What decides vulnerability
Most of the protection for groundwater comes from the ground above it. GSI's classes depend mainly on two things:
- How thick the subsoil is over the rock or the water-bearing gravels. More subsoil gives more time for pollutants to be filtered out.
- How permeable that subsoil is. Water passes quickly through sand and gravel, and slowly through dense clayey till.
Features where surface water drains straight underground, such as swallow holes in karst limestone, make the groundwater around them extremely vulnerable whatever the subsoil.
The five classes
| Class | Roughly means |
|---|---|
| Extreme (X) | Rock at or very near the surface, or karst. Little or no natural protection. |
| Extreme (E) | Thin subsoil over the rock or gravels. |
| High | Moderate depth of subsoil, or deeper subsoil that lets water through easily. |
| Moderate | Deeper subsoil of moderate permeability. |
| Low | Thick, low-permeability subsoil, such as deep clayey till. |
The exact depth thresholds behind each class are set out in GSI's groundwater protection methodology. The map is drawn at a regional scale, so the boundaries between classes are approximate on the ground.
Why it matters for a site
- Septic tanks and treatment systems. The EPA's Code of Practice for domestic wastewater treatment systems uses groundwater vulnerability, together with the aquifer type, to decide what level of treatment and what site assessment a new system needs. On Extreme or High vulnerability sites, expect closer scrutiny and possibly a more advanced treatment system.
- Soakaways and surface water. Where vulnerability is high, the council may want to know how run-off from roofs, yards and roads will be handled.
- Farm and commercial uses. Fuel stores, slurry and anything that could leak are assessed more strictly on vulnerable ground.
What it can't tell you
- It's a regional map, not a site investigation. A trial hole and percolation test are what actually decide whether a septic tank system can work on a site.
- It says nothing about how deep the water table is. Vulnerability is about protection, not depth.
Sources
- Geological Survey Ireland, Groundwater Vulnerability map (1:40,000), gsi.ie
- EPA, Code of Practice: Domestic Waste Water Treatment Systems (Population Equivalent ≤ 10)
Last reviewed 2026-10-02.