Quantitative analysis of Cenozoic faults and fractures and their impact on groundwater flow in the bedrock aquifers of Ireland | Analyse quantitative des failles et fractures cénozoïques et de leur impact sur l’écoulement des eaux souterraines dans les aquifères de roches dures d’Irlande Análisis cuantitativo de fallas y fracturas cenozoicas y su impacto en el flujo de aguas subterráneas en los acuíferos de basamento en Irlanda 爱尔兰基岩含水层新生代断层和裂缝的定量分析及其对地下水流动的影响 Análise quantitativa de falhas e fraturas Cenozóicas e seu impacto no fluxo de água subterrânea nos aquíferos rochosos da Irlanda

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 post-Devonian 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 scale-independent 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.