There is more water in the ground than in all the world鈥檚 rivers and lakes
combined. To use this valuable resource to good effect, we need to understand
its place in the water cycle
WATER is one of the most commonplace compounds on Earth. Yet, thousands of
people die every day because they do not have enough, or because their supply
is contaminated. How can this happen on a planet that has an estimated 1400
million cubic kilometres of water? That is 250 000 million litres for every
man, woman and child (1 cubic kilometre is 1012 litres). The
problems arise not from a shortage of water, but from its unequal
distribution. So, where is all this water?
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By far the largest proportion is in the oceans, which hold roughly 1370
million cubic kilometres of salt water.
The largest store of freshwater is in glaciers and icecaps 鈥� particularly
in Antarctica 鈥� which hold about 30 million cubic kilometres of water as ice.
Rivers, lakes, soils and the atmosphere, the obvious sources of freshwater,
contain about 200 000 cubic kilometres of water 鈥� less than one-fiftieth of 1
per cent of the world鈥檚 total water supply.
There is another important store of water, much of it fresh, that is easy
to overlook. The rocks of the upper part of the Earth鈥檚 crust contain many
holes. Some are caverns, but most of them are tiny pores 鈥� such as the spaces
between grains of sand in a sandstone, or networks of equally fine cracks.
Most pores are filled with water. After the oceans, porous rocks contain the
Earth鈥檚 largest store of water; one calculation puts the total at more than 50
million cubic kilometres, of which at least 4 million cubic kilometres is
freshwater.
Some rocks are more porous than others. More important, the pores in some
rocks are either large or join up so that water can flow through them easily.
Such rocks are said to be permeable; sandstones and gravels are good
examples of permeable rocks. In other rocks, water can hardly flow at all:
clay has very small pores, whereas pumice is full of good-sized holes but they
rarely link up. These and similar rocks are impermeable. Layers of rock that
are porous and permeable enough to store water and let it flow through them
easily are called aquifers.
In temperate countries such as Britain, the absence of water in the
landscape is often a good clue to the presence of one of these great
underground stores.
Where there are rocks of low permeability at the surface, such as clays or
granites, or altered rocks such as slates, only a little rain soaks into the
ground. Most of the rainwater flows straight to streams or rivers. These
impermeable rocks form landscapes with rivers that rise quickly, even flooding
their banks after heavy rain, but diminish or even dry up after a spell of dry
weather.
In areas of permeable rocks, most rain soaks into the ground. It reaches
rivers only after passing slowly through an aquifer. Permeable areas usually
have only a few streams, but they flow with little variation throughout the
year and rarely flood.
Far below the Earth鈥檚 surface, the rocks are so compressed that the pores
are closed. Water cannot easily flow in these impermeable rocks. They
effectively mark the bottom of the storage space for water in the Earth鈥檚
crust. One of the difficulties in estimating how much water is stored in the
ground is knowing how deep this 鈥渇loor鈥� is. Geologists have found water deeper
than 10 kilometres, but the greatest depth at which water actively circulates
is usually about a kilometre.
Above this floor, rain soaks down to recharge an aquifer. The aquifer fills
up with water until water reaches the surface of the land in one or more
places. This generally happens where the ground is lowest, usually in river
valleys. There, water flows from the aquifer as springs or seepages. The
aquifer therefore becomes saturated to a level where the water flowing out
balances the recharge that comes in from rainwater.
The place where the water leaves the ground may be a long way from where
recharge occurs. An aquifer may therefore carry water from a humid area to a
dry one, even a desert. In this case the water flowing from the ground forms
an oasis. At Kufra in the Libyan Desert, for example, water emerges from an
aquifer called the Nubian Sandstone to form small lakes. The presence of such
oases made human travel and nomadic existence possible in arid lands.
The top of the saturated rock is called the water table, and the water that
saturates the rocks beneath the water table is groundwater. The part of the
aquifer that lies above the water table is termed the unsaturated zone
The water table is not horizontal. It slopes towards places where water
leaves the aquifer 鈥� the river valleys 鈥� and away from where water is coming
in 鈥� the hill sides and high ground. It is thus higher beneath hills than
beneath valleys, following the landscape in a subdued way.
In most countries, groundwater flows from aquifers throughout the year but
rainfall replenishes them for only part of the time. So the water table is not
at a constant level, but rises and falls during the year as the amount of
water stored in the aquifer increases and decreases.
Down hills and up dales
Artesian wells
AN AQUIFER often lies on top of a layer of less permeable rock. This stops
or slows the downward flow of groundwater, and helps to separate the water in
this aquifer from flow in other aquifers that may be underneath it. And unlike
water in rivers or streams, groundwater is under hydrostatic pressure that is
greater than atmospheric pressure. Thus it can flow upwards, as well as
downwards or sideways, just like water in the plumbing system of a house.
This is especially important when an aquifer dips beneath another layer
that is much less permeable. As usual, rainwater fills up the aquifer and
water flows out from it where it meets the surface. Below the impermeable
layer, water is trapped in the aquifer, which is said to be confined. The
water presses on the less-permeable confining layers above and below it.
If you drill a borehole into a confined aquifer, water will rise up the
borehole until the column of water is enough to balance the pressure in the
aquifer. Most major aquifers have a confined portion, where the aquifer is
covered by impermeable rock layers, and an unconfined portion, where the
aquifer is covered simply by soil, or river or glacial deposits. Rainwater
enters the aquifer through the unconfined part.
If we drilled many boreholes into the aquifer and found the level of water
in all of them, we could imagine a surface made by joining all the individual
levels. In the unconfined part of the aquifer this surface would be the water
table. It separates the saturated part of the aquifer from the unsaturated
zone above it. But in the confined part of the aquifer, the spaces between
mineral grains, every pore in fact, is filled with water: there is no
unsaturated zone. Here, the surface is an imaginary one called the pressure
surface, or potentiometric surface. It passes through the confining layer
somewhere above the aquifer.
If the unconfined part of the aquifer is beneath high ground, and the
confined part beneath low ground, then the potentiometric surface may be above
ground level. If we drill a borehole into the aquifer the groundwater will be
under sufficient pressure to overflow from the borehole. Such a borehole is
called an artesian well. But the borehole must go deep enough to reach the
aquifer, even if this means drilling far below the potentiometric surface.
Otherwise it will yield little or no water from the impermeable confining
layer.
In many dry or semi-arid parts of the world, the availability of water from
artesian wells makes agriculture possible. In Australia, many ranches use
artesian wells, which can be relied upon to provide water unattended to stock
in remote areas. At the beginning of this century people in North and South
Dakota even used artesian wells to power machinery 鈥� until they realised the
dangers of taking too much water and depleting the aquifers.
When groundwater flows naturally or is pumped from an unconfined aquifer,
it drains from the pores. When water flows out from an artesian well in a
confined aquifer, none of the pore space drains completely. The water that
flows out comes from elastic storage. Groundwater in a confined aquifer is
compressed elastically, rather like air in a tyre. If you open the valve on
the tyre, air flows out until the pressure inside the tyre equals that
outside. Then the flow ceases although the tyre still contains air.
Although water is far less compressible than air, the aquifer holds a lot
of water compressed in this way because it is much larger than a tyre. Also,
the aquifer itself is slightly elastic; the pressure of the groundwater it
contains forces apart the mineral grains and increases the volume of the pores
slightly. Even if no more water were to flow into the aquifer, water would
flow out, like the air from the tyre, until the pressure was no longer enough
to lift the water above ground level. At that point we could still take water
from the aquifer, but we would have to pump it out in the same way as we would
from an unconfined aquifer. If we pumped long enough we could eventually bring
the potentiometric surface below the top of the aquifer. At this stage, part
of the aquifer would no longer be saturated. But before we reached that point,
we might have other problems.
When the air escapes from a tyre the tyre goes flat. Although it still
contains air, it can no longer support the weight of the car. In the same way,
the pressure of the groundwater in a confined aquifer helps to support the
overlying rock layers. When this pressure drops, more of the weight is
transferred to the rock of the aquifer itself. If the rock is weak, or
contains weak layers, it may be compressed and the ground above could subside.
As a result, in parts of California and Mexico City the ground has subsided by
several metres causing spectacular and costly damage to buildings.
Long-term residence
Natural storehouses
EXCEPT where rocks are heavily cracked or fissured, it usually takes many
years for water to find its way down through the soil and the unsaturated zone
to the saturated zone of an aquifer. Once there, it can take tens or thousands
of years for that water to emerge from a spring or a well. Australian
hydrogeologists have used radioactive isotopes as natural tracers to determine
how long water resides in the Great Artesian Basin of Australia. They can now
say that some of it has been in the ground for more than a million years.
The time that water spends in aquifers is an indication of their importance
as natural stores of water. The water cycle is often depicted as a simple
system of circulation that takes water from the sea as vapour, deposits it on
land as rain or snow and returns it quickly to the sea by way of rivers. Here,
the ocean is the only large store of water and the only place where a water
molecule stays for any length of time. The rest of the cycle is portrayed as a
continuous path with no storage. In fact, the cycle often transfers water
between two stores, the ocean store (the primary and largest one) and the
secondary store (the icecaps, glaciers and groundwater). The residence time
in, for example, an aquifer can be millions of years, and it may be even
longer in an icecap at high latitudes.
Virtually no water infiltrates deserts such as the Sahara at present, but
there are some large aquifers beneath them. Some of the water in these
aquifers comes from recharge in wetter areas nearby, but much of it is 鈥渇ossil
water鈥�, rain that fell in the last Ice Age. In Libya, a huge pipeline is under
construction to carry water from desert wells to coastal towns and farms.
Schemes such as these are the equivalent of 鈥渕ining water鈥�, that is to say
using a resource that is not replenished.
Some of the water in very deep aquifers may not have entered them as rain.
It may have been trapped in the pores of rocks that were formed beneath the
sea; it is saline, and not good for drinking. Another theory is that some is
juvenile water, released from molten rock deep within the Earth鈥檚 crust, often
during volcanic eruptions. In fact, much so-called juvenile water is probably
either rainwater that has circulated to great depths, or water that was
trapped in rocks that have been carried down at subduction zones (see Inside
Science, No. 6, 23 February 1988).
If we include all the deep groundwater that we can rarely use, as well as
the water trapped in pores in sediments such as clays, the total volume of
groundwater is probably more than 50 million cubic kilometres. Of this, 4
million cubic kilometres is a reasonable estimate for the freshwater we could
extract. It excludes water that will not drain from small pore spaces, saline
water, and water in deep confined aquifers.
Safe to drink
How much treatment?
WHATEVER its origin, groundwater spends a long time in an aquifer. It has
time to dissolve minute quantities of minerals which can give it definite
characteristics such as hardness or taste. Because of this, many people find
groundwater more pleasant to drink than water from rivers and lakes. It is
often bottled and sold as mineral water.
Another attraction of groundwater is its safety. The long residence time
generally means that any harmful bacteria that enter an aquifer do not live
long enough to pose a threat to anyone using groundwater. So groundwater
rarely needs treatment.
This is particularly useful for small communities and in developing
countries, where special water treatment is not available or prohibitively
expensive. In countries such as Bangladesh, for instance, there have been
major campaigns to give each village its own borehole and hand pump. The
consequent supply of water free from pollution has greatly reduced the
incidence of waterborne diseases such as diarrhoea and cholera, which formerly
claimed many lives.
The long residence time even means that short-lived radioactive substances,
such as those released by the Chernobyl accident, will be largely harmless by
the time they emerge from an aquifer.
The long, slow filtration process does not remove anything dissolved in the
water. But this time lag, combined with dilution by water already in the
aquifer, means that if a pollutant enters the ground, it may not affect water
supplies for many years. And this may be the greatest threat to our reserves
of groundwater. Groundwater may become contaminated without our knowledge. By
the time we realise the danger, large amounts of pollutants may already be in
an aquifer.
One pollutant that has raised concern is nitrate (see Inside Science number
37). The nitrate problem has taught us that to protect our drinking water, we
must monitor the unsaturated zone. This gives us an early warning of the
pollutants that may arrive in our groundwater in the future. We must also
study the processes by which pollutants decay and change. Researchers are now
looking at what happens to pesticides, and to spillages of fuel and industrial
solvents, which have become the latest major threat to groundwater quality.
Aquifers in action: wet towels and dry summers
IF YOU hold a dishcloth, a towel or a capillary tube with its lower end
just touching a container of water it will draw up water until an equilibrium
is reached. At this point, the forces of surface tension between the water and
the fabric, or between the water and the tube, balance the weight of water.
Water will not drain from the fabric or the tube when you lift it clear of the
surface. If you could measure the pressure of the water in the capillary tube
or in the pore spaces of the cloth, you would find that it was below
atmospheric pressure. The water was sucked into the pores. If you now add
water to the top of your suspended capillary tube or towel, water will drain
from the bottom until equilibrium is regained.
Water in the pores of rock and soil in the unsaturated zone of the ground
is held in place by the same forces of surface tension. This water is also at
pressures less than atmospheric.
Left to itself, water in the unsaturated zone would reach an equilibrium in
the same way as the water in the capillary tube or the towel. But over a
period of time, two things disturb this equilibrium: evaporation and
infiltration. Evaporation, directly from the ground, or from the leaves of
plants, which draw water from the ground, will take water from the top of the
unsaturated zone. This will increase the suction effect slightly and can make
water move upwards, much as a lamp wick draws up oil to replace that being
burned. More important, in most areas, is infiltration. Rain entering the soil
disturbs the equilibrium in the same way as water added to the top of the
capillary tube or towel; water drains through the unsaturated zone to the
water table.
In regions with marked wet and dry seasons, or where rainfall is evenly
distributed, but the winters are cool and there is a distinct growing season,
the behaviour of the unsaturated zone becomes very important. In the dry or
growing season, plants take water from the unsaturated zone. This dries out
downwards, just as a wet towel dries out if it is placed with its top in a
current of warm air. Hydrologists measure the drying out of the soil in terms
of the amount of rainfall that would be needed to make water just begin to
move downwards again. This amount is called the soil moisture deficit. This
deficit must be satisfied before any water can infiltrate downwards through
the soil to replenish the aquifer. Recharge normally happens in the winter or
rainy season. During summer, the soil moisture deficit usually prevents
rainfall from reaching the aquifer, and the water table falls as water flows
from the aquifer and is not replaced.
Sometimes the rainfall in winter is barely enough to cancel out the soil
moisture deficit. Then water levels in the aquifer do not recover, but may
continue to fall as summer approaches. The storage in aquifers is usually much
greater than that in surface reservoirs, so for one year this is not too
serious. But a spell of hot dry summers with high soil moisture deficits, and
of winters with low rainfall, would be serious for most aquifers.
Some of the world鈥檚 most important aquifers have recently been affected by
this problem. Much of south-east England depends on the Chalk aquifer for
water. Here, low rainfall over the four years 1988-1992 caused record low
water levels in wells. The Ogallala aquifer, which supplies many farms in the
US Midwest, has suffered a similar problem, as have aquifers in southern
Africa and in Spain, Greece, France and elsewhere in Europe.
It is probably too early to blame climatic change for these effects. Since
the dry periods, many of the affected areas have experienced exceptionally
heavy rainfall, and in some cases severe flooding. it is possible that we have
just been seeing some extreme 鈥渂lips鈥� in the natural variation in rainfall,
without the long-term average being significantly changed. But it is possible
that global warming is leading to more severe extremes of climate. In the
latter case, we may have some difficult decisions ahead; in the Chalk for
example, there are large reserves of water left even in a drought, but they
cannot be pumped out without lowering the water table so that many streams dry
up. Which do we want 鈥� more water on tap or water in the environment?
Water supplies: groundwater on tap
GROUNDWATER is useful in many ways. It sustains the flow of rivers, from
which we take water for drinking and many industrial uses. And by sinking a
well or borehole into an aquifer, groundwater can be pumped to the surface.
The borehole can often be sited precisely where the water is needed 鈥� in a
village or factory compound, for example 鈥� so avoiding the need for expensive
water mains.
Rather than operate a borehole pump whenever water is needed, the usual
arrangement is to pump water from the borehole at regular intervals to a tank
or distribution reservoir on high ground or on top of a water tower. From here
the water can flow by gravity to wherever it is needed. A distribution
reservoir holds enough water to supply the area it serves for about a day.
Pumping water from an aquifer lowers the water table. This may reduce or
stop the natural 鈥渙verflow鈥� from the aquifer, causing springs or small streams
to dry up. In Libya, pumping from wells around the Kufra Oasis has reduced the
natural outflow. In South-east England, some streams on chalk flow less than
they used to do; in part this is the result of abstraction. People must decide
which they value most 鈥� a cheap supply of water, or preserving the countryside
exactly as it was.
But some changes are blamed unfairly on abstraction. In the l850s, when
there was still little pumping from the Chalk aquifer, water levels in the
Chalk in southern England were nearly as low as they were in 1992. Low
rainfall was the main cause then, and has been the cause of low water levels
recently in many of Europe鈥檚 aquifers.
Although pumping out water can bring problems if aquifers do not have a chance
to recharge, groundwater is such an enormous resource that it would be
unthinkable for us to stop using it. But we should also be aware of the cost
of alternatives. We simply could not build, nor afford to build enough
reservoirs to replace the aquifers we depend on today. Reservoirs provide a
relatively small supply of water and they are expensive to build, but
relatively easy to understand. Aquifers hold an enormous amount of water, and
we do not have to raise the money to build them. In contrast, we do have to
invest time and money in understanding groundwater and its place in the water
cycle. And in the light of increasing concern about the possibility of global
warming, the more we understand about our water, the better.
Further reading
For a more complete but still simple introduction, see Introducing
Groundwater, by Michael Price (Chapman and Hall, 拢13.95). A more general
treatment of the hydrological cycle is provided in the third edition of
Principles of Hydrology, by R. C. Ward and M. C. Robinson (McGraw-Hill, 1990,
拢15.95). Groundwater Hydrology, by D. K. Todd (second edition Wiley,
1980, ppb 拢21.50), offers a more advanced treatment.