Groundwater is water stored
inside the Earth's soil and rock
Layers which hold usable amounts of water are called aquifers
Porosity and Permeability
Two important factors help determine how good an
• 1. Porosity
The percentage of the volume of a rock layer
which is empty space (called pore space) and
thus can hold water. Some rock layers are highly
porous, with up to 40% of their volume made of
pore space; others have low porosities of less
than 1%. However, even in the least porous
rock, there can be water..
• 2. Permeability
More or less, how easy it is for water to flow
through a rock layer. Permeability is partially
controlled by porosity and partially by how the
pores are connected and such.
Essentially, the higher the porosity, the more water
the rock layer can hold, and the higher the
permeability, the easier it is to extract that water.
Good aquifers will have high porosity and
Unsaturated VS Saturated
• Typically, the pore space in the shallow
soil beneath our feet is not full of water,
but contains some air. This shallow part is
called the unsaturated zone. Beneath the
unsaturated zone, the pore space is full of
water, and this zone is called the
• Groundwater begins
as precipitation that
is absorbed into the
ground. A lot of it is
taken up by plant
What makes it
beyond to root zone
is pulled down by
gravity until it
reaches the water
table. Below the
water table all the
pore space in the soil
is filled with water.
This is the saturated
• The boundary between the unsaturated
and saturated zones is called the
groundwater table or water table. Clearly,
if you are going to drill a well for water, you
want to get well below the water table into
the saturated zone.
• Some layers are highly porous and permeable (aquifers),
but others are very impermeable - water does not
penetrate or flow through these layers, which are often
made of clay or shale. These layers are called
aquicludes or aquitards.
White residual clay pit southeast of Cold Spring, Augusta County.
• Some aquifers have an aquiclude beneath
them but not above them; these are called
unconfined aquifers and are easily filled
with infiltrating rainwater.
• Other aquifers are sandwiched between
aquicludes (confined); the water in such
aquifers is often pressurized, and if a well
is drilled into one, the well will flow freely
with no pumping. Such wells are called
• Recharge is adding water to an aquifer and
discharge is removing water from an aquifer. If
recharge exceeds discharge, the aquifer will
slowly fill with water. If discharge exceeds
recharge, the aquifer will slowly drain of water.
Cone of Depression.
• If you pump water from an aquifer at a rate
which exceeds the rate of recharge, you
will lower the water table near your well;
this region of lowered water table is called
a cone of depression.
• If you pump water too quickly for too
long, your well's cone of depression
may deepen to the bottom of the well,
and your well will run dry. You will
either have to stop pumping and wait
for recharge or drill a deeper well.
Cone of Depression
• Overpumping can cause
problems at the surface,
as well. If you drain the
water from an aquifer, the
pore space may collapse,
causing the material to
shrink and the surface to
become depressed. This
surface depression is
called subsidence, and
is a major problem for
some parts of the world.
• In many areas of the arid
Southwest, earth fissures
are associated with land
fissures can be more than
100 feet deep and
several hundred feet in
length. One extraordinary
fissure in central Arizona
is 10 miles long. These
features start out as
narrow cracks, an inch or
less in width. They
drainage and can erode
to widths of tens of feet at
• Venice is slowly subsiding into the
northern Adriatic Sea because of over
pumping of groundwater; Mexico City is
sinking into the former lake bed on which it
is built. In some areas of the world, the
Earth's surface is subsiding at a rate of
one foot per year as a result of over
Clearly, if groundwater flows that slowly, anything which
contaminates the water will be there for a long time. Among the
sources for groundwater contamination are:
• Leaking gasoline storage tanks
A new law was recently put into effect requiring gas stations to
upgrade from single-walled underground storage tanks to double-
walled tanks, to try to reduce gas leakage into groundwater. (This is
why many gas stations have been either torn down or temporarily
• Poorly-maintained septic tanks
Oh, the stench of it all!
• Unlined landfills
If you dump trash and toxic materials into a
landfill, gunk will eventually make its way into
groundwater. However, if you build a clay or
rubber liner under your landfill, you can slow or
stop this process.
Pesticides and other agricultural contaminants
can enter groundwater over time.
• Mining waste
Often as mines were dug (and still are in
some places), big piles of leftover rock and
debris were left behind. Lots of heavy
metals are present in these tailings, and
rainwater can also leach acids out of them.
Neither heavy metals nor acids are stuff I
want in my drinking water.
Water and tin mining waste - a noxious mixture
Superfund Sites /
Virginia’s 30 Superfund
cleanup sites increase
the risk of soil,
especially in areas with
multiple listings such as
York County and the city
• Water Quality
Pollution and development
have compromised the Old
Dominion’s water sources:
Virginia ranks sixth highest in
the nation for amount of
developmental toxins released
into the state’s waters. The
problems affect people as well
as natural ecosystems: 81
percent of Virginia’s coastal
waters and 30 percent of
rivers, streams, and creeks are
under fish consumption
advisories due to chemical
contamination, and half of the
state’s 55 watersheds are at
high risk for loss of wetland
and aquatic species.
• Radioactive waste
We generate lots of high-level nuclear waste in
bomb-making and reactors. While nuclear power
is a good way to generate electricity, the waste
must be dealt with. One proposal is to drill a
series of giant tunnels and caverns into Yucca
Mountain in the south/central Nevada desert and
entomb the waste for 10,000 years. Of course,
you have to worry about the water table and
contamination, and billions of dollars have been
spent on this project. What will ultimately happen
is still not clear.
• In USA high-level civil wastes
all remain as spent fuel stored
at the reactor sites. It is
planned to encapsulate these
fuel assemblies and dispose of
them in an underground
engineered repository about
2010, at Yucca Mountain,
• What you need to know:
1.What is an aquifer?
2. Define Porosity and Permeability.
3. What is the an unsaturated zone verses a saturated zone?
4. What is a groundwater table?
5. What is an aquicludes or aquitards?
6. Explain the differences between a confined aquifer and an
7. What does recharge and discharge mean?
8. What is the Cone of Depression?
9. What is subsidence? And how does it occur?
10.Understand how contaminates can affect our water supply?