Recharge velocity and geochemical evolution for the Permo-Triassic Sherwood Sandstone, Northern Ireland

The Triassic Sherwood Sandstone Group is a major European aquifer system. It is also the principal groundwater source in Northern Ireland. However, key aspects of its regional hydrogeology, such as age distribution and geochemical evolution, remain largely unknown. Here the groundwater geochemistry and isotopic composition were investigated in order to evaluate groundwater recharge and flow processes in a complex regional hydrogeological setting. The dominant geochemical processes, such as dissolution of carbonate cements were determined from the major and trace element chemistry. Stable and radio-isotope measurements were taken as residence time and flow path indicators and, together with physical and geochemical groundwater modelling, revealed groundwater ages of up to 9000 years. The importance of infiltration from overland flow from springs deriving water from the adjacent Cretaceous chalk aquifer and subsequently re-infiltrating into the Sherwood Sandstone was confirmed. In addition, evidence was found of a slow recharge component through low conductivity mudstones that yielded significant groundwater resident times throughout the Lagan Valley. These findings provide improved understanding of groundwater flow processes in Northern Ireland and serve as an example of methods that can be applied to water management elsewhere.