A global helium clock for groundwater residence time.

Area of Science:

• Geochemistry
• Isotope Hydrology
• Hydrogeology

Background:

• Radiogenic helium (⁴He) is a potential groundwater age indicator, but its use is limited by site-specific factors integrating complex processes.
• Krypton-81 (⁸¹Kr) is a precise chronometer for old groundwater, but paired ⁴He-⁸¹Kr data are scarce, hindering quantitative helium age applications.

Purpose of the Study:

• To compile and analyze a global dataset of paired radiogenic helium (⁴He) and krypton-81 (⁸¹Kr) groundwater ages.
• To establish a robust, transferable empirical scaling relationship between dissolved ⁴He and groundwater residence time.
• To validate helium as a first-order proxy for groundwater age across diverse aquifer systems.

Main Methods:

• Compilation of an expanded global dataset of groundwater samples.
• Measurement of dissolved radiogenic helium (⁴He) concentrations.
• Independent determination of groundwater residence times using krypton-81 (⁸¹Kr) dating.

Main Results:

• Dissolved ⁴He concentrations show a monotonic increase with groundwater age across nearly three orders of magnitude.
• A simple empirical scaling relationship for waters older than ~50 kyr was identified and validated across diverse basins.
• Over 80% of helium-derived ages agreed with ⁸¹Kr residence times within a factor of three.

Conclusions:

• Radiogenic helium (⁴He), when calibrated with an absolute chronometer like ⁸¹Kr, serves as a transferable proxy for groundwater residence time.
• This approach provides a practical method for global groundwater age screening.
• It enables targeted application of high-precision dating methods in hydrogeological studies.