Aquifer and Its Types

Aquifers are underground reservoirs that are either unconfined or confined. Aquifers have a recharge area, the land from which water percolates to replace groundwater.

In unconfined aquifers, the recharge area is directly above them because the layers of rock above are permeable and allow surface water directly seep downward, replacing the aquifer contents. The upper boundary of an unconfined aquifer is called as water table. The water table is sandwiched between upper soil surface and lower rock surface. The later, rock surface has sediments and cracks saturated with groundwater. The water table varies in depth depending on the amount of precipitation occurring in an area for e.g. in case of deserts, the water table is generally far distant from the surface. In contrast, wetlands, lakes and streams have the water table that intersects with the surface. In dry conditions, the water table of a well is dropped lower than the depth of the well.

A confined aquifer, also known as artesian aquifer, is a groundwater storage area between impermeable layers of rock. The water in a confined aquifer is trapped and often under positive pressure. In contrast to unconfined aquifers, the recharge area may be hundreds of kilometers away. Generally, groundwater resources are considered as nonrenewable because they have taken hundreds or sometimes thousands of years to accumulate, and typically only a minor portion of it is replaced every year by percolation of precipitation. The confined aquifers are recharged particularly slowly.