The nature and extent of bomb tritium remaining in deep vadose zone: A synthesis and prognosis

Tritium present in deep vadose zones is a useful tracer for estimating groundwater recharge, but its full utility is constrained by not knowing where and for how long the tritium tracing method remains applicable. We obtained 44 tritium profiles from 17 globally distributed sites with vadose zone thicknesses of 12-624 m and used transport models to estimate the number of years that tritium may still be useful. Results show that the method may still be usable for 26 of 44 soil profiles surveyed, mainly in China, Australia, the United States, South Africa, and Senegal, with a remaining useful period of between 6 and 83 years. We also developed a statistical model that uses outputs from a hydrological model to predict the applicability of the tritium tracing method. Global implementation of the statistical model showed that the method remains usable at 20% of Earth's land mass (excluding Antarctica and Greenland) over the next few decades. Soil tritium profiles were compiled to estimate current 1963 bomb peak distributions in deep vadose zones.Bomb-peak tritium had migrated by an average of 20 m, depending on each site's soil and climate conditions.Climate and vadose zone thickness suggest a narrowing window of time for tritium tracing applicability.