Dating of groundwater with 85Kr, 39Ar and 81Kr

⁸⁵Kr (t_1/2=10.7 a), ³⁹Ar (t_1/2=268 a) and ⁸¹Kr (t_1/2=229 ka) are valuable isotopes for radiometric dating of groundwater, especially due to their gaseous properties and chemical inertness. Together with ¹⁴C, these radioisotopes cover the age range from present back to 1.5 million years. Due to their extremely low environmental abundances of 10^-17…10^-11, corresponding to only a few thousand atoms per kilogram of water or ice, the detection of these isotopes is very challenging. In the recent two decades, the laser-based method Atom Trap Trace Analysis (ATTA) has succeeded in measuring these radioisotopes in water and ice samples of <10 kg, enabling applications in groundwater, ocean water and glacier ice.

Here, we present dating of groundwater with ⁸⁵Kr, ³⁹Ar and ⁸¹Kr, using ATTA for the radioisotope measurement. Recent progress on high precision ⁸¹Kr analysis has closed the dating gap between ¹⁴C and ⁸¹Kr, allowing for ⁸¹Kr dating of groundwater from the last glacial maximum to the beginning of the Holocene. Crucial advances in the ATTA instruments have moreover enabled a sample size reduction down to 1 kg of water or ice, allowing for dating of groundwater also under special conditions, such as in fractured rock aquifers with very low flow rate. The smaller sample size also facilitates simplified sampling schemes, e.g. sampling water directly instead of degassing it in the field.