[advantager]

To expand on this

Groundwater "age" is typically considered in terms of residence time, that is the period of time between entering the subsurface and exiting at some discharge location. These analyses are often based upon the hydrogeochemistry and the decay rate of known isotopes, hence the mass spectrometer.

Shallow, unconfined aquifers often contain "young" groundwater, on the order of decades or centuries. For example snow melts on a mountain, downhill into a flatter coarse grained area where it enters the groundwater system. The groundwater then moves downdip along the bedrock contour through and into the deeper alluvial basin, eventually naturally discharging into a river or is pumped out at a well.

It would be interesting to read their paper to understand how they (i.e. what isotope) can date back to 1.6 billion years.

https://books.gw-project.org/groundwater-in-our-water-cycle/...