It’s not just the oceans we see from space that make us the blue planet. Underneath the Earth’s land masses is groundwater which starts life as rain entering underground rock systems we can’t see (aquifers). Not all groundwater is old but some of it has been there for a very long time.
Australia is a good place to investigate how long because we have some of the oldest landscapes in the world - and we are living on a very hot, flat continent. Groundwater in Australia therefore moves very slowly.
But how do we know how long? It’s called age dating.
The key to age dating comes from substances dissolved in the water. They are like footprints in the sand, a trace we can follow – and the substances we use for this we call “tracers”. Environmental tracers used until now suggest that groundwater in the south west of the Great Artesian Basin is up to two million years old. That means it entered the underground at the time the first members of the genus Homo appeared on Earth.
The radioactive decay of some substances dissolved in the water – such as isotopes of carbon and chloride – gives us a signature that we can use to calculate time since the rain fell. These isotopes are radioactive and have constant half-lives – their number drops by a factor 2 over a constant time interval – that are reliable like a watch.
So, if we know we had 100 atoms of these substances per litre in the rain water, and find only 50 atoms in the sample, then we know one half-life in time has passed. Nearly each element in the periodic table contains some of these isotopes that are radioactive, however their half-lives are vastly different. Some are good stopwatches for very young groundwater, say days or weeks old.
Others are good for much older groundwater, millennia and older. Some elements, like carbon, may not only originate from rain but can also get into the water from dissolving rocks, which disturbs our watch enormously.
But science is a process of constant refinement and we now have a new, more accurate way of age dating Australian groundwater with the Southern Hemisphere’s first Noble Gas Facility.
Source: Sydney Morning Herald