Spring formation; infiltration gallery

Formation of springs

A spring is a natural outflow of ground water at the ground surface. It forms when the water table meets the ground surface, or when ground water is forced out by an impervious layer. The main circumstances are:

1. Depression spring: the ground surface dips below the water table in a valley or hollow.
2. Contact (gravity) spring: an impervious layer below a pervious layer on a hill slope blocks the downward movement and water emerges at the contact.
3. Fault spring: a fault or fracture brings a confined aquifer in contact with the surface.
4. Artesian spring: confined water under pressure rises through a crack in the overlying impervious layer.
5. Fissure or solution spring: water comes out through joints and cavities in rock such as limestone.

 Contact spring           Artesian spring
  ___________              ground
 / pervious  \___        ____  ____
/____________    \__ o  ====impervious====
 impervious  -----> spring   confined aquifer

Infiltration gallery

An infiltration gallery is a horizontal perforated pipe or porous conduit laid below the water table, usually along a river bank or in a water-bearing sand layer, which collects water by seepage and delivers it to a sump well.

  river ~~~~~~~~~~~~~~~~
  ground ===========\===
       gravel+sand   \
   o  o  o  o  o  o  -> [Sump well]
   perforated pipe, 1 in 500 slope
   ------- impervious layer -------

• Trench 3 to 6 m deep, pipe diameter 0.3 to 0.9 m laid on a slight slope to the sump.
• Pipe surrounded by graded gravel and sand; the top is sealed with clay to stop surface pollution.
• Manholes are provided for cleaning.
• Water is naturally filtered, so it needs little treatment.