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GROUNDWATER Powered By Docstoc

 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
  aquifer is:
• 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
  saturated zone.
• Groundwater begins
  as precipitation that
  is absorbed into the
  ground. A lot of it is
  taken up by plant
  roots and
  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
               Groundwater Table
• 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.
         Unconfined Aquifers
• Some aquifers have an aquiclude beneath
  them but not above them; these are called
  unconfined aquifers and are easily filled
  with infiltrating rainwater.
           Confined Aquifers
• 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
  subsidence. Earth
  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
  intercept surface
  drainage and can erode
  to widths of tens of feet at
  the surface.
            Sinking Venice
• 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
Groundwater Contamination
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
  closed recently.)
• Poorly-maintained septic tanks
   Oh, the stench of it all!
 Groundwater Contamination
• 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.
• Agriculture
  Pesticides and other agricultural contaminants
  can enter groundwater over time.
 Groundwater Contamination
• 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
            Industrial Waste
Superfund Sites /
  Toxic Waste
Virginia’s 30 Superfund
cleanup sites increase
the risk of soil,
groundwater, and
surface water
especially in areas with
multiple listings such as
York County and the city
of Portsmouth
     • 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
• 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.
           Radioactive Waste

• 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
  unconfined aquifer.
  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?

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