It is often said, as if to encourage the hope of increasing our future water resources, that there is more water within the top half-mile of the earth’s crust than in all the rivers and lakes combined. True as this may be, what really concerns us is the proportion of ground water that can be made economically available. At present we do not know how much this is, but we do know that it is only a small fraction of the total. Some ground water lies so deep that it is uneconomic to raise it to the surface; much of it is too far away from the supply area; and often it is too heavily mineralized, or dangerously polluted by surface water.
Because ground water is normally invisible, the study of its distribution and movements has been much neglected and misunderstood in the past. This has often led to overpumping, which has depleted the amount stored. Today the dangers of exhausting this extremely valuable source are better realized. Obviously, no more water should be taken out of the ground than goes in, and when replenishment is slow, less can be withdrawn than when it is fast. But there is more to the effective use of ground water than this. To withdraw ground water safely, we need to know how much of the rainfall percolates through the soil, how fast ground water moves, how one area of ground water feeds another, and so on. In recent years a complex science has developed, including detailed geological surveys, test boreholes over a wide area, and the use of radio waves, in order to relate these variables.
Aristotle believed that all ground water derived from the condensation of atmospheric water vapour underground. We now know that only small amounts of ground water form in this way, mainly where there are heavy mountain fogs, desert sand dunes, or limestone caves. A small quantity of ground water also exists where surface water has been trapped during the formation of sedimentary rocks, as under the Californian valley. This connate water is usually very saline and therefore useless. Just as useless is juvenile water, formed deep in the earth’s crust by chemical and physical action. It was the Roman engineer Vitruvius who first proposed that ground water was derived from the infiltration of rain, but it was not until the 17th century A.D. that this was confirmed by experiment.
Strictly speaking, the term ground water includes all water below the earth’s surface, but in practice the term is only used for that which can be withdrawn by man. We begin by discussing how rain reaches the ground water zone. When rain falls on the surface of reasonably dry and permeable ground, some of it enters small channels, or interstices, and travels through them into the 50/7 zone. There are many such interstices in fertile soil, formed by the successive moistening and drying of the soil, by the boring of worms, by tunnels left by decayed plant roots, and by the expansion of water on freezing. In the soil zone, water clings to soil particles by surface tension, while some soil particles absorb water like a sponge. After a light rainfall, all the water may be retained nn the soil zone, so that none is left to travel farther downward. The proportion held also depends on the dryness of the soil, and a short, heavy downpour after a dry spell may all be retained. It is even possible that during a month in which the total rainfall has been fairly high, all the water is held in the soil zone if frequent showers have been interspersed with intense dry periods. The maximum amount of water that can be absorbed by the soil zone— called the field capacity—is important agriculturally, because it denotes the maximum amount of water available for plant growth.
When the field capacity is exceeded, the surplus water drains down into the transition zone, or zone of aeration. Any water not held in this zone continues to travel downward until it meets an impermeable layer, above which it accumulates to form the ground water zone. The layer through which ground water moves is called the aquifer, and its upper surface is known as the water table. In practice the water table may not be a well-defined surface, because water sometimes travels up from the aquifer for a few feet by capillarity to form a capillary fringe. This water may be absorbed by plant roots, but since it cannot be withdrawn by man, it is not considered part of the aquifer.
To be of any value, an aquifer must have two properties: it should hold a plentiful supply of water, and the water should flow reasonably quickly. The amount of water stored depends on the porosity, which is a measure of the size and number of spaces in the rock. Not all water contained in these interstices can be withdrawn, because some water is held in this zone in the same way as in the soil Zone. The rate at which ground water flows is determined by the aquifer’s permeability, and varies from a few feet per day to a few feet per year. Permeability depends on both the size of the pores and the extent to which they are interconnected. Without ground water flow, wells and boreholes would never be replenished, and ground water could not be exploited. In those cases where ground water drains away very slowly, the water table may rise as high as the land surface, so that the topsoil becomes waterlogged.
To abstract water economically, an aquifer should have both high permeability and porosity. Clay, for example, makes a bad aquifer because it has a very high porosity but negligible permeability. Aquifers with high porosities and permeabilities are sands, gravels, some sandstones, and alluvial deposits. The opposite is true of some volcanic rocks, such as granite. Formations such as basalt and some limestones are good aquifers not because they are porous or permeable but because they are full of cracks; in such rocks water flows very quickly—so quickly that there is little time for contaminants to be filtered out, and thus such aquifers may be polluted. Because each aquifer has a unique size, shape, porosity, permeability, stratification, inclination, and supply of rain, each must be evaluated individually to decide its potential. In the dense, unfissured limestone of Malta a borehole yields about 0.1 mgd, and the millstone grits of England yield 0.5 mgd at best, while a borehole in many red sandstones yields as much as 6 mgd.
Ground water strata fall into two distinct categories . In an unconfined aquifer, water percolates down until it meets an impermeable layer, above which it accumulates. The water table is under atmospheric pressure, which means that it rises and falls according to the amount of inflow and outflow. The water-levels in a group of wells sunk into the aquifer indicate the level of the water table.
A confined, or artesian, aquifer is sandwiched between two impermeable strata, and generally lies beneath an unconfined aquifer. Here the ground water pressure is greater than atmospheric, because water enters the aquifer at a height. The water pressure in a well is roughly equal to the difference in elevation between the well and the point where water enters the aquifer. It also depends to a small extent on the weight of the overlying impermeable layer. In confined aquifers, water rises under pressure up a well to a level higher than the aquifer itself. The level that water would’reach under pressure for every point in the aquifer is called the piezometric surface; it is calculated by sinking boreholes with sealed tops and recording the water pressures and thus the water-levels in the boreholes.
In the past, when small quantities of water were laboriously lifted from ordinary wells by man or beast, artesian wells were discoveries of great value, yielding large quantities for little effort. When the piezometric surface occurred above ground, water actually gushed from these boreholes, produced without any effort other than that of building. Artesian wells are not so important today, because many have been overdrawn, but some are still indispensable in arid areas. Of the six large artesian basins of Australia, the Great Basin in the east covers 678,000 square miles and supplies 410 mgd—the only source of water during the long dry season. Most of the water is used for stock because it is usually too saline for crops.
The area where water enters the ground to replenish an aquifer is called the recharge area. The rate of recharge depends on the area, the average precipitation, and the proportion of the precipitation that enters the soil and travels past the soil zone. So far man can do very little to increase the natural recharge of aquifers; experiments to increase rainfall, for example, have had little success. But man can do much to decrease recharge, especially by overgrazing and deforestation. In many developed countries, former recharge areas are now covered with houses and impermeable roads, from which precipitation discharges into storm drains instead of recharging the aquifer. This may well become a serious problem in some areas.
There is also a movement of water from aquifers into rivers, lakes, and seas. Water that escapes into the sea is, of course, lost to man, but that entering rivers and lakes serves a useful purpose by raising their levels. Ground water replenishment of rivers, such as the Thames, maintains the high flow needed during a dry summer. In some rivers, ground water regulates the flow in the following way: during the flood period, water leaves the permeable river bed to augment the ground water; when the river level falls, ground water returns through the river bed.
If the lower end of an aquifer is exposed on a sloping land surface the result is a spring, which is distinguished from a seepage area, such as a marsh, by the fact that water flows as a definite current. Most springs yield either small quanti- ties or large amounts for only part of the year. They seldom produce enough to meet the needs of a large community. One large spring that yields large quantities of water all through the year is the Thousand Springs of the Snake River plain in Idaho, U.S.A. Such springs invariably emerge from certain basalts or sandstones, although a few, such as the Silver Springs in Florida , are formed in limestone. The most profuse of all is the spring at Vaucluse, France, whose estimated yield of 2000 mgd would be enough to supply all the domestic consumers in Britain.
Throughout history, man has increased the escape of ground water by sinking wells. Today there are hundreds of thousands of wells and boreholes all over the world, as well as many infiltration galleries. Hand-excavated wells are usually a few feet in diameter and up to 50 feet deep; some of the largest—up to 60 feet in diameter and 250 feet deep—were built by the Romans in Arabia, and by the Russians in Bulgaria. Boreholes, 4 to 60 inches in diameter, may be drilled to a depth of more than 1000 feet. Although the design of wells and boreholes varies, they all penetrate some distance into the aquifer so as to tap as much of the waterbearing face as possible. At the region where water percolates into the well, screens prevent particles from entering the well and pump; the upper part of the well is carefully sealed to prevent the entry of polluted surface water.
One way of abstracting ground water is by deliberately overpumping, which amounts to mining water, like iron ore or petroleum. There is nothing wrong with this method if there are new sources available when the wells dry up. The mining of ground water is inevitable when the rate of replenishment is extremely slow—as in parts of the Sahara, where water has taken about 30,000 years to travel from the recharge area. Even so, overpumping poses problems before the aquifer is finally exhausted: pumping costs increase as water is overdrawn and the water-level is depressed, and water at increasing depths may be heavily mineralized. Land subsidence may also occur, as in Alameda Square in the centre of Mexico City, which is sinking at the rate of nearly one foot a year because of continuous pumping from the underlying waterlogged volcanic ash.
Ideally, the water table or piezometric surface should not be allowed to fall permanently. Since there is a time lag between water entering the recharge area and reaching the well, minor temporary falls do not matter, which is just as well in areas whose rainfall varies widely from year to year. Here the aquifer acts as a concealed storage reservoir from which water can be overdrawn in dry years and then left to recharge during a wet year.
Although confined and unconfined aquifers recharge in much the same way, they behave differently during abstraction. In unconfined aquifers, withdrawal reduces the amount stored so that the water table drops. In confined aquifers a reduction in pressure accompanies a reduction in the amount stored. The overlying impermeable layer is partly supported by the pressure of the contained water, and as the pressure falls, the aquifer is compressed. If the aquifer is elastic, as it often is, the pressure builds up again as soon as pumping stops. In other cases, the aquifer may be permanently deformed, as has happened in certain parts of the world, resulting in subsidence of the land, and buildings that lean at crazy angles.
Ground water is usually almost entirely free from bacteria, which gives it a valuable advantage as a source for public water supplies; it is also of even temperature, which is very useful for certain industries. Generally, however, ground water contains more salts than does surface water, since it has more contact with, and therefore more time to dissolve, part of the soil and rocks. Mineralization starts as soon as water percolates through the soil zone, because absorption and transpiration of water by plants concentrate the salts in the soil water. In arid areas, rapid evaporation from the soil seriously increases the salt concentration; if there is in addition a slow rate of flow in the aquifer, the ground water may quickly become too mineralized and saline to be used.
Although ground water is often quite heavily mineralized, large quantities of it are used in several countries. Ground water is the only source in Malta; in Germany it constitutes over 70 per cent of the total, in Israel about 54 per cent, and in America and Britain about 20 per cent. Sometimes the water needs only chlorin-ation before use, but even when it contains such objectional minerals as iron and manganese, to remove them, as is common practice in Germany, may still be worthwhile. But the compound that cannot be easily removed is ordinary salt . Of great importance, therefore, is the contamination of aquifers by sea-water intrusion. In an aquifer that discharges on the shoreline, there is a fairly distinct line of contact a short way inland between fresh and salt water . The fresh water floats on top of the denser sea water with very little mixing between the two. As long as the water table is above sea-level and slopes toward the sea, water flows seaward and aquifers along coastlines and on islands yield fresh water. But with rapidly increasing demands for fresh water in many populous coastal areas and islands, the naturally seaward-sloping gradient of the water table has been reduced, or even reversed, drawing the salt/fresh-water line inland.
One of the earliest examples of sea-water intrusion in England occurred in 1855, when saline water entered wells in London and Liverpool from the tidal reaches of their rivers. Since then, overpumping has led to serious contamination of coastal aquifers in Germany, the Netherlands, Japan, and many parts of America. In 1964 the extensive Californian coastline contained about 13 contaminated aquifers in an area of 150 square miles. It is the price paid for using ground water to supply a large population regardless of hydrological conditions.
The various methods used to overcome saltwater intrusion are all as expensive as they are necessary. The simplest solution is to postpone abstraction until the water table rises again above sea-level, but this is very inconvenient and costly, because water has to be brought in from other sources. For this reason, Malta is commissioning a 1 mgd desalination plant to allow its contaminated aquifers time to regain their natural fresh-water balance. Another method is to flush out the salt and raise the water table by artificially recharging the contaminated aquifer with surface water, although this again requires clean water from other sources. However, California is considering the use of flood water and purified waste water that would otherwise discharge into the sea. Californians are also trying to repel sea water by creating an artificial fresh-water ridge adjacent and parallel to the coast, fed by a line of recharge wells.
In many areas, ground water is artificially recharged for the benefit of water supplies. Where the land is flat, water may be spread in a thin sheet over a large spreading area surrounded by banks or ditches. In alluvial plains a series of shallow basins can be built alongside the river, which is connected to the upper basin by a canal. As the first basin fills, water spills into the second, and so on, excess water flowing from the lowest basin back into the river. As part of the water supply of Amsterdam and The Hague, water is diverted into basins constructed in coastal sand dunes, which not only help to filter the water but also prevent sea-water intrusion. The main disadvantage of artificial recharge is that the ground surface becomes clogged with silt, algae, or both, so that it is necessary to rest the beds for long periods, or to renew the upper layer. In Sweden the water is filtered to speed up recharging—a practice that is becoming increasingly common in many parts of the world. When an impermeable stratum restricts the flow of water to an aquifer, recharge wells are used to transfer water underground, an action in reverse of the normal supply well.
More than 1100 recharge wells, for example, have been drilled on Long Island, New York City, which also prevents sea-water intrusion. Despite the- frequent mismanagement of ground water in the past, it is fortunately not too late to rectify our mistakes. By applying the results of modern research, existing aquifers can be used more efficiently and new ones developed. More countries will probably adopt the approach of such places as California and Mid-Cheshire in the conjunctive use of ground and surface water —that is, with ground water kept as a reserve for dry periods. Ground water development on a much more intensive scale is likely in the future, even if this sometimes involves artificial recharge. Aquifers are natural reservoirs that can be simply tapped by sinking boreholes and pumping water to the surface; surface reservoirs, on the other hand, are costly to build, occupy valuable land, and often involve building long aqueducts. By using aquifers as concealed underground reservoirs, there are no dangers of dam failure, no silting-up of surface reservoirs and— of especial importance in hot countries—no wasteful losses by evaporation.