Abstract - Faults and fractures are a critical store and pathway for groundwater in Ireland’s limestone bedrock aquifers either directly as
conductive structures or indirectly as the locus for the development of karst conduits. From the quantitative analysis of postDevonian faults and fractures in a range of lithological sequences, this report describes the principal characteristics of Cenozoic
strike-slip faults and joints, the youngest and the most intrinsically conductive fractures within Irish bedrock. Analysis of these
structures in more than 120 outcrop, quarry, mine and cave locations in a range of bedrock types, provides a basis for: (1)
definition of quantitative models for their depth dependency, lithological control, scaling systematics and links to preexisting
structure, (2) conceptualisation of their impact on groundwater behaviour, and (3) estimation of groundwater flow parameters.
The quantitative models provide constraints on fracture-controlled flow connectivity. Commonly observed decreases in sustainable flows and water strike interceptions with depth are attributed to increasing confinement and decreasing fracture connectivity
and dissolution. Faults and joints have quite different end member geometries, with faults having strongly heterogeneous scaleindependent properties and joints more often showing scale-dependent stratabound properties. The highest and most sustainable
groundwater flows are usually associated with the complexity of structure of Cenozoic faults and of preexisting Carboniferous
structures (on which conductive fracturing localises), enhanced by karstification and strongly jointed limestone bedrock particularly in the near-surface. Increased groundwater flow is promoted within bedded, rather than massive (i.e. unbedded), limestone
sequences, characterised by bedding-parallel fractures and karst connecting otherwise subvertical fractures and subvertical wells.
Hydrogeology Journal, doi: 10.1007/s10040-021-02395-z, 2021.