A Hidden Reserve: Groundwater

1
A Hidden Reserve Groundwater

• Much of the global H2O, resides below the surface
  of the Earth in what geologists refer to as
  groundwater.
• Although in the subsurface, it has a large impact
  on society and the features that we see on the
  surface of the Earth.
• In general, surface water gets into the
  subsurface by infiltration.

Central-pivot irrigation utilizing groundwater,
Jordan
The Winter Park, Florida sinkhole
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Groundwater Why do We Care?

• Groundwater is an important resource.
• Drinking water for people and livestock.
• Irrigation.
• Industry.
• It has been used
• for millennia.

3
The Underground Reservoir

• Some precipitation enters the subsurface via
  infiltration.
• Soil properties and vegetation govern
  infiltration rate.
• Infiltrated water adds to soil moisture and
  groundwater.
• Soil moisture wets the soil.
• Some is wicked up by roots, some is evaporated.

4
The Underground Reservoir

• Some infiltrated water percolates to a deeper
  level.
• It is added to water that fills subsurface void
  spaces.
• This is groundwater.

5
Porosity

• Only a very small amount of the global
  groundwater flows in subterranean rivers
• or lakes
• Most groundwater
• resides in pores,
• small open spaces within rocks
• Porosity refers to the total volume of a rock
  that is empty space and is usually quoted as a
  percentage (e.g. rock A has 25 porosity).
• Therefore, the porosity of a rock is the
  percentage of the rock that is empty space
• Porosity greatly varies between different rock
  types

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Primary vs. Secondary Porosity

• Primary Porosity This is the porosity of the
  rock after it first lithifies/forms based on the
  spaces between grains.
• Fine grained sediment has a lower porosity
  because the little grains can fill in the spaces.
• Crystalline rocks have very low primary porosity.
• Secondary Porosity New pore space created in
  the rock at some time after the rock formed
• E.g. Joints, Faults, Dissolution.
• Because of secondary porosity any rock could
  potentially have some porosity.

Primary porosity
Secondary porosity
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Porosity vs. Permeability

• If solid rock completely surrounds a water-filled
  pore, then the water cannot flow.
• For groundwater to flow, pore spaces must be
  interconnected.
• The ability of a rock to allow a fluid to flow
  through an interconnected network of pores is
  called Permeability.
• If a rock has a high porosity it does not
  necessarily have a high permeability. The pores
  must have interconnected conduits!
• E.g. porous cork, is nearly impermeable
• Permeability depends on
• Number of available conduits
• Size of conduits
• Straightness of conduits

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Aquifers and Aquitards

• Hydrogeologists distinguish between rocks that
  transmit water easily and rocks that do not
  easily transmit water.
• Aquifer A rock that easily transmits water
• Aquitard A rock that does not transmit water
  easily (i.e. retards water motion)
• Aquiclude A rock that does not transmit water
  at all

• Unconfined Aquifer An aquifer that has direct
  access to the surface of the Earth
• Can be quickly recharged by meteoric water

• Confined Aquifer An aquifer that is trapped
  below an aquitard

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Hydrogeologic Zones

• Unsaturated Zone/Vadose Zone The portion of the
  subsurface where some of the pores are filled
  with only air.
• Saturated Zone/Phreatic Zone The portion of the
  subsurface where the pores are completely filled
  with water.
• Water Table The boundary between these two
  zones
• How deep does the saturated zone go?
  Hydrogeologists are not sure
• At some depth (10-20km) water is utilized for
  metamorphic reactions.

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Topography of the Water Table

• The water table is not a flat surface that never
  changes
• It may have seasonal oscillations (wet dry
  seasons) and rise and fall
• Underneath mountains and hills, the water table
  follows a similar but subdued shape

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Perched Water Tables

• A locally present aquitard may create a Perched
  Water Table, a localized phreatic zone
  (saturated) above the regional water table.
• Can form springs if there is topography.

12
Recharge and Discharge

• Groundwater flows downward in areas of Recharge,
  and upward in areas of Discharge.
• But what causes groundwater to flow??
• Hydraulic Head A measure of the potential energy
  available to drive the flow of a given volume of
  groundwater.
• Groundwater flows from locations of high
  hydraulic head to low hydraulic head.

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Hydraulic Head and Hydraulic Gradients

• In an unconfined aquifer, hydraulic head can be
  thought of being due to the weight of the water
  above it. Similar to air pressure.
• Water will always flow from regions of high head
  to regions of low head
• In a confined aquifer the hydraulic head is more
  complex.
• Hydrogeologists measure hydraulic head by
  drilling holes into the ground and measuring the
  level to which water fills the hole.
• The rate at which groundwater flows is determined
  by the Hydraulic Gradient, which is the change in
  head from one location to another along the flow
  path.

• To calculate discharge, a French engineer, Henry
  Darcy, coined what we now call Darcys Law.
• If you know the hydraulic gradient (?h/j), the
  hydraulic conductivity (K), and the area through
  which the water is flowing (A), you can calculate
  the discharge by
• Q K(?h/j)A
• Sometimes we simplify this and say that Discharge
  Slope of Water Table Permeability

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How Fast Does Groundwater Flow?

• Water in an ocean current 3 km/hr
• Water in a river - up to 30 km/hr
• Groundwater 0.01 - 1.4 m/day (4-500 m/yr)
• Why so slow? - Conduits are very curved and
  small, so groudwater must flow in a very crooked
  path and friction with conduit walls slows it
  down.
• Hydrogeologists measure flow in some regions by
  injecting a dye, radioactive element, or bacteria
  (all types of tracers) and monitor its movement.
• Some groundwater may emerge after months or
  years, but some may not emerge for thousands to
  tens of thousands of years.

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Wells How We Get To Groundwater

• Since water is important to society, access to
  groundwater is important.
• We access groundwater through wells and springs
  (where groundwater percolates out at the surface
  of the Earth).
• An ordinary well penetrates to a depth below the
  water table where an aquifer allows access to
  flowing water. We then either pump it out (right)
  or manually pull up the water (below).

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Well Drawdown Cones of Depression

• When a well is drilled to access an aquifer,
  drillers like to make the well as shallow as
  possible to save .
• If a well pumps out water faster than it is
  replaced by normal groundwater flow, it draws
  down the water table in what is called a Cone of
  Depression.
• Cones of depression can make nearby wells
  temporarily dry.

• So when drilling a well, drillers must consider
  both the flow rate in the aquifer and the pumping
  rates of nearby wells.

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Artesian Wells

• In some places, groundwater does not need to be
  pumped out of a well if water freely flows out
  of the well it is called a flowing artesian well.
• To see why this happens, lets look at a city
  water supply and water towers.
• Cities first pump water from the local
  aquifer/source into a high reservoir tank.
• This high tank is connected to the houses in town
  by a network of underground pipes.
• The pressure in the elevated tank provides the
  push to make water rise out of the pipes of town.
  The level to which the water will rise is called
  the Potentiometric Surface.

• Therefore, the water company doesnt have to pump
  water to your house, just to the raised tower.
• So if you are on city water, you are likely
  getting it because you have an artesian
  connection to the city water.

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Artesian Wells in Nature

• An artesian well can occur in nature when a well
  penetrates a confined aquifer that is under great
  pressure.
• If the potentiometric surface is above ground,
  the well will be a flowing artesian well.
• If the potentiometric surface is above the water
  table, but below the ground, it will be a
  non-flowing artesian well.

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Springs What Conditions Cause Them to Form?

• Spring A location where groundwater is
  discharged from the ground
• The various conditions shown here cause a spring
  to form.

Impermeable layer
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Springs What Conditions Cause Them to Form?

• The various conditions shown here cause a spring
  to form.

• Note that an Artesian Spring is a natural feature
  while and artesian well is drilled by man.

21
Oasis...Mirage or Geology?

• Folded aquifers and faults can cause an Oasis to
  form.
• These are important stops for people traveling
  across the Sahara.
• Faults can bring deep water up to the surface of
  the Earth forming a hot spring.

22
Karst Landscapes and Groundwater

• Groundwater can dissolve calcite bearing rocks
  such as limestone.
• When CO2 mixes with water is makes a weak acid
  called carbonic acid that speeds this process.
• Over time, changes in the water table may form
  complex networks of caves.
• If a large cave becomes near to the surface of
  the Earth (usually by erosion), it can collapse
  forming a sinkhole.
• Terrain dominated by sinkholes is called Karst
  Landscape or Karst Topography.

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How Does Groundwater Flow Through Limestone?

• Although limestone is nearly impermeable, it is
  commonly jointed.
• The joints provide a secondary porosity and allow
  groundwater to flow through.

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Karst Landscapes
Arecibo Observatory, Puerto Rico
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GroundwaterInfinite or Finite?

• Although on scales of tens of thousands of years,
  groundwater is renewable, if usage is high, it
  can be a big problem on scales of years to
  hundreds of years.

An Industrial well, lowers the water table and
dries up a river
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Groundwater Problems

• Large wells can change the direction of
  groundwater flow moving contaminants into unsafe
  places.

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Groundwater Problems

• Saltwater is more dense than freshwater so it
  stays below the fresh water table.
• Pumping and drawdown can cause saltwater influx
  into what would naturally be freshwater aquifers.

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Groundwater Problems

• Saltwater influx is a huge problem in Florida.

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Groundwater Problems

• Groundwater pressure holds grains of rock apart.
• When water is removed, the once wet layer may
  become compacted, causing subsidence above the
  aquifer.

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Course Evaluations

• Course GLY1101-101
• Instructor ID 016
• I hope you enjoyed the course!