Groundwater recharge and residence times evaluated by isotopes of hydrogen and oxygen, noble gases and CFCs in a mountain catchment in the Jizera Mts., northern Czech Republic

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Groundwater recharge amounts and residence times were estimated in the quaternary aquifer of the Jizera Mountains, Czech Republic, using noble gases, ²H, ¹⁸O, ³H and CFCs. Tracer ages from years to decades were determined for water in a shallow sedimentary aquifer (three boreholes of 10, 20 and 30 m depth). Approximately 90–180 mm of precipitation (7–14% of annual precipitation) infiltrates to deep percolation. A mean percolation velocity of 0.6 m/y in the sedimentary Uhlířská aquifer was estimated based on an apparent groundwater age of 40 years at 30 m depth. A lumped parameter approach was used to calculate groundwater residence times based on synoptic evaluation of ³H/³He, ³H and CFCs. Groundwater apparent ages in the shallowest borehole (10 m depth) determined by ³H/³He are younger than those determined by CFCs. This discrepancy is caused by partial re-equilibration of water with modern air, and by admixing of water that has zero age. The pronounced variations of δ¹⁸O values in the borehole may be ascribed to a young component quickly recharged from the air-exchanged stream water or through preferential flowpaths.