What is an aquifer?
The EU Water Framework Directive (WFD, 2000/60/EC) defines an aquifer as a:
"subsurface layer of … geological strata ..[which allows].. either a significant flow of groundwater or the abstraction of significant quantities of groundwater".
The meaning of 'significant' varies depending on the use of the groundwater. In Ireland, because of our dispersed rural population who are reliant on private wells for their homes and farms, nearly all bedrock types are classified as aquifers since they are able to provide water in sufficient amounts for houses and farms.
Types of aquifers in Ireland
There are two main types of aquifer in Ireland – bedrock aquifers, and sand and gravel aquifers. In sand and gravel aquifers, which underlie <5% of the land area, groundwater is stored and flows between the sand and gravel grains. This is known as intergranular porosity and permeability.
In contrast, nearly all bedrock aquifers in Ireland have secondary fissure permeability only. This means that groundwater is stored in and travels through fractures, joints and other weaknesses in the rock. Any primary intergranular porosity that may have existed has been obliterated. This is because nearly all of Ireland's bedrock is comparatively old (>300 million years old), and has undergone more than one phase of burial and deformation. (The oldest rocks found in Ireland have undergone many deformation, burial and uplift phases.)