Transition Zone

Transition Zone

​​​​​​​​​​​​​​​​​​The transition zone - the broken, weathered zone at the top of bedrock - is widely recognised as a significant pathway for groundwater flow and potential movement of contaminants.  However, the properties, extent and variability of the transition zone across Ireland are poorly understood.  Geological Survey Ireland aims to better understand this important interface at a national level.​

​Transition Zone definition

Geological Survey Ireland has developed the following definition of the transition zone:

The transition zone is the broken, weathered zone between the subsoil (Quaternary deposits) and competent, unaltered bedrock (see Figure 1).  It may be formed by chemical weathering (Figure 2), or physical processes (Figure 3), or both.  It can have different hydraulic properties to the subsoil and bedrock, and may act as a significant pathway for groundwater and contaminant flow.  It may also serve to attenuate potential contaminants in some cases, and can rapidly change in thickness over very short distances and may be absent in places. 

Transition zone processes may be of particular importance in poorly productive bedrock aquifers in Ireland, where the transmissivity of the transition zone may be greater than the deeper bedrock transmissivity.  This means, simply, that most of the flow in poorly productive aquifers will be in the broken up portion of the top of the bedrock.​

How is Geological Survey Ireland studying the transition zone?

Building upon previous transition zone research, the current project aims to characterise the transition zone in Ireland. Existing desk study information from publicly available reports will be supplemented with new field data collected during this project. Fieldwork examining the transition zone will focus on each of Ireland's main hydrostratigraphic units, and will comprise:
  • systematic logging of exposures and cores;
  • geophysical surveys;
  • and hydraulic testing.

The Transition Zone Story Map

 The Transition Zone Story Map is a visual georeferenced library of photographs of the transition zone. Geological Survey Ireland use it to map the transition zone's variability across different geological settings and to focus their field work.  The story map is populated with photos from hydrogeologists and engineers across Ireland.  Everyone is encouraged to submit their photographs of the transition zone to the story map.  ​Visit the Transition Zone Story Map.