By Trista McKenzie
Since returning from fieldwork in Sydney, I have been spending a lot of time in the lab processing samples. Radium is one type of sample we are analyzing, which is a naturally-occurring radioactive tracer for groundwater. One of the advantages of using radium is that there are four naturally-occurring isotopes, all with different half-lives. One half-life represents the amount of time for half of the unstable atom in the nucleus of a radioactive isotope to undergo radioactive decay. This physical property can also be understood as an exponentially decreasing relationship over many half-lives, where by the time an isotope has undergone seven half-lives, less than 1% of the initial radioactivity still exists (Figure 1).
The span of half-lives represented between the four radium isotopes (Table 1) can help us understand not only groundwater discharge but also tidal dynamics and how the groundwater transits once discharged to surface water.
Table 1: Radium isotopes analyzed and associated half-lives.
To measure the concentrations of radium isotopes in water, we take the manganese-fibers that our water samples were passed through and analyze them on a Radium Delayed Coincidence Counter (RaDeCC; Moore & Arnold, 1996). The first run (analysis of 223Ra and 224Ra) is completed immediately after sample collection to capture the isotopes with shorter half-lives. The fibers are first cleaned and washed with radium-free water and then placed into columns for counting. The counting process here is fairly quick, and all of my samples were able to be analyzed within two labor-intensive days.
After this, the fibers are placed into air-tight containers for at least one week for 226Ra analysis, which will allow for ingrowth of 222Rn proportional to the 226Ra concentration (Peterson et al., 2009). These are then analyzed on the same instrument following a modified procedure. Compared to 223Ra and 224Ra analysis, 226Ra is much more time intensive - only 8 samples (or 16 if you start the day early) can be run per day.
Once 226Ra is analyzed, provided enough time has passed since sample collection, the samples can be analyzed for 228Ra following a similar protocol to the initial run.
I have one more week in Australia and it feels like I still have a lot to finish!