Aquifers

• Aquifer


  • Body of rock that holds water which is exploited as a resource
  • For a rock to form an aquifer it must have certain features
• Porosity


  Measure of the proportion of a rock's volume that is space and could therefore hold water
• Porous rocks that can form aquifers

  • Chalk
  • Limestone
  • Sandstone
• Permeability


  Measure of the ease with which fluids may flow through a rock because of the interconnections between the spaces and their size
• Impermeable rocks


  • Granite
  • Clay
• Aquifer recharge


  1. Natural flow or seepage of water into the aquifer

  2. Water that flows out of the aquifer
• Abstraction of water by humans greater than recharge rate


  Volume of water in the aquifer will decline
• Most aquifer recharge takes place as precipitation landing on the ground surface infiltrates and percolates through soil and rock to the aquifer
• Water abstraction from aquifers that are recharged by surface water can be sustainable as long as it does not exceed the recharge rate
• Some aquifers were recharged during the last ice age 10-20,000 years ago when the global climate was wetter
• Ancient aquifer water is often saline and can cause soil salinization as irrigation water evaporates, leaving the salt behind
• Lowering of water table

  Reduces or stops the outflow of water into wetlands, causing them to dry up
• Lowering of water table

  Plants with higher water requirement will die or become less abundant as they fail to compete with plants that have a lower requirement for water
• Wetlands drying out


  Aquatic or semi-aquatic animals will die
• Overexploitation of aquifer


  Seawater from the sea, or in the rock under the sea, flows sideways into the aquifer to replace the freshwater, making the aquifer water unsuitable for irrigation
• Water no longer present in the interstitial spaces in the rock

  Rock particles will be compacted by the weight of material above, causing subsidence at the surface
• Extraction rate exceeding recharge rate

  Volume of water available for abstraction will be reduced
• Nearly half the world's population live in countries that are over-exploiting their aquifers
• GRACE satellites


  NASA operates two satellites that can detect changes in the mass of water in aquifers below their flight path
• Over-exploited aquifers


  • High Plains Aquifer in the USA
  • North China Plain in China
  • Malta