Geoscience Reference
In-Depth Information
Artesian well
Unconfined or perched aquifer
Aquitard or aquiclude
Confined aquifer
Figure 4.5
A confined aquifer. The height of water in the well will depend on the amount of
pressure within the confined aquifer.
Well
Water table
Figure 4.6
An unconfined aquifer. The water level in the well is at the water table.
within a confined aquifer is normally under pressure
and if intersected by a borehole will rise up higher
than the constricted boundary. If the water reaches
the earth's surface it is referred to as an
artesian
well
. The level that water rises up to from a confined
aquifer is dependent on the amount of fall (or
hydraulic head) occurring within the aquifer. This
is analogous to a hose pipe acting as a syphon. If
the syphon has a long vertical fall between entry
of the water and exit then water will exit the hose
pipe at a high velocity (i.e. under great pressure). If
there is only a short vertical fall there is far less
hydraulic head and the water exits at a much slower
velocity. To continue the analogy further: if you
could imagine that the end of the hose pipe was
blocked off but that you punctured the hose, then
you would expect a jet of water to shoot upwards.
This jet is analogous to an artesian well.
An
unconfined aquifer
has no boundary above it and
therefore the water table is free to rise and fall
dependent on the amount of water contained in the
aquifer (see Figure 4.6). The lower boundary of the
aquifer may be impervious but it is the upper
boundary, or water table, that is unconfined and may
intersect the surface. It is possible to have a
perched
water table
or
perched aquifer
(see Figure 4.5) where
an impermeable layer prevents the infiltration of
water down to the regional water table. Perched
