Groundwater is the water found beneath the Earth's surface in the soil and rock pore spaces and fractures and in unconsolidated material such as gravel, sand and silt. As seen earlier on in the water cycle diagram, groundwater is the water that soaks into the ground. The level where groundwater can be found depends on the level of the water table, which varies from place to place. Below the water table, the soil and rock pore spaces and fractures and unconsolidated material are saturated with groundwater, whereas above the water table, there is no saturation. The name given to the substrates that hold this water is 'aquifer' and water can be extracted from these using a well.
Groundwater exists due to two simple phenomena:
- Gravity – like with everything else on Earth, when something such as water lands on the surface of the Earth, it is pulled downwards towards the centre
- The underlying bedrock – water collects in the pore spaces and fractures of the rock, which happens in rocks such as sandstones, or water dissolves the rock due to chemical reactions and collects in the resulting cavities, which happens in rocks such as limestone. The latter is important in Ireland, as large parts of Ireland are covered in limestone bedrock (43% of the Republic of Ireland and 38% of the island of Ireland) and karst landscapes dominate in these areas. Karst is the name given to such landscapes, where the rock can be dissolved by water.
Ireland relies heavily on groundwater; it supplies 20-25% of our drinking water supplies and, in some counties, it supplies up to 50%. In rural areas not served by water schemes, it is the only source of drinking water with these areas relying on wells and springs. More information about using groundwater as a drinking resource i.e. bottled water, karst and water quantity and quality can be found in the Topics section.
Geological Survey Ireland's Groundwater Programme is involved with projects such as aquifer recharge and vulnerability, karst landscapes and water quality and natural hazards such as groundwater flooding and sinkholes. More details on their current activities can be found here.
Below is an activity from the United States Geological Survey (USGS) on how to visualise groundwater:
Get two sponges and lay one on top of the other. Pour water (respresenting precipitation) on top and it will seep through the top sponge downward into the bottom sponge. If you stopped adding water, the top sponge would dry up and, as the water dripped out of the bottom sponge, it would dry up too. Now, put a piece of plastic wrap between the sponges, creating your "confining layer" (making the bottom sponge an impermeable rock layer that is too dense to allow water to flow through it). Now when you pour water on the top sponge, the water will seep downward until it hits the plastic wrap. The top sponge will become saturated, and when the water hits the plastic wrap it won't be able to seep into the second sponge. Instead, it will start flowing sideways and come out at the edges of the sponge (horizontal flow of groundwater). This happens in the earth all the time, and it is an important part of the water cycle.