A layer of permeable rock below the water table that stores and transmits groundwater is called a/an:

An aquifer is an underground layer of permeable rock, sediment or soil that can hold and transmit significant amounts of groundwater. Rocks such as sandstone, limestone with cracks, gravel and sand are good aquifers, because they have spaces (pores) or fractures that allow water to be stored and to flow. Water from an aquifer can be drawn using wells and tube wells.

Rain water and water from rivers and lakes seep down into the ground. The upper surface of the zone where the ground is fully saturated with water is called the water table. The process by which water enters an aquifer is called recharge.

Types of aquifers:

Unconfined aquifer: lies directly below the land surface, with no impermeable layer above it. Its water table rises and falls with rainfall and pumping.

Confined aquifer: lies between two impermeable layers, so the water in it is under pressure.

When a well is dug into a confined aquifer whose recharge area is higher than the well, the pressure can push water up to the surface without pumping. Such a well is called an artesian well, named after the region of Artois in France. The Great Artesian Basin of Australia is one of the largest in the world.

Layers that do not let water pass easily are called aquicludes (such as clay) or aquitards (which let water pass very slowly).

In India, groundwater supplies a large share of water for irrigation and drinking. Over-extraction has caused the water table to fall sharply in states such as Punjab, Haryana and Rajasthan. The Atal Bhujal Yojana (2019) and the Central Ground Water Board work to manage groundwater.

Why not the other options?

• ✗An aquiclude is an impermeable layer, such as clay, that does not transmit water.
• ✗An artesian well is a well from which water flows up under natural pressure.
• ✗A geyser is a hot spring that throws out jets of hot water and steam at intervals.