overuse of water

• Saltwater encroachment is due to saltwater being more dense than freshwater, they meet at a point where saltwater goes underneath the fresh.  If freshwater is abstracted too quickly, saline water intrudes into the aquifer instead of freshwater recharging - the boundary between the two types of water rises and reaches the bottom of wells. 
• Solutions to saltwater intrusion:

  • Stop using well. 
  • Pump freshwater back into well to build up freshwater depth - limited effect 
  • Plan a network of shallow wells so salt water takes longer to intrude. 
• Surface subsidence is the removal of water that has a similar effect as removing rock, water lost due to:

  • Over-abstraction 
  • Natural drying out of rock
  • Compaction of rock due to weight of buildings 
• Surface subsidence:

  Pore pressure is exerted by water on the grains in an aquifer by keeping them apart. When water is lost there is no force keeping the grains apart so the grains compact and repack due to overlying weight - reducing porosity and volume. Usually to small to notice (sandstone).
• Clay subsidence:

  • Clay has 50% porosity so there is a more significant change in pore space and reduction in thickness. 
  • Clay holds water by surface tension so hydrostatic pressure is distributed throughout. If pressure reduces due to abstraction clay packs closer and water is forced out - causing subsidence. 
• Solutions:

  • Water can be pumped back into aquifers and boreholes however porosity will have decreased 
  • Clay subsidence cannot be solved other than stopping abstraction 
  • Reduction of impermeable surfaces over recharge zones (block paving, permeable matting in carparks)