Define and explain the following terms:

1
WATER SUPPLY

• Define and explain the following terms
• Porosity
• Permeability
• hydrostatic pressure
• hydraulic gradient
• Aquifers
• water table.

2
Porosity and Permeability

• ? What is meant by the terms porosity and
  permeability?
• Porosity Amount of pore space between grains 
• Porosity Volume of pore space x 100
• Total volume of rock

3

• Factors influencing porosity
• Grain shape
• Packing
• Amount of cement and smaller grains between
  grains.
• Sedimentary rocks have a greater porosity as a
  rule than igneous and metamorphic rocks that are
  made of interlocking crystals.
• Some examples
• Limestone 10
• Sandstone 18
• Shale 18
• Clay 45
• Sand 35
• Gravel 25
• igneous rocks 1

4
Permeability

• Definition The ability of a liquid to pass
  through a rock.
• Permeable water can pass through, varies from
  high to low.
• Impermeable water cannot pass through.
• The larger the pore spaces the more permeable the
  rock will be especially if the pore spaces are
  well connected.
• A moderately porous rock may not necessarily be
  permeable, as the pore spaces may not be
  connected.
• E. g. shale has a porosity of 18 similar to
  sandstone but clay minerals hold onto the water
  and so it is impermeable.

5
Post Depositional Changes

• These can affect porosity and permeability
• Faulting increases the permeability
• Jointing increases the permeability
• Cementation decreases permeability
• Metamorphism will crystallise the rock and reduce
  the porosity.

6
Hydrostatic Pressure

• Normal water pressure exerted at any point in a
  water body at rest.
• This increases with depth.

7
Hydraulic Gradient

• Forms when areas of low and high hydrostatic
  pressure are connected e.g. within an artesian
  well.
• The water in the high-pressure zone will flow
  towards the low-pressure zone (at the surface).
• In artesian systems the well is dug into the
  aquifer below the water table.
• This creates a strong hydraulic gradient and
  water keeps flowing.

8
Aquifers

• The water table tends to follow the surface
  topography.
• Water Table Surface of the saturated groundwater
  zone.
• Where a steep surface and the water table meet
  then springs will occur.
• This is particularly common in limestone, which
  can dissolve in water forming cave systems.
• Springs will occur where the permeable aquifer
  meets an impermeable rock below.

9
Aquifers 2
There are different types of aquifer

• Confined aquifers This is an aquifer between 2
  impermeable rocks.
• The 2 sandwiching rocks are called aquicludes.
• Unconfined aquifers Not covered by impermeable
  rock.
• Water table level varies depending on the season.

10
Aquifers 3

• Perched aquifers Smaller aquifers that hold
  amounts of water higher than the regional water
  table e.g. valley sides.
•  
•   

These can often lead to the formation of
springs.   Water can be extracted from these
aquifers in a number of ways
11
WELLS

• These are dug or drilled down into an unconfined
  aquifer and must extend below the water table.
• The water is either pumped out mechanically or
  drawn out by hand.

The main problem is that over time if used
excessively output may exceed the main meteoric
input and water table level can drop locally.
12
ARTESIAN WELLS

• A well is dug or drilled into a confined aquifer.
• This time no pumping is needed as water in the
  aquifer is under considerable pressure.
• The well acts like a pressure release valve and
  water flows out under its own pressure.
• E. g. the Great Australian Artesian basin in
  Queensland.

13
Artesian Wells 2

• In artesian systems the well is dug into the
  aquifer below the water table.
• This creates a strong hydraulic gradient and
  water keeps flowing.
• When the aquifer is folded producing a broad
  synform then an artesian basin is formed e.g. the
  Australian basin covers 1.5 million km2.
• Water is picked up in the mountains in the east
  and the water slowly moves through the rock west
  towards an arid region, where wells up to 1.5 km
  deep.