I’m here at Buick Street in Petone
where there is an artesian well. Lots of people come here to get their fresh drinking water. But where did this water come from?
And how long did it take to get here? Our team at GNS cience can date water to find out the answers to these questions. It turns out that the water at Petone
came from here, ten kilometres up the valley at Taita. You can see that most of the water travels on the surface of the river, but some of it percolates through
the gravel and enters the groundwater system. Now I’m going to take you to the
water dating laboratory at GNS Science to show you how we measure how long the
water travels underground to get from here to Petone. Here we are at the lab. This is where we enrich and isolate a natural isotope in the water called tritium What is tritium and where does it come from? Cosmic rays from outer space are bombarding nitrogen which is in the atmosphere. This converts a very tiny amount of the nitrogen into tritium. This is a radioactive isotope of hydrogen, only found in very tiny quantities. When tritium combines with oxygen atoms, like normal hydrogen it forms a water molecule. This water molecule is included in the rain that falls. Some of this water goes down into the groundwater system. Because tritium is an unstable
isotope, it breaks down over time. How do we measure the age of water? The groundwater travels from point A to point B.
We can take a sample of the water and measure the amount of tritium atoms
in that sample. Tritium has a half-life of 12.3 years. It means after 12.3 years, only half of the tritium atoms which were once in that sample are left. We can figure out how many years it has taken the water to travel in the ground between the two sites By looking at the number of
tritium atoms in the two samples. So imagine that one business card
represents a hydrogen atom. we could stack business cards one on top of the other, all the way to the Sun 100,000 times and in those stacks, if one of those business cards was a tritium atom, we could detect it. At GNS Science, we have the most accurate water dating lab in the world. So we know how long the water travelled to get here underground. It’s about 18 years