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Shallow and deep wells

Short answer

Shallow wells tap the upper unconfined aquifer, are up to 15 m deep, have small variable yield and poor bacteriological quality; deep wells tap confined aquifers below impervious layers, are deeper, with large steady yield and good quality. For public supply, prefer a deep well because of its reliable yield and better water quality.

Differentiate between shallow and deep wells (with neat sketches), and describe their characteristics/suitability with respect to water quality and quantity. (Which of them would you prefer for a public water supply scheme and why?)

A well is a hole or shaft sunk into the ground to reach water-bearing strata. Wells are classified as shallow or deep by the stratum they draw from.

Sketches

  SHALLOW WELL            DEEP (TUBE) WELL
  |  |  ground level      |  | ground level
  |  |                    |  |
  |~~|~ water table       |  | unconfined layer
  |__|                    |==|== impervious layer
  /////  impervious       |  |
                          |__| confined aquifer
                          (strainer at bottom)

Differences

BasisShallow wellDeep well
DepthUp to about 10 to 15 m30 m to hundreds of metres
Tapped stratumTop (unconfined) aquifer above the first impervious layerConfined aquifer below an impervious layer
ConstructionDug, lined with masonry or concrete ringsDrilled/bored with casing and strainer
YieldSmall (about 1 to 5 lps), depends on rainfallLarge and steady
QualityPoor; likely to be polluted by bacteriaGood, free from bacteria, constant; often harder and may contain iron
VariationDries in the dry seasonRarely fails
CostLowHigh
UseHousehold and small rural supplyMunicipal and industrial supply

Suitability

  • Shallow well water is filtered only by a thin soil layer, so bacteriological pollution from latrines and surface drainage is likely. The quantity is limited and varies with the water table.
  • Deep well water is naturally filtered through a long path in the soil and is protected by the impervious cover. The quantity is large and dependable, though the water can be hard or contain iron because it is in contact with minerals for a long time.

Preferred choice

For a public supply scheme a deep well (tube well) is preferred because of its larger and more reliable yield, better quality (less treatment) and long life. Shallow wells serve only individuals or very small groups.

✓ Answer checked by Claude Opus 5.5 (high effort)

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