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 category assigned to a site or an area 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.

Conceptually therefore, the vulnerability can be related to the recharge acceptance rate or the recharge potential at any given site or area:

• In areas where recharge occurs more readily, a higher quantity of introduced contaminants will have access to groundwater;
• In areas where recharge is rapid, contaminants may quickly enter groundwater. ​

Click here for Geological Survey Ireland's Guidelines for Assessment and Mapping of Groundwater Vulnerability to Contamination report, 2003