Besides Tritium, the most frequently used radio-active environmental isotope is Carbon 14. Both isotopes makes it a suitable tool for the dating of ground water, ranging from months to about 50 years for Tritium, and about 1.000 years to maximum of 50.000 years for Carbon 14. The age of ground water commonly used for water supplies fits into the range. Beside this re-calculating the mean residence time, you can also calculate, or get information, about the travel time, and about the flow velocity.