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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

Abstract:

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 Uhlírská 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 d18O values in the borehole may be ascribed to a young component quickly recharged from the air-exchanged stream water or through preferential flowpaths.