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

Abstract Lough Gur is a shallow groundwater fed eutrophic lake situated within a small agricultural catchment containing volcanic and karst rock features in mid‐west Ireland. Seasonally active conduits linking two spring discharge locations from the lake under high flow conditions were revealed using dye tracing and a terrestrial geophysical survey, highlighting the architecture of the conduit flow path from Lough Gur to its discharge spring. A radon survey combined with a lake geophysical survey identified the locations of in‐lake discharge springs and thickness of the lakebed sediments. Falling head hydraulic characterization experiments illustrated the heterogenous nature of lakebed sediments and hydrograph analysis coupled with stable isotopes of water ( δ 18 O and δ 2 H) revealed significant surface water ‐ groundwater interaction during high flow periods. Significantly, δ 18 O and δ 2 H signatures plot above the global meteoric water line and local meteoric water line indicating hydration of silicate minerals and direct isotope exchange of δ 18 O between water and rock minerals. Groundwater δ 18 O and δ 2 H signatures during low flow periods indicate that recharge sources are influenced by enriched surface waters and precipitation while a wider range 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 signatures confirm widespread surface water‐groundwater interaction/mixing. Hydrochemical analyses confirm both silicate weathering and carbonate dissolution as primary geochemical processes with Mg/Ca ratios suggesting greater groundwater residence time during low flow periods. Correlations between δ 13 C DIC and dissolved organic carbon suggest a seasonal switch in the source of DIC to groundwaters between the oxidation of organic matter in summer and dissolution of carbonate minerals in winter. The SI saturation index for calcite (SI C ) illustrates calcium carbonate precipitation along with CO 2 evasion to be a perennial processes. Finally, the spatial variation for nitrate isotopic signatures ( δ 18 O NO3‐ and δ 15 N NO3‐ ) 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.