Groundwater depletion
Groundwater is a valuable resource throughout
the world. Where surface water, such as lakes and rivers, are scarce or
inaccessible, groundwater supplies many of the hydrologic needs of people
everywhere.
In the United States, it is the source
of drinking water for about half the total population and nearly all of the
rural population. it provides over 50 billion gallons per day for agricultural
needs.
Groundwater depletion, a term often
defined as long-term water-level declines caused by sustained groundwater
pumping, is a key issue associated with groundwater use.
Excessive pumping can
overdraw the groundwater "bank account"
The water stored in the ground can be
compared to money kept in a bank account. If you withdraw money at a faster
rate than you deposit new money you will eventually start having account-supply
problems.
Pumping water out of the ground faster
than it is replenished over the long-term causes similar problems. The volume
of groundwater in storage is decreasing in many areas as a result of over
pumping.
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primarily caused by
sustained groundwater pumping.
What are some effects
of groundwater depletion?
Pumping
groundwater at a faster rate than it can be recharged can have some negative
effects of the environment and the people who make use of the water:
Some of the negative effects of
groundwater depletion are:
·
drying up of wells
·
reduction of water in streams and lakes
·
deterioration of water quality
·
increased pumping costs
·
land subsidence
Lowering of the water table
The
most severe consequence of excessive groundwater pumping is that the water table, below which the ground is
saturated with water, can be lowered. For water to be withdrawn from the
ground, water must be pumped from a well that reaches below the water table.
If
groundwater levels decline too far, then the well owner might have to deepen
the well, drill a new well, or, at least, attempt to lower the pump. Also, as
water levels decline, the rate of water the well can yield may decline.
Increased costs for the user
As
the depth to water increases, the water must be lifted higher to reach the land
surface. If pumps are used to lift the water (as opposed to artesian wells), more energy is required to
drive the pump. Using the well can become prohibitively expensive.
Reduction of water in streams and lakes
There
is more of an interaction between the water in lakes and rivers and groundwater
than most people think. Some, and often a great deal, of the water flowing in
rivers comes from seepage of groundwater into the streambed.
Groundwater
contributes to streams in most physiographic and climatic settings. The
proportion of stream water that comes from groundwater inflow varies according
to a region's geography, geology, and climate.
Groundwater
pumping can alter how water moves between an aquifer and a stream, lake, or
wetland by either intercepting groundwater flow that discharges into the
surface-water body under natural conditions, or by increasing the rate of water
movement from the surface-water body into an aquifer.
A
related effect of groundwater pumping is the lowering of groundwater levels
below the depth that streamside or wetland vegetation needs to survive. The
overall effect is a loss of riparian vegetation and wildlife habitat.
Land subsidence
The
basic cause of land subsidence is
a loss of support below ground.
In
other words, sometimes when water is taken out of the soil, the soil collapses,
compacts, and drops. This depends on a number of factors, such as the type of
soil and rock below the surface. Land subsidence is most often caused by human
activities, mainly from the removal of subsurface water.
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Deterioration of water quality - saltwater intrusion
One
water-quality threat to fresh groundwater supplies is contamination from
saltwater intrusion.
All
of the water in the ground is not fresh water; much of the very deep
groundwater and water below oceans is saline.
In fact, an estimated 3.1 million
cubic miles (12.9 cubic kilometers) of saline groundwater exists compared to
about 2.6 million cubic miles (10.5 million cubic kilometers) of fresh
groundwater .
Under
natural conditions the boundary between the freshwater and saltwater tends to
be relatively stable, but pumping can cause saltwater to migrate inland and
upward, resulting in saltwater contamination of the water supply.
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