Types of geological formation

The Diagram below shows a hypothetical geological situation in which different sources of groundwater may (or may not) be tapped by dug wells or boreholes.

A) Perched aquifers:
At site A, a shallow dug well may provide a little water from a ‘perched’ aquifer in the weathered zone above relatively impervious (low porosity) mudstones. If this well was extended into the mudstones it might produce very little additional water. A perched aquifer is normally limited in size and lies on an impervious layer higher than the area’s general water table.

B) Shallow unconfined aquifers:
The term ‘unconfined’ refers to an aquifer within which the water is open to atmospheric pressure: the so-called piezometric surface (pressure head level) is the same as the static water level (SWL) in the borehole. At site B, a borehole extracts water from an unconfined sandstone aquifer, the SWL of which is somewhat lower than the level of flow in the river. This sandstone aquifer is in a good catchment area because of recharge from the river.

C) Confined aquifers
A ‘confined’ aquifer may hold groundwater under greater pressure, so that when punctured by a borehole, the SWL rises to the higher piezometric level. If the piezometric surface happens to be above ground level (which is not uncommon), water will flow out of the borehole by itself: this is known as ‘artesian water’. Deep borehole C intersects the sandstone aquifer and a deeper confined aquifer in fissured limestone; because of overpressure in the limestone aquifer, the SWL in C may be
at the same or higher elevation than in B. The limestone aquifer may have no source of replenishment; so the water in it is ancient, or ‘fossil,’ and could be exhausted if over-exploited.

D) Fracture zone
Borehole D, which has been drilled into fractured granite (shaded area), finds water held in the fracture zone. Fracture zones develop during geological times as a result of the severe mechanical stress, caused by tectonic movements, that is exerted on non-plastic formations.

E) Hydrogeological basement
Site E, a borehole sunk into massive granite on top of a hill, is dry. In this situation, it would be a waste of time and money to extend a deep borehole (such as C) into the metamorphic basement, which is generally known as the ‘hydrogeological basement’ or ‘bedrock.’ The bedrock marks the level below which groundwater is not likely to be found.