92 / Basics of Environmental Science
forming a ‘cone of depression’. If the rate of abstraction exceeds that at which the aquifer is recharged,
the water table will fall over a wide area, eventually to a level at which the yield from the borehole
decreases and the aquifer is exhausted. In the United States, there are parts of the Great Plains,
California, and southern Arizona where the severe depletion of aquifers for irrigation now threatens
future water supplies and also reduces water quality. Quality is affected because, in coastal regions,
as the water table falls salt water enters to recharge it, and anywhere that toxic mineral salts dissolve
in ground water, reducing the volume of water may increase their concentration, so the water requires
more extensive, and therefore costly, processing to render it drinkable (RAVEN ET AL., 1993, pp.
279–281). Pollution of this kind is natural, although caused by human over-exploitation of a resource,
but ground water can be polluted by industrial or domestic wastes.
Lowering the water table can also cause ground subsidence due to the reduction in volume of the
material comprising the saturated layer as this dries. Between 1865 and 1931, groundwater abstrac-
tion in London caused the ground to subside at 0.91–1.21 mm yr-1, producing a total subsidence of
0.06–0.08 m. In Tokyo, the ground subsided 4 m between 1892 and 1972, at a rate of 500 mm yr-1,
and Mexico City is sinking at 250–300 mm yr-1 for the same reason (GOUDIE, 1986, p. 207).
Not all aquifers require pumping. An unconfined aquifer is one into which water drains freely from
above, but where two approximately parallel impermeable layers are separated by a layer of porous
material, the resulting aquifer is said to be confined. Natural undulations in a confined aquifer produce
low-lying areas in which water is under pressure from the water at a higher level to either side (see
Figure 3.1B). This water will flow without pumping from a borehole drilled into the aquifer through
the upper impermeable layer and it will continue to flow provided the aquifer is constantly recharged
by water draining into the hollow. The result is an ‘overflowing’ or ‘artesian’ well.
Where the water table reaches the surface, water will flow spontaneously, as a spring, and on sloping
ground it will form a stream and eventually, through the merging of many small streams, a mighty
river. Rivers also supply water, but since long before our ancestors invented wheeled vehicles and
built roads for them they have also been used to convey people and goods. It is no coincidence that
most of the world’s major inland cities are located beside large rivers. Almost any river might serve
as an example, but the Rhine is an especially good one, because it flows across a densely populated
continent for a distance of 1320 km. Figure 3.2 shows the river together with some of its more
important tributaries and the principal cities that border it.
Over the centuries the cities along the Rhine prospered and grew, and as Europe industrialized several
of them became important manufacturing centres. Most industries use water and produce liquid
wastes, and humans produce sewage, a mixture of urine, faeces, and water that has been used for
washing and cooking. At one time all this was poured into the river, which removed it, and wastes
discharged into the Rhine were joined by those discharged into its tributaries, including the Emscher,
which drains the Ruhr and enters the Rhine north of Düsseldorf.
Rivers have a remarkable capacity for cleaning themselves, because their waters are continually
replenished and contaminants removed by extreme dilution, precipitation and burial beneath
other sediment, or, most of all, by bacterial activity that breaks down large, organic molecules
into simpler, biologically harmless compounds. In the case of rivers such as the Rhine, however,
transporting foul water merely delivers it to the next city downstream, where it must be treated
before it can be used, and the further downstream people live, the more their drinking water will
cost them. In modern times the problem has been addressed, but it was not simple. As Figure 3.3
shows, water drains into the Rhine from an area of 220150 km^2 in six countries. Why should the
Swiss pay more to treat effluent prior to discharge for the benefit of the distant Netherlands?
Why should the French regulate discharges from their chemical industries in Alsace when the