Using tritium and other geochemical tracers to address the old water paradox in headwater catchments

Documenting the changing sources of water during high streamflows following rainfall is important for understanding catchment behaviour. This study uses tritium (H-3), major ion concentrations, and delta O-18 values to identify the sources of water that sustained streamflow in the upper Yarra and Latrobe catchments of southeast Australia over two high flow events in March 2016 and August 2016. The variations in geochemistry imply that direct contribution of rainfall to streamflow was minor. Rather, the baseflow that sustained the streams prior to the events was augmented by water derived from the soils or regolith. A two component mass balance based on H-3 indicates that the soil/regolith water constituted 60-100% of total streamflow at the peak of the events. A mass balance based on silica yields similar estimates (50-70%). The H-3 activities of the soil water imply residence times of up to six years. These are sufficiently long for evapotranspiration, dissolution of organic matter, and mineral weathering to increase the major ion concentrations above those of rainfall and for mixing to homogenise delta O-18 and delta H-2 values. This study helps resolve the paradox that the water contributing to high flow events in streams is commonly not rainfall but is unlikely to be only increased inflows of groundwater due to the time required for hydraulic loading to occur.