In many catchments, changes in stream geochemistry during high streamflows following rainfall events implies little dilution of groundwater inflows by event water. Rather, much of the water contributing to the high streamflows appears to be older water from within the catchment (the "old water paradox"). However, it is unclear whether this represents water displaced from shallow stores in the soils or regolith or older regional groundwater.
Here we use tritium, major ions and stable isotopes to assess sources of water during high streamflows in streams from the Latrobe and Yarra catchments (southeast Australia). Most major ion concentrations do not vary systematically with streamflow. However, nitrate concentrations increase and silica concentrations decrease during the events and there are small but systematic changes in stable isotope ratios. Tritium activities increased from 1.4 TU to up to 2.4 TU close to the peak in streamflow and then decline over several days to pre-high flow values. The peak tritium activities are lower than those of the rainfall that generated the high flow events (~3 TU) but within the range of tritium activities commonly recorded in soil water in southeast Australia (2.0 to 2.6 TU). The combined geochemical data imply significant water input to these streams during high streamflows from shallower stores with residence times of <5 years rather than deeper regional groundwater.
Catchments contain multiple stores of water (old regional groundwater, younger shallow groundwater, soil water, interflow). Thus, a multi-tracer approach is required to apportion the contribution of water from these stores during high streamflows . Tritium provides the opportunity to directly assess how the average residence time of water varies across high streamflow events and through this address the old water paradox. Understanding the changing sources of water during high streamflows is important for catchment water balances, flood forecasting, and predicting contaminant transport.