The Groundwater Subsoil Permeability map classifies how easy water can infiltrate subsoils downwards at any point in the land surface. Permeability across the country is classified as either ‘High’, ‘Moderate’ or ‘Low’. This map is used to determine groundwater vulnerability categories.

Groundwater Vulnerability is a term used to represent the natural ground characteristics that determine the ease with which groundwater may be contaminated by human activities. More scientifically, groundwater vulnerability embodies the characteristics of the intrinsic geological and hydrogeological features at a site that determine the ease of contamination of groundwater.The groundwater vulnerability concept is based largely on the question ‘can water and contaminants move in the subsurface materials (soil and subsoil) and/get down to groundwater easily?’The vulnerability categories drawn-up therefore show those assigned to sites and areas across Ireland, and is thus based on the relative ease with which infiltrating water and potential contaminants may reach groundwater in a vertical or sub-vertical direction. As all groundwater is hydrologically connected to the land surface, it is the effectiveness of this connection that determines the relative vulnerability to contamination. Groundwater that readily and quickly receives water (and contaminants) from the land surface is considered to be more vulnerable than groundwater that receives water (and contaminants) more slowly, and consequently in lower quantities. Also, the slower the movement and the longer the pathway, the greater is the potential for attenuation of many contaminants.A ranking of subsoil permeability therefore classifies the relative ease with which water can infiltrate downwards at any point in the land surface, and the subsoil permeability map has grouped permeabilities across the country as either ‘High’, ‘Moderate’or ‘Low’.The data gathered and used in the production of this map are intended for use at a scale of no greater than 1:40,000.Application of the data: Local details are generalised to fit the original mapping and interpretation scale of 1:40,000. Evaluation of specific sites and circumstances will normally require further and more detailed assessments, and will often require site investigations.