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Groundwater System
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Introduction:
- Water is continuously collected, purified, and distributed
in the hydrologic cycle.
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- As water precipitates in the form of rain or snow, some of
the water infiltrates the ground and fills the spaces
and pores in soil and rock - this is called groundwater.
The water table is the upper surface of the zone of saturation
where all of the pore spaces are filled. The ability of the soil
or rock layers to absorb and hold water depends on how porous
and permeable it is. Porous, well-saturated layers of sand, gravel,
or bed rock are called aquifers. A confined aquifer
is sandwiched between layers of rock that have a very low permeability
to water. An unconfined aquifer collects on top of a impermeable
layer of rock. An artesian well may flow without pumping
because the confined aquifer is under pressure from water many
miles away at a higher elevation. Most other wells need a pump
of some type to bring the water to the surface.
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Water Table Draw Down or Groundwater "Mining":
- Most aquifers are replenished naturally by infiltration of
water from precipitation of rain and snow in the recharge area
which may be many miles from the point of withdrawal from wells.
If water is pumped from many wells at a withdrawal rate in an
aquifer that exceeds the natural recharge rate, the water table
drops. As shown in the diagram, a cone of depression may form
around a well. Depending upon the depth, other wells in the area
may go dry. If this situation prevails for any significant amount
of time, this is called water "mining". This may happen
from rapid withdrawals for irrigation purposes from so called
"fossil aquifers" which get very little if any recharge.
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- Such "fossil aquifers" underlie the Sahara and
Kalahari deserts in Africa, the Great Artesian Basin in Australia,
the central Asia basins, and the Ogallala Aquifer in the western
part the Midwest of the United States.
Circular Irrigation Plots in the Plains States
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Salt Water Intrusion:
When fresh water is withdrawn at a faster rate than it can
be replenished, a draw down of the water table occurs with a
a resulting decrease in the overall hydrostatic pressure. When
this happens near an ocean coastal area, salt water from the
ocean intrudes into the fresh water aquifer as shown in the diagram.
The result is that fresh water supplies become contaminated with
salt water as is happening to communities along the Atlantic
and Gulf coasts.
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