determines the quantum of the surface flow called runoff. If for any reason the underground aquifers are less capable to absorb enough water from precipitation the surface flow will be more and may cause floods. In course of time nature establishes a kind of balance among the various links in the hydrological cycle. As an integral part of this cycle groundwater also develops a regime of its own. At times this balance and the regime may get disturbed by geological and cosmic extreme events like earthquakes, droughts, heavy rains and abrupt shifts in the climatic cycle. But nature, when left to itself, has the capacity to restore them or to re-establish a new balance and regime.

Since the industrial revolution men are exploiting groundwater at an ever increasing rate to meet their ever increasing fresh water needs and thereby interfering too much in its natural regime. Now in many areas of the world the rate of withdrawal of groundwater from aquifers is much in excess of the rate of their recharging. Such over-exploitation of groundwater has serious consequences. Continued over-extraction of groundwater will cause the water table to fall resulting in the ultimate depletion of aquifers. Renewal of water in rivers, for example, takes only a few days whereas in case of groundwater it takes 1400 years. Thus if an aquifer gets depleted due to overdrawl of its water in excess of the rate of its recharging it will need these many years to get replenished. And what will happen in the mean time to the vast population which depends on this source for its survival and the material civilization it has built upon it? That civilization will come crashing like a house of cards and that population will be extinct in a matter of days.

Other adverse effects of such over exploitation of groundwater are deprivation of rivers, streams and lakes of an important source of their supplies, subsidence or caving in of land above the aquifers, drying up of wetlands and other small water bodies disastrously affecting the ecosystems they support and salt water intrusion in coastal regions rendering them unusable. Many areas of the world are already experiencing these adverse effects and in days to come many more areas will follow suit. Due to over-drafting of groundwater parts of Mexico City, for instance, have subsided as much as 10 metres in the past 70 years, resulting in a host of problems in its water supply and sewerage systems. According to the US Geological Survey in the United States more than 17,000 square miles have been directly affected by subsidence.

Over-abstraction may give rise to another danger. An aquifer which is used by more than one country or region may become a bone of contention amongst those countries or regions. Excessive irrigation on the other hand may cause water table to rise and lead to water logging making the land saline. We often ignore the geological aspect of groundwater and the adverse effects that its over-abstraction may have on the underground geological structure of an area. Engineers must consider groundwater when planning almost any kind of structure, either below or above the ground. Ignoring the slope stability can be costly and dangerous. Groundwater is a major force in geological changes. The fluid pressure exerted by groundwater, for example, play an important role in the occurrence of earthquakes. Geologists also know that the movement of water through underground geologic formations controls the migration and the accumulation of petroleum and the formation of some ore deposits.

We are threatening groundwater not only quantitatively but also qualitatively. Water is never found in a pure state in nature and both groundwater and surface water may contain many constituents which make their use without appropriate treatment unfit for specific purposes. The suitability of water for a given use depends on many factors and considerations important for surface water are equally applicable to groundwater. But one very important difference between the two is that as groundwater flows through an aquifer it is naturally filtered. This filtering, combined with the long residence underground, makes groundwater usually free from disease-causing micro-organisms. Natural filtering also means that groundwater usually contains less suspended material and un-dissolved solids than surface water. This is the reason why it is generally assumed that it is safe and is therefore used mostly without any treatment. But it may not be so. The rain water which charged the aquifer itself might have been polluted by the polluted atmosphere. The acid rain is an example.

Again, one of the most important natural changes in groundwater chemistry occurs in the soil and in course of its journey through the aquifer it dissolves certain substances it encounters. Who knows if arsenic, the greatest menace now found in groundwater throughout the world, is due to this? Most dangerous are the innumerable ways in which we are contaminating groundwater by our various activities and making it unsafe. There is a popular misconception that as it is not exposed like surface water it is less vulnerable to pollution.

Human activities which are mainly responsible for pollution of groundwater are those related to agriculture and industry. Modern agriculture is very intensive and uses heavy doses of chemical fertilizers, pesticides and herbicides containing highly toxic compounds. Industrial processes also involve the use of many toxic materials. Through our agricultural and industrial activities we also generate huge amounts of hazardous wastes both liquid and solid. They pollute not only the soil, air and surface water but also find their way to the underground aquifers. Other factors responsible for groundwater pollution are leaking pipelines and storage tanks containing petroleum products, septic systems, municipal landfills, livestock wastes, leaky sewer lines, wells for disposal of liquid industrial wastes, toxic substances from mining sites, dumping grounds for disposal of solid and liquid wastes, fly ash from coal-fired power plants, contaminants in rain water etc.

In cold countries road salt is a major source of contamination. Groundwater pollution is a serious problem and is very costly and difficult, if not impossible, to solve. That is so because of its relative inaccessibility, huge volume and slow flow rate. Once pollutants enter an aquifer, the environmental damage can be severe and long lasting because of the very long time needed to flush out the pollutants, if it is at all possible to do so. Contamination of groundwater is also very insidious, it takes many years to show up in water withdrawn from wells. By the time it is detected, it may be too late to prevent serious contamination. And as a major link in the hydrological cycle polluted groundwater pollutes surface water also.

Thus it is obvious that though the groundwater reserve is huge we cannot use it as much as and in any manner we like. Because of the threats to groundwater and its importance not only for the maintenance of the present standard of our material civilization and a sustainable development, but also for our very survival it is urgently necessary to stop its overdrawal and pollution. There should be continuous monitoring to detect and prevent them. Withdrawal of water from aquifers found polluted should be immediately stopped and appropriate steps should be taken for its early purification. It is a very difficult task for various reasons. It is a highly technical job and there is a great shortage of groundwater experts throughout the world. Costs involved are also huge. Even in a rich country like the U.S.A. many states are unable to bear the burden of all the functions involved and some of it, like the UIC or Underground Injection Control Program, is done for them by the federal government.

Exploration of groundwater started much later than surface water and sufficient and reliable data are therefore not often available. Finally, the root cause of excessive demands not only on groundwater but also on all the physical resources of the earth is the abnormal growth of population. Unless that cause is removed it is impossible to solve the problems of groundwater. And unfortunately the rate of population growth is comparatively much higher in poorer countries. To cap it all, many of these poor countries happen to be poor in fresh water resources also. According to the UNEP Report, Global Environment Outlook-2000, India is one of those countries which are currently withdrawing 20 to 40% and will be withdrawing around 40% of their water resources by the year 2025 AD when they will face a severe fresh water crisis. According to the Report of the World Commission on Dams in India the groundwater table is falling at an alarming rate of one metre per year. It is therefore needless to say that we have a special reason to be careful in the use of our water resources. To save ourselves we have to save our groundwater too.