Tritium- tracing the flow beneath us

I’m so excited about today’s song because you get to compose your own!  I will warn you that it can be quite addictive but it’s a fun way to explore today’s Earth theme. Music by you courtesy of The Radioactive Orchestra.

Having survived the Christchurch earthquake, I have witnessed the myriad changes going on in the city and its people. And while the shaking has diminished, it seems there’s no end to the turmoil that they’ve caused through changes to the structure which lies beneath the city.

The floods earlier this month were an added pressure that many locals could have done without. This brief video (from Movie467) needs no narrative or sound accompaniment.

That’s a lot of water!

Where does all that water go?

Quite a lot runs off straight out to sea. A recent satellite image from Nasa has captured it nicely.

But what happens to the rest? Has it changed since the earthquake? These are some of the questions that scientists are working on.

The city sits on top of a large artesian aquifer which supplies Christchurch’s drinking water. An artesian aquifer is an underground reservoir in which the water is under pressure due to the weight of the water above it. It remains under pressure because the water flows slowly through some layers of rock which are porous, but is confined from above and below by impermeable rock layers. If wells are drilled down through the impermeable rock layer (aquitard) it releases the pressure. Often the water is under enough pressure that it will reach the surface.

Surface water trickles down through the surface eventually makes its way into the groundwater. It continues to move sideways through the rock layers along particular flow paths. Scientists can trace the different sources of water that make their way into the aquifer and measure differences in the ages of the waters within it. They do this by using a radioactive isotope of hydrogen.

Wait! Radioactive? I feel another song coming on! (Radioactive by Imagine Dragons).

Enjoy that? Great – but let’s get back to those isotopes!

The nucleus of an atom is made up of protons and neutrons and is surrounded by an electron cloud. Isotopes are atoms with different numbers of neutrons. The element hydrogen has three: H (hydrogen), ²H (deuterium) and ³H (tritium). The small numbers indicate how many neutrons it has. For measuring water age, in an aquifer, scientists use tritium as a tracer. Tritium is created naturally when cosmic rays interact with gases in the atmosphere. Rain water absorbs any isotopes from the atmosphere so the content in the atmosphere got a boost from nuclear testing in the 1950s.

Radioactive decay happens when the nucleus of an atom loses some energy and gives off radiation. Luckily, tritium emits low energy rays and those rays can’t penetrate your skin. The result of that energy loss is often a transformation of that element into another one – in this case ³He (an isotope of helium).

Each radioactive element has a half-life which is the number of years it takes for half of that initial (parent) element to transform into the new one (daughter). The half life for tritium is 12.3 years. This makes it particularly useful for dating recent waters entering the groundwater. Imagine a bucket of 100 parent red balls sitting on the back porch. If you came back in 12.3 years half of those balls will have turned daughter blue. In another 12.3 years another half of the red balls will have turned blue so three quarters would now be blue and only one quarter would be red. It is the same with the decay of tritium in the water. Water travelling slowly through the aquifer will have less parent tritium and more daughter ³He in it due to the decay. But the amount of tritium in the atmosphere is also reducing since nuclear testing was halted. How can the hydrologists tell the difference between reduced tritium due to smaller amounts in the atmosphere and reduced amounts due to decay? Well, they use the ratio of tritium to ³He to estimate the ages of the waters. Records of the atmospheric content of ³H have been kept so they can establish which effect it is. In this way, use of these tracers can help map and track any changes in the pathways of the water flowing beneath the surface.