Characterization of seasonal groundwater origin and evolution processes in a geologically heterogeneous catchment using geophysical, isotopic and hydro-chemical techniques (Lough Gur, Ireland)

Lough Gur is a shallow groundwater fed eutrophic lake situated within a small agri-cultural catchment containing volcanic and karst rock features in mid-west Ireland.Seasonally active conduits linking two spring discharge locations from the lake underhigh flow conditions were revealed using dye tracing and a terrestrial geophysicalsurvey, highlighting the architecture of the conduit flow path from Lough Gur to itsdischarge spring. A radon survey combined with a lake geophysical survey identifiedthe locations of in-lake discharge springs and thickness of the lakebed sediments.Falling head hydraulic characterization experiments illustrated the heterogenousnature of lakebed sediments and hydrograph analysis coupled with stable isotopes ofwater (δ18O andδ2H) revealed significant surface water - groundwater interactionduring high flow periods. Significantly,δ18O andδ2H signatures plot above the globalmeteoric water line and local meteoric water line indicating hydration of silicate min-erals and direct isotope exchange ofδ18O between water and rock minerals. Ground-waterδ18O andδ2H signatures during low flow periods indicate that rechargesources are influenced by enriched surface waters and precipitation while a widerrange of signatures during high flow periods indicates a greater variation of sources.D-excess signatures illustrate rapid rainfall infiltration under high flow conditions,thereby demonstrating the vulnerability of the groundwater, while lake water signa-tures confirm widespread surface water-groundwater interaction/mixing. Hydroche-mical analyses confirm both silicate weathering and carbonate dissolution as primarygeochemical processes with Mg/Ca ratios suggesting greater groundwater residencetime during low flow periods. Correlations betweenδ13CDICand dissolved organiccarbon suggest a seasonal switch in the source of DIC to groundwaters between theoxidation of organic matter in summer and dissolution of carbonate minerals in winter. The SI saturation index for calcite (SIC) illustrates calcium carbonate precipitation along with CO2 evasion to be a perennial processes. Finally, the spatial variation for nitrate isotopic signatures (δ18ONO3- and δ15NNO3-) suggests a number of nitrate sources to groundwaters including soil organic nitrogen, manure and/or domestic effluent with indications of denitrification processes under low flow conditions