TRIFIN: TRItium Facility via INgrowth

Tritium (³H₂O) is produced naturally in the atmosphere, where it enters the hydrologic cycle. Tritium concentrations in rain prior to nuclear testing were of the order of a few tritium units (TU) (3.1 TU in Australia). After the atmospheric nuclear bomb tests in the 1960s, tritium concentrations in precipitation peaked in the northern hemisphere during 1963, rising to a few thousand TUs in most mid-continental locations. In the southern hemisphere, the peak tritium concentrations in precipitation was lower at around 70 TU, resulting in more challenging measurement requirements of lower detection limits. Current tritium levels in precipitation vary between 2-3 TU across Australia. After deposition on the continents, tritium entered into the vadose zone, surface water and groundwater. At the simplest level, the presence of tritium in groundwater implies that at least a fraction of the water in the aquifer has been recharged since the beginning of the bomb era. As a result, information on time scales for hydrologic processes can be obtained from tritium measurements. In hydrology, tritium has proven to be useful in studies on the movement of water through the unsaturated zone, on time scales for physical processes occurring in surface waters and groundwaters, and interactions between groundwater and surface water systems.

Tritium in environmental samples such as water naturally decays to Helium-3 (³He) which can be measured by noble gas mass spectrometry using CSIRO’s TRIFIN (TRItium Facility via Ingrowth). Based on the unique features of mass spectrometry combined with a highly-automated and precise preparation of water samples, tritium measurements via ³He will have much greater sensitivity, require smaller sample volumes and complete the measurement in a shorter turn-around time compared to conventional methods.

Owing to these features, the TRIFIN facility will accelerate innovation in hydrological sciences in a number of areas, including:

• Ability to measure tritium at lower detection limits owing to the lower concentrations of bomb-tritium in rainwater on the Southern Hemisphere
• Better detection limits will improve the assessment of different age components in mixed waters
• Water-limited environments including dry unsaturated zones are rarely sampled because insufficient sample volumes can be extracted for traditional tritium measurements; tritium measurements via ingrowth with He-3 require smaller sample volumes thus unlocking new opportunities for unsaturated zone science
• An ingrowth device delivers greater throughput at lower costs: (1) more samples can be sampled and analysed, delivering more comprehensive data sets for system conceptualisation and groundwater model calibration, (2) time-series with high temporal resolution can be established for monitoring streamflow (e.g. low flows)
• The ability to accurately measure more samples at lower cost provides opportunities for involving non-specialists in sampling, thus promoting citizen science.

Noble gas and tritium measurements are also useful in lake studies or oceanography to characterise water masses, gas exchange, and the temperature of last equilibration with the atmosphere. The TRIFIN facility will be able to measure tritium with excellent detection limits via the ingrowth method of ³He.