What is Groundwater?

Groundwater is the largest available reservoir of fresh water.  The majority of fresh water is locked away as ice in the polar ice caps, continental ice sheets and glaciers.  Water in rivers and lakes only accounts for less than 1% of the worlds fresh water reserves.

Water falls as rain and snow onto the land, precipitation; and soaks into the soil, with excess rainfall flowing overland to rivers. Once the needs of plant roots and soil moisture have been satisfied, the remaining water continues its journey downward to rock layers beneath the soil. These underground rock layers have the capacity to let water flow through them, either through large cracks and openings in the rock, or through tiny inter-connected spaces between individual rock grains.

The water contained in these rocks is groundwater; and these bodies of rock are known as aquifers. Perhaps the best way of imagining an aquifer is as a giant sponge. Water aided by gravity naturally fills the aquifer from the bottom upwards. This bottom part of the aquifer has spaces that are completely filled by water and this is known as the saturated zone of the aquifer. In the top part of the aquifer the rock spaces contain air as well as water, here the aquifer zone is called unsaturated. The margin where the aquifer changes from unsaturated to saturated is marked by the water table.

Water in an aquifer does not sit still, pulled by gravity and pushed by the force of the water above and behind it, it flows through the spaces and cracks. The water moves from an area where water enters the aquifer, the recharge zone to an area where water exits the aquifer the discharge zone. Where discharge happens, springs may appear, and the aquifer will contribute groundwater to support the flow of rivers and maintain important habitats like fens and marshlands.

The movement of groundwater through the aquifer has the effect of removing a lot of impurities from the water, filtering it through the rock so that groundwater is generally much cleaner than surface water. As groundwater is generally very clean it often requires little or no treatment before being used, the level of treatment depends on what it is to be used for. This makes groundwater a very cheap source of 'raw water' for public supply. This freshwater can be accessed by drilling down into the water-bearing rock layers and pumping the water out.

Aquifers close to, or outcropping at, the ground surface are more vulnerable to pollution or physical damage that could harm both the quality and flow of the groundwater. The flow of groundwater is slower than surface water, and the deeper into an aquifer the water is, the slower it moves. This means that if groundwater becomes polluted and the pollution moves deep into the aquifer, the water can potentially remain polluted for a very long time. This could subsequently lead to a deterioration in the quality of drinking water supplied from a groundwater source or damage vulnerable groundwater dependent rivers and ecosystems.