Infiltration wells and galleries

Infiltration wells
Infiltration is defined as the flow of water from above ground into subsurface. Infiltration wells are also called interception wells. They are structures that allow surface run-off to drain through underground piping. They do not have a direct water inlet at ground level. However, they improve the ground's infiltration capacity because of installation of porous material.
Sometimes, a coiled drain is placed between the soil surface and the underground piping. Rocks, coarse
wood chips or straw are sometimes used as filtering material.

Infiltration wells are useful in the following cases:

• Poor drainage of small, circular depressions
• Localised resurgence of subsurface runoff or unconfined groundwater

Infiltration wells are normally installed at lowest point of depressions. The idea of an infiltration well is based on the concept of natural water balance as part of hydrological cycle. When the ground surface is fully covered with vegetation and is permeable, some precipitation infiltrates naturally and the remaining flows along the ground surface as run-off. Due to rapid industrialisation and urbanization more and more ground surface is being covered by impervious layers with roofs and pavements causing an increased run-off and less infiltration to recharge the underground aquifers. By the technique of using infiltration wells the surface run-off enters the infiltration wells serve as a temporary storage facility for the surface run-off to infiltrate and join groundwater.

Dimensions of an infiltration well depends upon:

• Surface coverage area
• Intensity of rainfall
• Hydraulic conductivity
• Dominant duration of precipitation
• Shape factor
• Time lag of precipitation
• Groundwater and surface water distribution
• Existing wells distribution (Infiltration and dug wells)

Advantages of infiltration wells

• Quantity of unconfined groundwater can be conserved
• Surfacelevel of unconfined groundwater stays stable
• Area of ponding is minimised
• Dimension of drainage network is minimised
• Groundwater quality is improved
• Land subsidence is preventer
• Salt water intrusion in coastal areas is avoided

Infiltration galleries

An infiltration gallery is a horizontal drain made from open jointed or perforated pipes or a block drain which is laid below the water table and collects groundwater. Infiltration galleries need soils that are permeable to allow sufficient water to be collected. The gallery should be surrounded with a gravel pack to improve flow towards it and to filter any large particles that might block its perforations.Infiltration galleries can be used to collect sub-surface flows from rivers. 

Water is taken to a collection well, or sump, and then either withdrawn directly or pumped to a storage tank.

Galleries are often used in conjunction with other water supplies as a means of increasing the quantity of water quality intake in areas of poor water yield. 

Sometimes, one or more galleries are built which drain into a central point. These are called collector wells. An infiltration gallery is protected from contamination by locating it uphill and at a safe distance from any latrines. A distance of 30 m has been suggested. The gallery should be constructed to ensure that unfiltered surface water cannot enter. 

Infiltration galleries vary in size from a few meters to several kilometers forming an integral part of an urban water supply.