Work

A controlled and scalable noble gas injection method for quantitative tracer tests in hydrogeological studies

Year: 2026

Type: article

Abstract: • Noble gases are ideal artificial hydrogeologic tracers in sensitive environments • Their widespread adoption has been limited by complex injection, sampling and analysis techniques • We develop a method whereby controlled amounts of noble gases are quickly injected into the subsurface • Noble gas breakthrough allows accurate determination of flow characteristics and tracer recovery • Performance of noble gases as artificial tracers compares favourably with fluorescent dyes Dissolved noble gases have been recognized for decades as ideal artificial hydro(geo)logical tracers, as they are chemically inert, invisible, and non-toxic. However, their widespread adoption has historically been limited by difficult tracer injection, sampling, and analysis procedures. Developments in portable, high-resolution dissolved gas measurement technology over the last two decades have rekindled interest in the use of gas tracer methods for routine hydrogeological investigations, such as well-to-well tracer tests, intra-well tests, or studies of river infiltration towards alluvial aquifers. The application of gases in aqueous environments still poses unique challenges compared to other tracer methods, as potential exsolution and degassing needs to be accounted for, and if possible avoided, during tracer injection. Here we present a simple and efficient methodology that addresses these challenges and allows the efficient, on-site preparation and injection of highly concentrated tracer solutions with controlled dissolved gas concentrations. We applied the method in a large drinking water wellfield and performed well-to-well tracer tests in an unconfined aquifer using helium-4 ( 4 He), neon-20 ( 20 Ne) and krypton-84 ( 84 Kr). Known tracer quantities were injected together with fluorescent dyes into an observation well upgradient of a pumping well. Gas tracer breakthrough was monitored in the pumping well with a portable mass spectrometer. Breakthrough curves of 4 He and 84 Kr compared favorably with fluorescent dye tracers, and enabled reliable estimates of groundwater flow velocities, travel times, and tracer recovery. These findings illustrate how noble gases can substitute or complement other artificial tracer methods, even in large-scale settings. The methodology can be extended to other gases (e.g., neon-22, xenon isotopes, light hydrocarbons), significantly expanding the range of artificial tracers available for routine hydrogeological investigations.

Source: Water Research

DOI: 10.1016/j.watres.2026.125505

Authors: Morgan Peel, Kai Solanki, Philip Brunner, Daniel Hunkeler, Oliver S. Schilling, Rolf Kipfer