Terrestrial and marine electrical resistivity to identify groundwater pathways in coastal karst aquifers

Groundwater movement in karst aquifers is characterised by high-velocity fissure and conduit flow paths, and in coastal karst aquifers, these act as pathways for saline intrusion and freshwater discharge to the sea. This paper examines groundwater movement in two neighbouring catchments in the west of Ireland that represent canonical coastal karst aquifers dominated by discharges in the intertidal zone and at offshore submarine springs. Terrestrial and surface-towed marine electrical resistivity tomography, coupled with ancillary hydrogeological data, identifies the influence of faulting and conduits on groundwater egress/saltwater ingress. The on-shore and off-shore subsurface geometry of major fault zones is identified, and the tidal influence of seawater and groundwater flow is demonstrated in these zones and karst springs. Imaging of these sub-surface structures is a pre-requisite for numerical modelling of current and future climate-driven freshwater-seawater interactions in karst coastal aquifers.