Groundwater, Aquifers
and Water
Movement
.
Aquifers and Groundwater
 |
| hole at the beach |
I hope you appreciate my spending an hour in
the blazing sun to dig this hole at the beach.
It is a great way to illustrate the concept of how, below
a certain depth, the ground, if it is permeable enough to hold water, is
saturated with water.
The upper surface of this zone of saturation is called
the water table. The saturated zone beneath the water table is called an
aquifer, and aquifers are huge storehouses of water.
What you are looking at in this picture is a
"well" that exposes the water table, with an aquifer beneath it.
Of course, I am cheating here, as at the beach, the level
of the water table is always at the same level as the ocean, which is just
below the surface of the beach.
Groundwater is one of our most valuable resource - even
though you probably never see it or even realize it is there.
As you may have read, most of the void spaces in the
rocks below the water table are filled with water.
But rocks have different porosity and permeability
characteristics, which means that water does not move around the same way in
all rocks below ground.
When a water-bearing rock readily transmits water to
wells and springs, it is called an aquifer.
Wells can be drilled into the aquifers and water can
be pumped out. Precipitation eventually
adds water (recharge) into the porous rock of the
aquifer.
The rate of recharge is not the same for all aquifers,
though, and that must be considered when pumping water from a well.
Pumping too much water too fast draws down the water in
the aquifer and eventually causes a well to yield less and less water and even
run dry.
In fact, pumping your well too much can even cause your
neighbor's well to run dry if you both are pumping from the same aquifer.
In the diagram below, you can see how the ground below
the water table (the blue area) is saturated with water.
The "unsaturated zone" above the water table
(the greenish area) still contains water (after all, plants' roots live in this
area), but it is not totally saturated with water.
You can see this in the two drawings at the bottom of the
diagram, which show a close-up of how water is stored in between underground
rock particles.
Sometimes the porous rock layers become tilted in the
earth. There might be a confining layer of less porous rock both above and
below the porous layer.
This is an example of a confined aquifer. In this case,
the rocks surrounding the aquifer confines the pressure in the porous rock and
its water.
If a well is drilled into this "pressurized"
aquifer, the internal pressure might (depending on the ability of the rock to
transport water) be enough to push the water up the well and up to the surface
without the aid of a pump, sometimes completely out of the well.
This type of well is called artesian. The pressure of
water from an artesian well can be quite dramatic.
A relationship does not necessarily exist between the
water-bearing capacity of rocks and the depth at which they are found. A very
dense granite that will yield little or no water to a well may be exposed at
the land surface.
Conversely, a porous sandstone, such as the Dakota
Sandstone mentioned previously, may lie hundreds or thousands of feet below the
land surface and may yield hundreds of gallons per minute of water.
Rocks that yield freshwater have been found at depths of
more than 6,000 feet, and salty water has come from oil wells at depths of more
than 30,000 feet.
On the average, however, the porosity and permeability of
rocks decrease as their depth below land surface increases; the pores and
cracks in rocks at great depths are closed or greatly reduced in size because
of the weight of overlying rocks.
Pumping can affect the level of the water
table
Groundwater occurs in the saturated soil and
rock below the water table.
If the aquifer is shallow enough and permeable enough to
allow water to move through it at a rapid-enough rate, then people can drill
wells into it and withdraw water.
The level of the water table can naturally change over
time due to changes in weather cycles and precipitation patterns, streamflow
and geologic changes, and even human-induced changes, such as the increase in
impervious surfaces on the landscape.
The pumping of wells can have a great deal of influence
on water levels below ground, especially in the vicinity of the well, as this
diagram shows.
If water is withdrawn from the ground at a faster rate
that it is replenished, either by infiltration from the surface or from
streams, then the water table can become lower, resulting in a "cone ofdepression" around the well.
Depending on geologic and hydrologic conditions of the
aquifer, the impact on the level of the water table can be short-lived or last
for decades, and it can fall a small amount or many hundreds of feet.
Excessive pumping can lower the water table so much that
the wells no longer supply water—they can "go dry."
Water movement in aquifers
Water movement in aquifers is highly
dependent of the permeability of the aquifer material.
Permeable material contains interconnected cracks or
spaces that are both numerous enough and large enough to allow water to move
freely.
In some permeable materials groundwater may move several
meters in a day; in other places, it moves only a few centimeters in a century.
Groundwater moves very slowly through relatively impermeable materials such as
clay and shale.
After entering an aquifer, water moves slowly toward
lower lying places and eventually is discharged from the aquifer from springs,
seeps into streams, or is withdrawn from the ground by wells.
Groundwater in aquifers between layers of poorly
permeable rock, such as clay or shale, may be confined under pressure.
If such a confined aquifer is tapped by a well, water
will rise above the top of the aquifer and may even flow from the well onto the
land surface.
Water confined in this way is said to be under artesian
pressure, and the aquifer is called an artesian
aquifer.
RELATED
POSTS:
.
The Science of
Sinkholes
CLICK HERE . . .
.
.
Water dowsing
What is water dowsing?
CLICK HERE . . .
.
Groundwater
is the name used to describe water
which is stored underground in permeable rocks
CLICK HERE . . .
.
 |
 |
GS Series Submersible Pumpsource:
water.usgs.gov |
No comments:
Post a Comment