Groundwater dating applied for geological disposal of radioactive waste

—A review of methods employed worldwide—

Abstract

Groundwater dating methods employed in projects to develop deep geological repositories for radioactive wastes, or to research technologies and methods that may be used when developing a repository, have been reviewed. The reviewed projects are being, or have been, undertaken in Japan, Finland, Sweden, Belgium, the UK, Germany, France, Switzerland, Canada and the U.S.A.. A wide range of actual and potential repository host rock types and hydrogeological settings have been investigated. The most commonly used dating methods are based on physical hydrogeology and measurements of δ¹⁸O/¹⁶O, ³H, ¹⁴C, ³⁶Cl and ⁴He in groundwater or pore water. Only two of the reviewed projects have used noble gas data to estimate recharge temperatures, and hence deduce the timing of recharge. A single project used ¹²⁹I. Analyses of U-series isotopes have generally not been used successfully, owing to the complex radiological and redox processes that influence U migration, which complicates the interpretation of such U-series data. None of the reviewed projects have used methods based on Tritium (³H)–Helium-3 (³He),Chlorofluorocarbons (CFCs)or ⁸⁵Kr. These methods have not been needed because they indicate the presence of water recharged within the last ca. 50 – ca. 60 years, which could be identified readily at all the sites by the presence of ³H. In the case of ⁸¹Kr, sampling and analytical difficulties have probably prevented its use. Recently, improved analytical techniques enable smaller samples to be analyzed, but ⁸¹Kr gives similar information to ³⁶Cl, which can be investigated more easily. Whatever the combinations of methods that have been used in a particular program, their results have invariably been interpreted in combination with one another and in the context of other information that is available for the investigated site. That is, a particular groundwater dating method is never applied in isolation.