The Issue of Very Old Groundwaters

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

70 million years. This may well be the age of the water entrapment due to tectonic events that disconnected the system from its former drainage system. 

The accumulation of radiogenic helium in confined groundwaters is mainly determined by the duration of water entrapment. Applying the data available on uranium and thorium concentrations in the host rocks and rock water ratios, reliable, though semiquantitative, dating is possible in the range of 104 10s years. 

Contributions of mantle helium (mainly via magmas and water-dominated fluids) may be detected and corrected by 3He 4He ratios. 

In order to date groundwaters, they have to be studied for possible mixing of waters of different ages. Helium concentrations should be applied for the calculation of meaningful ages only for nonmixed water or for reconstructed end members. 

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