Groundwater and geology
Groundwater is also
important quite apart from its value as a resource or its close
connection with surface water supplies. Engineers must consider
groundwater when planning almost any kind of structure, either above or
below the ground. Ignoring the effect of groundwater on slope stability
can be both costly and dangerous. Geologists see groundwater as a major
force in geological change. The fluid pressures exerted by groundwater,
for example, play an important role in the occurrence of earthquakes.
Geologists also know that the movement of water through underground
geologic formations controls the migration and the accumulation of
petroleum and the formation of some ore deposits.
Groundwater and engineering
Groundwater can also
have dramatic implications for engineering and geotechnical studies. The
study of groundwater is essential for engineers who construct dams,
tunnels, water conveyance channels, mines, and other structures.
Groundwater must be considered whenever the stability of slopes is
important, whether the slope is natural or constructed. Groundwater must
also be taken into account when devising measures to control flooding.
In all of these situations, groundwater flow and fluid pressure can
create serious geotechnical problems.
Groundwater, for
example, may create structural weaknesses in dams, or it may flow
underground right around the structure as it did at the Jerome Dam in
Idaho. Water flowed so efficiently through the rock formations
surrounding the reservoir that the dam would hold no water, even though
it was structurally sound.
In another case, when
geological exploration was being carried out in preparation for the
construction of the Revelstoke Dam in British Columbia, geologists and
engineers were concerned about an old landslide on the bank of the
proposed reservoir. They suspected that the water held in the reservoir
could increase groundwater pressures enough to make the slide unstable.
The solution was to increase drainage around the slide to ensure that
groundwater pressures did not increase. In 1963, these same conditions
at the Vaiont reservoir in Italy caused a slide which killed
2500 people.
Other problems result
from the excessive use of groundwater. Over drafting occurs
when people draw water out of an aquifer faster than nature can
replenish it. The most obvious problem created is a shortage of water.
Over drafting, however, can also create significant geotechnical
problems. Although not an issue in Canada, at many locations around the
world over drafting has caused land subsidence. This can
produce severe engineering difficulties. Parts of Mexico City, for
instance, have subsided as much as 10 meters in the past 70 years,
resulting in a host of problems in its water supply and sewerage system.
Land subsidence may also occur when the water table is lowered by
drainage. In the early 1970s, for example, an entire residential
subdivision in Ottawa subsided when a collector sewer was constructed
nearby. The subsidence seriously damaged the residents' property.
Safeguarding our groundwater supply
Groundwater is an
essential resource. It exists everywhere under the Canadian landscape
and is vitally connected to our rich surface water resources.
Contamination of groundwater is a serious problem in Canada. Industrial
and agricultural activities are major sources of contaminants, but
Canadian households are equally important sources.
Groundwater moves so
slowly that problems take a long time to appear. Because of this, and
because it is so expensive to clean up a contaminated aquifer (if it can
be done at all), it is preferable by far to prevent contamination from
happening in the first place. For example, leaking underground storage
tanks can be replaced by tanks that will not corrode; landfills can be
sited in locations where leachates will not contaminate underlying
groundwater; and the impacts of spills of hazardous materials reduced by
restricting access to recharge areas.
All levels of
government in Canada are starting to take some of the actions necessary
to protect our groundwater supplies, but there is a long way to go
before these measures are fully effective. At the same time,
universities and government research institutes are investigating what
happens to water underground and what can be done to preserve it and
even improve its availability to us. Both as a society and as
individuals, we must keep in mind groundwater's susceptibility to
contamination.
Groundwater is just as
important as the sparkling lakes and rivers of our postcard image of
Canada. This national treasure may be "hidden," but it must not be
forgotten.
 Groundwater
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