Groundwater depletion, deterioration of water quality
and saltwater intrusion
.
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.
Groundwater depletion is 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For water to be withdrawn from the ground, water must be pumped from a well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.
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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source: water.usgs.gov
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