Groundwater is the water that soaks into the ground from rain and can be stored beneath the ground. An aquifer is a body of rock and/or sediment that holds groundwater. There are two main types of aquifer in Ireland – bedrock aquifers, and sand and gravel aquifers.

Bedrock is the solid rock at or below the land surface. Over much of Ireland, the bedrock is covered by materials such as sands and gravel. The sands and gravels occur naturally on top of the bedrock. They were laid down by meltwater from melting ice sheets, by rivers, or by wind.

There are two main types of bedrock aquifer. In most of them, groundwater flows through fractures and fissures. In about half of the limestone rocks, groundwater flows through cavities and caves. This type of limestone is called karst.

Not all sand and gravel layers are aquifers. This is because some of them are very thin or are dry. If the sands and gravels are saturated with water, they have the potential to supply large volumes of water through wells or springs.

The aquifer maps show the potential of areas in Ireland to provide water supplies. There are three main groups based on their resource potential:

Regionally important – the aquifers are capable of supporting large public water supplies sufficient to support a large town;

Locally important – the aquifers are capable of supporting smaller public water supplies or group schemes;

Poor – the aquifers are only capable of supporting small supplies, such as houses or farms, or small group schemes.

The three main groups are broken down into nine aquifer categories in total. Please read the lineage for further details.

Information used to assign bedrock aquifer categories include: rock type (Hydrostratigraphic Rock Unit Groups - simplified bedrock geology with similar hydrogeological properties), yield (existing wells and springs), permeability and structural characteristics. All of the information is interpreted by a hydrogeologist and areas are drawn on a map to show the aquifers.

The Sand and Gravel Aquifer map is to the scale 1:40,000 (1 cm on the map relates to a distance of 400 m).

It is a vector dataset. The sand and gravel aquifer data is shown as polygons. Each polygon holds information on the aquifer code, description, name, comments and confidence level associated with the delineation of the area as an aquifer.

The Aquifer Geological Lines shows the details of the structural geology; faults and thrusts. Faults are the result of great pressure being applied to rock across a whole continent or more. These rocks break under the pressure, forming faults. Faults are recorded as lines where the break in the rock meets the surface. A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.

Geologists map and record information on the composition and structure of rock outcrops (rock which can be seen on the land surface) and boreholes (a deep narrow round hole drilled in the ground). Lines are drawn on a map to show the structure.

To produce this dataset, the twenty one 1:100,000 paper maps covering Ireland were digitised and any inconsistencies between map sheets were fixed. We collect new data to update our map and also use data made available from other sources.

This map is to the scale 1:100,000 (1cm on the map relates to a distance of 1km).

It is a vector dataset. The Geological Lines data is shown as lines. Each line holds information on: description of the line, bedrock 100k map sheet number, line code and name (if it has one).