Determining water transit times in dynamic environments

Determining the time taken for water to pass through catchments from where it is recharged to where it discharges into streams or is sampled from within the soils or aquifers (the transit time) is vital for understanding catchment functioning. Near-surface environments are dynamic and transit times are likely to vary at different stages of the hydrological cycle. Because of the lower input of bomb-pulse tritium in the southern hemisphere it is possible to determine transit times from individual tritium measurements. Additionally, because tritium is radioactive, transit times can be estimated where the catchment is not stationary. While the transit times are subject to uncertainties, this approach allows transit times at different stages of the hydrological cycles in dynamic environments to be determined.

In several southeast Australian headwater catchments, the mean transit times of stream waters at low flows range from several years to decades. The tritium activities increase at higher flows, implying that there is an input of younger water at that time. However, the tritium activities generally remain below those of recent rainfall implying that simple dilution by recent rainfall is not occurring; that conclusion is consistent with the variation in the concentrations of other geochemical tracers at different streamflows. Rather, the variations in geochemistry are consistent with shallower younger stores of water from the soils and regolith being progressively mobilised as the catchments wet up during winter. These younger water stores typically have mean transit times of at least a few years. The generally long transit times imply that the southeast Australian headwater catchments have large storage capacities, probably due to the catchments being unglaciated and deeply weathered. The observation that the transit times at high flows are still relatively long suggest that, even though they may only be active for part of the year, the shallow water stores also have relatively large volumes.

Understanding the transit times improves our ability to predict the behaviour and management of these catchments. The large storage capacities result in the catchments being resilient to year-on-year variations in rainfall and many of the headwater streams in southeast Australia have continued to flow through recent droughts. Similarly, the streams are less susceptible to inputs of surface contamination but contaminants stored in the soil water or shallow groundwater may impact the streams over prolonged periods. As the bomb-pulse tritium decays over the next few decades, determining mean transit times from single tritium measurements will become possible in northern hemisphere catchments. This will enable a better global understanding of catchment functioning in a wider range of environments.