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J.M. Gil-Marquez

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Environmental dating tracers (3H, 3He, 4He, CFC-12, CFC-11, SF6) and the natural response of spring (hydrochemistry, water temperature, and hydrodynamics) were jointly used to asses mixing processes and to characterize groundwater flow in a relatively small carbonate aquifer with complex geology in South Spain. Results evidence a marked karst behavior of some temporary outlets, while some perennial springs show buffer and delayed responses to recharge events. There is also a general geochemical evolution pattern, from higher to lower altitudes, in which mineralization and the relation Mg/Ca rises, evidencing longer water-rock interaction. The large SF6 concentrations in groundwater suggest terrigenic production, while CFC-11 values are affected by sorption or degradation. The groundwater age in the perennial springs deduced from CFC-12 and 3H/3He point out to mean residence times of several decades, although the difference between both methods and the large amount of radiogenic 4He in the samples indicates a contribution of old groundwater (free of 3H and CFC-12). Lumped Parameter Models and Shape-Free Models were created based on 3H, tritiogenic 3He, CFC-12, and radiogenic 4He data in order to interpret the age distribution of the samples. The resulting groundwater-age distributions evidence the existence of two mixing components, with an old fraction ranging between 160 and 220 years. Some dating parameters derived from the mixing models and their correlation to physicochemical parameters permits to explain the hydrogeochemical processes occurring within the system. All these results prove that large time residence times are possible in small alpine systems with a clear karst behavior when the geological setting is complex, and they highlight the importance of applying different approaches, including groundwater dating techniques, to completely understand the groundwater flow regime within this type of media.