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Confined versus unconfined aquifer
Short answer
Unconfined aquifer: no impermeable top, water table at atmospheric pressure, direct recharge, large fluctuation. Confined aquifer: bounded by aquicludes, water under pressure, recharge only at outcrop, small fluctuation, may produce artesian flow.
Differentiate between confined and unconfined aquifers with a suitable diagram.
| Point | Unconfined (water-table) aquifer | Confined (artesian) aquifer |
|---|---|---|
| Boundary | No impermeable layer above; open to the atmosphere through the overlying pores | Between two impermeable layers (aquicludes) |
| Upper surface | Water table, free surface | Overlying confining bed |
| Pressure | Atmospheric at water table | Water under pressure greater than atmospheric |
| Water level in well | Same as water table | Rises above top of aquifer (piezometric surface) |
| Recharge | Direct from rain over the whole area | Only at outcrop (recharge area) |
| Fluctuation | Large with rainfall | Small |
| Pollution risk | High | Low |
| Storage | Specific yield (by draining pores) | Storage coefficient (compression of the aquifer and expansion of water) |
| Well | Water table well | May be a flowing artesian well |
Unconfined Confined
~~~~ water table Piezometric surface ......
////// sand //// -------------------------
////////////// ====== clay (aquiclude) =
==== clay ===== ///// sand aquifer /////
====== clay (aquiclude) =
✓ Answer checked by Claude Opus 5.5 (high effort)
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