Dating ground water and the evaluation of repositories for radioactive waste

The age of ground water is the length of time that the water has been isolated from the atmospheric portion of the hydrologic cycle. It is a theoretical concept only, because all ground water to some extent is a mixture of waters of different ages. In simple systems, however, relative ages of ground water from different portions of an aquifer can be determined by different methods, and the dates obtained are commonly in accord with each other and reflect systematic increases of water ages in downgradient directions. At least nine independent methods can be used to approximate groundwater ages. The methods vary widely in precision but all give useful information. In complex ground-water systems, as many dating methods as possible should be used. Discordant ''dates'' will result which, when properly interpreted, will not give a single water age but will give valuable information concerning the hydrodynamics of the ground-water system. Dating methods which use isotopic and other geochemical techniques will read the actual history of the water and will give direct information on average ground-water conditions over long periods of time. If these periods exceed several hundred years, geochemical methods which use past conditions to predict the future are superior to hydrodynamic methods which use an extrapolation of short-term data to predict long-term hydrogeologic conditions. 312 refs., 34 figs., 26 tabs