Tracing groundwater over the past one million years in deep aquifers of Argentina using noble gases and krypton-81

Abstract:

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 several hundreds to more than 1 km. This study provides insight into the Neogene aquifers by the application of ⁴He and ⁸¹Kr tracers for dating. 10 deep wells were sampled 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 and ¹⁴C were sampled and determined at external laboratories.

³H contents were not detectable in all of the cases, thus no young water components exist. ⁴He were determined to be around 10^-6 to 10^-5 cm³ STP/g, several orders of magnitude above the atmospheric level, indicating very old groundwater. Samples are beyond the ¹⁴C range along the borders of the Salado Basin, one of them was found to be 640 ky old using ⁸¹Kr. Ages increase to 1,050 ky at the basin axis. The large ⁴He concentrations, from 8 x 10^-6 to 2 x 10^-5 cm³ STP/g, reveal an external basal flux. The only well sampled in the Claromecó basin gives a ⁸¹Kr age of 630 ky. In the Colorado basin recharge takes place at the north, with ¹⁴C ages of 20 ky and ⁸¹Kr ages of 40 ky. Towards the south the maximum ⁸¹Kr ages of 920 ky were determined.

The Neogene sedimentation is related to the tectonic evolution of the basins since the opening of the Atlantic. The Pliocene marine ingression deposited a thick clay layer, isolating it from the surface. Recharge occurs close to the mountain ranges where these clays disappear, but in most of the area they are confined, with a slow movement and mixing with connate waters, resulting in ages around one million years.