Water stable isotopes, radiocarbon, noble gases and krypton-81 study of thermal groundwater from Upper to Mid-Pleistocene recharge age in deep aquifers of Argentina.

Recharge environmental conditions and residence time, can be studied by the application of different tracers. Several tracers are useful as proxies of the environmental recharge conditions, such as water stable isotopes deuterium and oxygen-18, and the dissolved noble gases. Other tracers are applied in order to know when the recharge occurred. Carbon-14 dating is a widely applied method for dating old groundwater, having an application range up to around 30 ky. Noble gases, as non-reactive and water-soluble substances, constitute useful tracers for studying different processes in hydrologic cycles. One of the applications is dating very old groundwater beyond the range of ¹⁴C. It can be done in a semi-quantitative way by the accumulation of ⁴He, and quantitatively through the radionuclide ⁸¹Kr (t_1/2 = 229,000 y), a more robust method for dating groundwater up to 1.3 million years.

The province of Buenos Aires, Argentina, hosts three deep sedimentary basins, from north to south, Salado, Claromecó and Colorado, with areas of 85,000 km², 3,100 km² and 125,000 km², respectively. In these basins, a thick continental sequence of Neogene sediments contains confined thermal aquifers, at depths from hundreds meters to more than 1 km. The recharge conditions and the water age of the Neogene aquifers are studied through water stable isotopes, ⁴He and ⁸¹Kr tracers.  10 deep wells were sampled for δ²H, δ¹⁸O, ³H, ¹⁴C, for noble gases using clamped copper tubes, and for ⁸¹Kr with a gas extractor. ⁴He analyses were performed at the IAEA laboratory by mass spectrometry, and ⁸¹Kr at the ATTA laboratory of USTC.

³H contents were not detectable in all of the cases, thus no young water components exist. By plotting the isotopic results in a δ²H vs δ¹⁸O diagram, four groups of samples can be recognized. Group 1 includes Colorado basin isotopically depleted samples (δ¹⁸O from -6.5 to -7.5 ‰) along a line parallel to the GMWL and the present LMWL, but with a higher deuterium-excess (d). Samples in G1 have a Ne/He ratio around 0.6. ¹⁴C and ⁸¹Kr ages were from 10 ky to 40 ky. Group 2 includes the samples of the borders of the Salado basin, being isotopically more enriched (δ¹⁸O from -3 to -4.5 ‰) and with a lower d than present precipitation, a Ne/He ratio from 0.2 to 0.8 and one sample with ⁸¹Kr age of 640 ky. Group 3 is formed by brines from the Colorado basin, a Ne/He ratio in the range of 1E-02 to 1E-04, and ⁸¹Kr in ages around 900 ky, and are along a line of slope 1.9, showing a ¹⁸O shift. Finally the Group 4 formed by samples at the axis of Salado basin, are isotopically enriched (δ¹⁸O from -0.5 to -3.7 ‰) along a line of slope 3.9 resembling and evaporation line. However, these samples of ⁸¹Kr ages of 1000 ky and Ne/He ratio of 2E-03, showed a high correlation Cl- vs δ¹⁸O, with increasing values from West to East. This suggest a mixing with a brine or an increasing water-rock interaction.