Groundwater age is a key aspect of production well vulnerability. Public drinking water supply wells typically have long screens and are expected to produce a mixture of groundwater ages. The groundwater age distributions of seven production wells of the Holten well field (Netherlands) were estimated from tritium‐helium (³H/³He), krypton‐85 (⁸⁵Kr), and argon‐39 (³⁹Ar), using a new application of a discrete age distribution model and existing mathematical models, by minimizing the uncertainty‐weighted squared differences of modeled and measured tracer concentrations. The observed tracer concentrations fitted well to a 4‐bin discrete age distribution model or a dispersion model with a fraction of old groundwater. Our results show that more than 75% of the water pumped by four shallow production wells has a groundwater age of less than 20 years and these wells are very vulnerable to recent surface contamination. More than 50% of the water pumped by three deep production wells is older than 60 years. ³H/³He samples from short screened monitoring wells surrounding the well field constrained the age stratification in the aquifer. The discrepancy between the age stratification with depth and the groundwater age distribution of the production wells showed that the well field preferentially pumps from the shallow part of the aquifer. The discrete groundwater age distribution model appears to be a suitable approach in settings where the shape of the age distribution cannot be assumed to follow a simple mathematical model, such as a production well field where wells compete for capture area.

One of the principal questions in hydrology is how and when water leaves the critical zone storage as either stream flow or evapotranspiration. We investigated subsurface water storage and storage selection of the Southern Sierra Critical Zone Observatory (California, USA) within the age‐ranked storage selection framework, constrained by a novel combination of cosmogenic radioactive and stable isotopes: tritium, sodium‐22, sulfur‐35, and oxygen‐18. We found a significant positive correlation between tritium and stream flow rate and between sulfur‐35 and stream flow rate, indicating that the age distribution of stream flow varies with stream flow rate. Storage selection functions that vary with stream flow rate are better able to reproduce tritium concentrations in stream flow than functions that are constant in time. For the Southern Sierra Critical Zone, there is a strong preference to discharge the oldest water in storage during dry conditions but only a weak preference for younger water during wet conditions. The preference of evapotranspiration for younger water, constrained by oxygen‐18 in stream water, is essential to parameterize subsurface storage but needs to be confirmed by isotopic or other investigations of evapotranspiration. This is the first study to illustrate how a combination of cosmogenic radioactive isotopes reveals the hydrochronology and water storage dynamics of catchments, constrains the subsurface architecture of the critical zone, and provides insight into landscape evolution.

The atmospheric abundance of ⁸¹Kr is a global integrator of cosmic rays. It is insensitive to climate shifts, geographical variations, and short‐term solar cycle activity, making it an ideal standard to test models of cosmic‐ray flux on the time scale of 10⁵ years. Here we present the first calculation of absolute ⁸¹Kr production rates in the atmosphere, and a measurement of the atmospheric ⁸¹Kr/Kr abundance via the Atom Trap Trace Analysis method. The measurement result significantly deviates from previously reported values. The agreement between measurement and model prediction supports the current understanding of the production mechanisms. Additionally, the calculated ⁸¹Kr atmospheric inventory over the past 1.5 Myr provides a more accurate input function for radiokrypton dating.