Argon-39: new possibilities for groundwater dating up to 1000 years from ATTA

Full text: While several methods exist to assess groundwater flow and transport on time scales below a century (3H, CFCs, SF6, 85Kr), a severe practical dating gap exists for centuries up to few millennia. This time scale is very important because it covers the strongest impact of man on groundwater systems, like land clearance or the agricultural revolution. The radioactive isotope argon-39 (39Ar) has a half-life of 269 years and is the ideal, and only, tracer for dating water in the time range of 50–1000 years. However, its routine application was difficult due to its extremely low atmospheric abundance (39Ar/Ar = 8.2 E-16). Over the last 30 years, Low Level Counting (LLC) was the only viable technique for detecting 39Ar with reasonable effort. In the last decade a laser-based atom counting method, Atom Trap Trace Analysis (ATTA), became a promising alternative for dating groundwater [Ritterbusch et al. 2014]. For LLC methods, 1-3 tons of water must be degassed in the field to obtain enough gas for the required 600 ml of argon, whereas the required sample size for ATTA is presently only 10–25 L water. This makes groundwater sampling practical by removing the need for degassing in the field, but it also allows applications in other environmental reservoirs, like dating of glacier ice or ocean water. Besides the new analytic method, various sampling protocols and preparation setups were developed to link the fieldwork with the new detection technique. In this talk, a description of the techniques required for Ar-39 dating of groundwater will be given, starting from sample degassing in the field, followed by the sample purification in the lab and the analysis with LLC or ATTA. The added value of 39Ar to studying hydrological processes will be demonstrated in recent groundwater studies as well as in an outlook to applications in other systems. (author)