Groundwater dating tools (3H, 3He, 4He, CFC-12, SF6) coupled with hydrochemistry to evaluate the hydrogeological functioning of complex evaporite-karst settings.

Abstract

The hydrogeological functioning of four different areas in a complex evaporite-karst unit of predominantly aquitard behavior in S Spain was investigated. Environmental dating tracers (3H, 3He, 4He, CFC-12, SF6) and hydrochemical data were determined from spring samples to identify and characterize groundwater flow components of different residence times. Results show a general geochemical evolution pattern, from recharge to discharge areas, evidencing longer water-rock interaction. Ne values show degassing of most of the samples, favored by the high salinity of groundwater and the development of karstification so all the considered gases were corrected according to the Ne measurements. The presence of modern groundwater in every sample was proved by the detection of 3H and CFC-12, while radiogenic 4He in most samples also indicates an old component. The 3H/3He dating method does not give reliable ages as a consequence of degassing and the large uncertainty of the 3He/4He ratios of the sources for the radiogenic Helium. The large SF6 concentrations suggest terrigenic production related to halite and dolomite. Binary Mixing and free-shape models were created based on 3H and CFC-12 data to interpret the age distribution of the samples. Two parameters (GA50 and >70%) were proposed as an indicator of that distribution, as they provide further information than the mean age. Particularly, GA50 is derived from the median groundwater age and is presented as a new way of interpreting mixed groundwater age data. A greater fraction of old groundwater (3H and CFC-12 free) was identified in discharge areas, while the proportion and estimated infiltration date of the younger fractions in recharge areas were higher and more recent, respectively. The application of different approaches has been useful to corroborate a previous conceptual model proposed for the study area and to test the applicability of environmental tracer in dating brine groundwater and karst springs.