Evolution of groundwater can be described based on hydrogeochemical reactions such as mineral dissolution and precipitation. These reactions can take place on timescales of days to thousands of years. Confirmation of the sequence in which they occur often relies on complex hydrochemical modelling. Alternatively, estimates of the mean residence time of groundwater samples based e.g. on environmental tracers such as tritium or 14C can provide a timeline, especially in cases where it is unclear whether the samples all fall along the same flow path. One potentially very useful tracer for this is 4He because of its chemical inertness and the wide time range (millennia to a million years) covered in comparison to other tracers.
Data sets of existing groundwater studies in Australia (Peel (WA), Beetaloo Sub-Basin (NT), Surat Basin (QLD), Pilliga (NSW)) were used to evaluate the usefulness of combining 4He with hydrochemistry records (major and minor ions). Mineral saturation indices (SI) of minerals related to the specific geochemical environment of the aquifer were calculated with PHREEQC. To increase the sensitivity of 4He as a time indicator in younger groundwater, excess air models were applied to separate the atmospheric and terrigenic 4He components.
Simple scatter plots of the SI as a function of 4He reveal the order in which minerals reach saturation either because of dissolution or once they start to precipitate. This method is thus useful to infer the sequence of hydrogeochemical reactions without the need for a kinetic reaction model or it can inform the building of such a kinetic model. 4He is currently not a very widely utilized tracer, partly because of the high cost, e.g. compared to hydrochemical analyses. However, the advent of new field-going mass spectrometers (MIMS = membrane-inlet mass spectrometers) will allow the measurement of 4He directly in the field at significantly lower costs.