Geology Reference
In-Depth Information
above the main water table forms. Water moving laterally
along the perched water table may intersect the surface to
produce a spring.
Lowering of the regional water table, because more
groundwater is being withdrawn than is being replenished, is
becoming a serious problem in many areas, particularly in the
southwestern United States where rapid growth has placed tre-
mendous demands on the groundwater system. As mentioned
earlier, some of the largest cities in the United States depend
entirely on groundwater for their municipal needs. Further-
more, some of the largest agricultural states are withdrawing
groundwater from regional aquifers that are not being suffi -
ciently replenished. Unrestricted withdrawal of groundwater
cannot continue indefi nitely, and the rising costs and decreas-
ing supply of groundwater should soon limit the growth of
some regions in the United States, such as the Southwest.
Water Wells
Water wells are openings made by digging or drilling down
into the zone of saturation. Once the zone of saturation
has been penetrated, water percolates into the well, fi lling it
to the level of the water table. A few wells are free fl owing
(see the next section); however, for most, the water must be
brought to the surface by pumping.
In some parts of the world, water is raised to the surface
with nothing more than a bucket on a rope or a hand-operated
pump. In many parts of the United States and Canada, one can
see windmills from times past that used wind power to pump
water. Most of these are no longer in use, having been replaced
by more effi cient electric pumps.
When groundwater is pumped from a well, the water
table in the area around the well is lowered, forming a cone
of depression (
Artesian Systems
The word artesian comes from the French town and province
of Artois (called Artesium during Roman times) near Calais,
where the fi rst European artesian well was drilled in 1126
and is still fl owing today. The term artesian system can be
applied to any system in which groundwater is confi ned and
builds up high hydrostatic (fl uid) pressure (
Figure 13.5). A cone of depression forms
because the rate of water withdrawal from the well exceeds
the rate of water infl ow to the well, thus lowering the wa-
ter table around the well. A cone of depression's gradient,
that is, whether it is steep or gentle, depends to a great
extent on the permeability of the aquifer being pumped.
A highly permeable aquifer produces a gentle gradient in
the cone of depression, whereas a low-permeability aqui-
fer results in a steep cone of depression because water
cannot easily fl ow to the well to replace the water being
withdrawn.
The formation of a cone of depression does not nor-
mally pose a problem for the average domestic well, pro-
vided that the well is drilled deep enough into the zone of
saturation. However, the tremendous amounts of water used
by industry and for irrigation may create a large cone of de-
pression that lowers the water table suffi ciently to cause shal-
low wells in the immediate area to go dry (Figure 13.5). This
situation is not uncommon and frequently results in lawsuits
by the owners of the shallow dry wells.
Figure 13.6).
Water in such a system is able to rise above the level of the
aquifer if a well is drilled through the confi ning layer, thereby
reducing the pressure and forcing the water upward. An ar-
tesian system can develop when an aquifer is confi ned above
and below by aquicludes, the rock sequence is usually tilted
to build up hydrostatic pressure, and the aquifer is exposed
at the surface, thus enabling it to be recharged.
The elevation of the water table in the recharge area and
the distance of the well from the recharge area determine
the height to which artesian water rises in a well. The surface
defined by the water table in the recharge area, called the
artesian-pressure surface, is indicated by the sloping dashed
line in Figure 13.6. If there were no friction in the aquifer,
well water from an artesian aquifer would rise exactly to the
elevation of the artesian-pressure surface. Friction, however,
slightly reduces the pressure of the aquifer water and conse-
quently the level to which artesian water rises, which is why
the pressure surface slopes.
An artesian well will fl ow freely
at the ground surface only if the
wellhead is at an elevation below
the artesian-pressure surface. In
this situation, the water fl ows out of
the well because it rises toward the
artesian-pressure surface, which is
at a higher elevation than the well-
head. In a nonfl owing artesian well,
the wellhead is above the artesian-
pressure surface, and the water will
rise in the well only as high as the
artesian-pressure surface.
One of the best-known arte-
sian systems in the United States
underlies South Dakota and extends
southward to central Texas. The ma-
jority of the artesian water from this
Dry wells
Well
Water table
Former water table
Cone of depression
Figure 13.5 Cone of Depression A cone of depression forms whenever water is withdrawn
from a well. If water is withdrawn faster than it can be replenished, the cone of depression will
grow in depth and circumference, lowering the water table in the area and causing nearby
shallow wells to go dry.
 
Search WWH ::




Custom Search