A global helium clock for groundwater residence time.

Radiogenic helium (⁴He) has long been recognised as a potential indicator of groundwater residence time, but its quantitative application has remained largely site-dependent because helium concentrations integrate production, crustal fluxes, transport and mixing processes. In contrast, krypton-81 (⁸¹Kr) provides a conservative chronometer for old groundwater, yet paired ⁴He-⁸¹Kr datasets have remained extremely scarce. Here we compile a uniquely expanded global dataset of groundwater samples with paired dissolved radiogenic ⁴He concentrations and independently determined ⁸¹Kr residence times across diverse aquifer systems. Dissolved ⁴He increases monotonically with groundwater age over nearly three orders of magnitude and follows a simple empirical scaling for waters older than ~ 50 kyr that is robust to the exclusion of individual basins. More than 80% of helium-derived ages agree with corresponding ⁸¹Kr residence times within a factor of three under conservative cross-basin validation. These results demonstrate that radiogenic helium, when anchored to an absolute chronometer, can serve as a transferable first-order proxy for groundwater residence time, providing a practical basis for global groundwater age screening and targeted application of high-precision dating methods.