CBC has this story and they mention it's zenon isotopes that are measured to determine the age of the water.
>The researchers estimated how old the water was based on an analysis of the xenon gas dissolved in it. Like many other elements, xenon comes in forms with different masses, known as isotopes. The water in the Timmins mine contained an unusually high level of lighter isotopes of xenon that are thought to have come from the Earth's atmosphere at the time it became trapped.
edit: I found this http://earthobservatory.nasa.gov/Features/Paleoclimatology_O...