Geospatial drivers of the groundwater δ¹⁸O isoscape in a temperate maritime climate (Republic of Ireland)

In recent years, the concept of “isoscapes” has been used to describe spatiotemporal stable isotope distributions within natural environments including groundwater systems at multiple scales. This study presents an updated groundwater δ¹⁸O isoscape for the Republic of Ireland and describes the climatic and geological drivers influencing ¹⁸O composition. In all, 142 geographically and geologically representative groundwater installations were sampled and analysed, in addition to 35 samples from six nested boreholes. Geospatially, Irish groundwater exhibits high δ¹⁸O values in southern and western coastal regions, becoming progressively depleted inland before reaching lowest measured values along the eastern coast, equating to a national groundwater δ¹⁸O profile range of approximately 3‰. Groundwater δ¹⁸O composition is primarily driven by location with respect to orographically influenced rainfall deposition and annual precipitation volume, with a bias towards winter recharge. Results also demonstrate that local/regional (hydro)geological setting exerts a secondary influence on δ¹⁸O composition via infiltration and recharge mechanisms. “Flashy” groundwater systems (e.g. karst limestones) are more likely to exhibit seasonal groundwater δ¹⁸O patterns, whereas low productivity systems with high residence times (e.g. granites), tend to display a dampened composition to groundwater recharge and exhibit more temporally constant δ¹⁸O values. The derived δ¹⁸O isoscape enhances current understanding of what is a geologically distinct groundwater setting, whilst also potentially serving as a δ¹⁸O “sentinel” for continental Europe, with respect to both groundwater and precipitation, due to Ireland's geographic location.