GSI's aquifer categories
Every location in Ireland has a bedrock aquifer category. In broad terms:
| Category | What it means |
|---|---|
| Regionally important | Can supply large public water schemes. Includes karstified limestone (water moving through dissolved channels) and well-fractured rock. |
| Locally important | Moderately productive, either generally or only in zones. Enough for local supplies. |
| Poor | Generally unproductive, except in local zones. Good for a house well at best. |
Sand and gravel aquifers are mapped separately, only where significant deposits exist. Many sites have none.
Irish bedrock aquifers mostly carry water in fractures, which are concentrated near the surface. That's why a deeper borehole doesn't necessarily find more water.
Source protection areas
Around many public water supply wells and springs, and some group water scheme sources, GSI and the local authority have mapped:
- Inner and outer source protection areas, based on how long water takes to travel underground to the source.
- Zones of contribution: the whole area whose rainfall eventually feeds the source.
Inside these areas, potential pollution sources such as septic tanks, fuel stores, farmyards and quarries are assessed more strictly, and some may not be acceptable at all.
Why it matters for a site
- Septic tanks: the EPA's Code of Practice combines the aquifer category with groundwater vulnerability to set the level of wastewater treatment needed. A regionally important aquifer with high vulnerability is the strictest combination.
- Your own well: the category gives a first idea of whether a well is likely to yield enough water. See also measured depths in the water table guide.
- Planning: a site inside a source protection area for a public supply will draw extra attention from the council and Uisce Éireann.
What it can't tell you
- Aquifer maps are regional. Whether a well on a particular site will yield enough water only comes from drilling it.
- Not every public or group water source has a mapped protection area. No mapped area doesn't mean no source nearby.
Sources
- Geological Survey Ireland: Aquifer maps (bedrock and sand/gravel); Public Water Supply and Group Water Scheme Source Protection Areas, gsi.ie
- EPA, Code of Practice: Domestic Waste Water Treatment Systems
Last reviewed 2026-10-02.