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   A Hidden Reserve: Groundwater
• On May 8, 1981, Rosa Owens looked outside
  her window and discovered that a large
  sycamore tree in the backyard of her Winter
  Park, Florida home had suddenly disappeared.
• Rosa Owens discovered that her whole
  backyard had become a deep gaping hole. The
  hole continued to grow for a few days until it
  swallowed Owens’s house and six other
  buildings.
• What happened in Winter Park, Florida?
• Groundwater the water that resides under the
  surface of the Earth, had gradually dissolved
  limestone underneath Winter Park creating a
  circular depression called a sinkhole.
• The Winter Park sinkhole serves as a reminder
  that significant quantities of water reside
  underground.
• Groundwater has increasingly become a major
  supply of water for homes, agriculture, and
  industry.
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  WHERE DOES THE GROUNDWATER
            RESIDE?
• The upper part of the Earth’s crust behaves like
  a giant sponge that can soak up water. Water
  soaks into the ground by a process called
  infiltration. The amount of infiltration depends
  on the vegetation cover and on the composition
  of materials making up the surface layer of the
  Earth—loose sand lets water sink easily, hard
  packed clay or unfractured rock does not.
  Porosity: The Home of Groundwater
• Most groundwater resides within the pore
  space of what might at first look like solid rock
  or sediment. Only a small percentage of
  groundwater flows freely as lakes and streams
  of cavern networks.
• A pore is any open space within the body of
  sediment or rock, and porosity refers to the
  total volume of empty space in a material,
  usually expressed as a percentage.
• For example, if we say that a block of
  sandstone has 30% porosity, then 30% of block
  of what looks like solid sandstone contains 30%
  open space.
• Geologist further distinguish between primary
  and secondary porosity.
• Primary porosity consists of space that
  remains between solid grains or crystals after
  sediment accumulates or rocks form.
• Secondary porosity refers to new pore space
  in rocks, produced some time after the rock first
  formed. For example, when rocks fracture.
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PERMEABILITY: THE EASE OF FLOW
• If solid rock completely surrounds a pore, the
  water in the pore cannot flow to another
  location. For groundwater to flow, pores must
  be linked by conduits (openings). The ability of
  a material to allow fluids to pass through an
  interconnected network of pores is called
  permeability.
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• The permeability of a material depends on
  several factors.
• Number of available conduits: As the
  number of conduits increases, the permeability
  increases.
• Size of the conduits: More fluids can through
  wider conduits than narrow ones
• Straightness of conduits: Water flows more
  rapidly through straight conduits than it does
  through crooked one.
• With the concept of permeability in mind,
  hydrologist, distinguish between aquifers,
  sediment or rocks that transmit water easily,
  aquitards, sediment or rock that do not
  transmit water easily and therefore retard the
  motion of water, and aquicludes do not
  transmit water at all.
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          THE WATER TABLE
• Geologist define a boundary, called the water
  table, above which pore space contain mostly
  air and below which they contain only water.
• In technical terms the subsurface above the
  water table is called the unsaturated zone or
  the vadose zone and the region below the
  water table is the saturated zone or the
  phreatic zone.
• The depth of the water table in the subsurface
  varies greatly with location.
• Rainfall affects the water-table depth. If the
  water table drops below the floor of a river or
  lake, the river or lake dries up because the
  water it contains infiltrates the ground.
• Note that “water table” refers to the position of
  the top of the groundwater reservoirs beneath
  the land.
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     Topography of the Water Table
• In hilly regions, If the ground has relatively low
  permeability, the water table is not a planar
  surface, but rather its shape mimics, in a
  subdued way the shape of the overlying
  topography.
• This means that the water table lies at a higher
  elevation beneath hills than it does beneath
  valleys.
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       GROUNDWATER FLOW
• What happens to water that has infiltrated down
  into the ground?
• In the zone of aeration, the region between the
  ground surface and the water table, water
  percolates straight down, like water passing
  through a drip coffee maker. The water moves
  down because of gravity.
• But in the zone of saturation, the region below
  the water table, water responds to downward
  pull of gravity and differences in pressure.
• Pressure may cause groundwater to flow
  sideways, or even upward. Pressure in
  groundwater at a specific point underground is
  caused by the weight of all the overlying water
  from that point up to the water table. The
  weight of the overlying rock does not contribute
  to the pressure pressing on the groundwater,
  because contact points between mineral grains
  bear the rock’s weight.
• Pressure provides potential energy, imagine a
  water-filled plastic bag, if you puncture the bag
  and then squeeze the bag to exert pressure,
  water spurts out.
• The potential energy available to drive the flow
  of a given volume of groundwater at a location
  is called the hydraulic head. To measure the
  hydraulic head, hydrogeologists drill a vertical
  hole down to a point and then insert a pipe in
  the hole. The height above a reference
  elevation to which water rises in the pipe
  represents the hydraulic head—water rises
  higher in the pipe where the head is higher.
• As a general rule, water flows from regions of
  high hydraulic head to regions of low hydraulic
  head.
• The location where water enters the ground is
  called the recharge area and the location
  where groundwater flows back up to the surface
  is called the discharge area.
• Groundwater that flows close to the Earth’s
  surface stays under ground for hours or weeks.
  Groundwater that flows several kilometers
  within the Earth stays underground for years.
  Groundwater that flows hundreds of kilometers
  within the Earth flows underground for centuries
  to millennia.
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       Rates of Groundwater Flow
• The rate at which groundwater flows, at a given
  location, depends on the permeability of the
  material containing the groundwater;
  groundwater flows faster in material with
  greater permeability than material with lesser
  permeability.
• The rate also depends on the hydraulic
  gradient, the change in hydraulic head per unit
  of distance between two locations as measured
  along the flow path.
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    TAPPING THE GROUNDWATER
              SUPPY
• Groundwater can be brought to the surface at
  wells or springs. Wells are holes that people
  dig or drill to obtain water. Springs are natural
  outlets from which groundwater flows.
• In an ordinary well, the base of the well
  penetrates an aquifer below the water table.
  Water from the pore space in the aquifer seeps
  into the well and fills it. The water surface in
  the well is the water table.
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• If users pump water out of the well too fast,
  then the water table sinks down around the
  well, a process called “drawdown” so that the
  water becomes a downward-pointing, cone
  shaped surface called a cone of depression.
  Drawdown may cause shallower wells that have
  been drilled nearby to become dry.
• An artesian well penetrates confined aquifers,
  in which water is under enough pressure to
  cause the water to rise on its own to a level
  above the surface of the aquifer.
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• To understand how an artesian well works we
  have to look at the configuration of a city water
  supply. Water companies pump water into a
  high tank that has a significant hydraulic head
  relative to its surrounding areas. If the water
  were connected by a water main to several
  vertical pipes, pressure caused by the elevation
  of the water in the high tank would make the
  water rise in the pipes until it reached an
  imaginary surface, called a potentiometric
  surface, that lies above the ground. This
  pressure drives water through water mains to
  household water systems without requiring
  pumps.
• Artesian wells will flow if the potentiometric
  surface lies above the ground surface, and
  nonflowing artesian wells form where the
  potentiometric surface lies below the ground.
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 GROUNDWATER USAGE PROBLEMS
• Since prehistoric times, groundwater has been
  an important resource that people have relied
  on for drinking, irrigation, and industry.
• Is groundwater a renewable resource? In a
  time frame of 10,000 years, the answer is yes,
  for the hydrologic cycle will eventually resupply
  depleted reserves. But in a time frame of 100
  to 1000 years—the span of a human lifetime or
  a civilization—groundwater in many regions
  may be a nonrenewable resource.
• What are some groundwater problems?
• Lowering the water table—lowering the water
  table can cause diversion of water from other
  areas to dry up.
• Reversing the flow direction of
  groundwater—the cone of depression creates
  a local slope to the water table and the resulting
  hydraulic gradient may be so great that flow of
  water is reversed, bringing in contamination.
• Saline intrusion—In coastal areas, fresh
  groundwater lies in a layer above saltwater. If
  people pump water out too quickly the
  saltwater/freshwater boundary rises leaving
  behind useless water.
• Pore collapse and land subsidence—When
  water fills the pore space of a rock it holds the
  grains of the rock apart, for water can not be
  compressed. The extraction of water from a
  pore eliminates the support for holding the
  grains apart. As a result, the grains pack more
  closely together. Such pore collapse
  permanently decreases the porosity and
  permeability of a rock and thus lessens the
  value of an aquifer.
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• Fortunately, in some cases, natural processes
  can clean up groundwater contamination.
  Chemicals may be absorbed by clay, oxygen in
  the water may oxidize the chemicals, and
  bacteria in the water may metabolize the
  chemicals thereby turning them into harmless
  substances.
• More recently, environmental engineers have
  begun exploring techniques of bioremediation:
  injecting oxygen and nutrients into a
  contaminated aquifer to foster growth of
  bacteria that can breakdown molecules of
  contaminates.
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