Realizing the Potential of ATTA to Enhance Groundwater Dating

The noble gas radioisotopes ⁸⁵Kr (T_1/2 = 10.74 yr), ³⁹Ar (T_1/2 = 269 yr), and ⁸¹Kr (T_1/2 = 229 kyr) are reliable tools for groundwater dating, covering a wide range of residence times. In particular, the combination of ⁸⁵Kr and ³⁹Ar can improve the constraints of transit time distributions (TTDs) in the young to medium age range, typical for many exploited aquifers. Recent analytical progress of the atom trap trace analysis (ATTA) technique now renders routine analysis of these tracers feasible. Sample size requirements have been reduced to a point that allows sample collection without the need for highly specialized equipment and experience. For ³⁹Ar, the collection of about 10 liters of water in pre-degassed gas bottles is a straightforward solution. Comparatively small and simple in-field degassing systems to handle the somewhat larger water amounts required for Kr isotopes and to facilitate sample shipment are also possible.

The first groundwater sampling based on 12-L propane gas bottles for ³⁹Ar water samples was conducted in May 2018 in the semi-arid coastal plain around the city of Salalah, Dhofar governorate in southern Oman. This multi-tracer study also comprised the tracers SF₆, CFCs, ³H, ¹⁴C, and noble gases. The results of the ³⁹Ar analyses proved to be of critical importance to constrain TTDs and mixing proportions as well as ages of young water, containing transient tracers, and old water free of these tracers. The results of the study are thus useful to obtain a better understanding of the regional aquifer system in order to enable its sustainable exploitation.

Additional groundwater studies based on ³⁹Ar dating are currently ongoing. One main goal is to combine the ³⁹Ar chronology with precise noble gas temperature determinations, in order to reconstruct soil temperature histories over the past millennium.