Physics for Geologists, Second edition

38 Gravity
Figure 3.6 Profile of the water table to a producing bore. The so-called cone of
depression reflects the loss of energy as the water flows radially in
towards the bore. The pump is not shown.
The reduction of energy around the borehole during production,
reflected by the cone of depression, as it is called, also leads to reduc-
tion of pore-water pressure, which leads to slight compaction and the
reduction of pore space around the borehole.
In theory the production never stabilizes, but that is a quibble.
Artesian basins
The general features of artesian basins are well-known, but from time to
time articles appear arguing that artesian basins are closed systems and that
production of water from them is exploiting a non-renewable resource. One
important aspect of such articles is the emphasis on the decline of production
after an artesian bore is drilled and during the development of an artesian
basin. This we have already discussed. One aspect often neglected is that the
total head - the level to which the water would rise in a borehole if it could -
of the bores drilled can be mapped, thus mapping the distribution of energy
or energy field over the basin, as in Figure 3.7. The higher this surface, the
greater the energy. If there is leakage or production from the basin these sur-
faces, called potentiometric surfaces, indicate the direction of flow - always
normal to the lines of equal energy (equipotential) on the surface and from
higher to lower energy. The flow may very well be slow but the volume-rate-
of-flow may be very large. If there is no flow, the potentiometric surface is
horizontal. We shall examine fluid flow in more detail in Chapter 12.
One other point: human activity can alter the behaviour of artesian basins.
For example, London's water used to come almost entirely from its under-
lying artesian basin, and there were springs emanating from the Cretaceous
chalk in and near the Thames river, downstream, at low tide. Consumption
threatened to reverse these springs, which would have contaminated the
Copyright 2002 by Richard E. Chapman