Time series of tritium, stable isotopes and chloride reveal short-term variations in groundwater contribution to a stream

摘要

A major limitation to the assessment of catchment transit time (TT) stemsfrom the use of stable isotopes or chloride as hydrological tracers, becausethese tracers are blind to older contributions. Yet, accurately capturing theTT of the old water fraction is essential, as is the assessment of itstemporal variations under non-stationary catchment dynamics. In this study weused lumped convolution models to examine time series of tritium, stableisotopes and chloride in rainfall, streamwater and groundwater of a?catchmentlocated in subtropical Australia. Our objectives were to determine thedifferent contributions to streamflow and their variations over time, and tounderstand the relationship between catchment TT and groundwater residencetime. Stable isotopes and chloride provided consistent estimates of TT in theupstream part of the catchment. A young component to streamflow wasidentified that was partitioned into quickflow (mean TT??≈??2?weeks)and discharge from the fractured igneous rocks forming the headwaters (meanTT??≈??0.3?years). The use of tritium was beneficial for determiningan older contribution to streamflow in the downstream area. The best fitsbetween measured and modelled tritium activities were obtained for a?mean TTof 16–25?years for this older groundwater component. This was significantlylower than the residence time calculated for groundwater in the alluvialaquifer feeding the stream downstream (?≈??76–102?years), emphasisingthe fact that water exiting the catchment and water stored in it haddistinctive age distributions. When simulations were run separately on eachtritium streamwater sample, the TT of old water fraction varied substantiallyover time, with values averaging 17?±?6?years at low flow and38?±?15?years after major recharge events. This counterintuitive resultwas interpreted as the flushing out of deeper, older waters shortly afterrecharge by the resulting pressure wave propagation. Overall, this studyshows the usefulness of collecting tritium data in streamwater to documentshort-term variations in the older component of the TT distribution. Ourresults also shed light on the complex relationships between stored water andwater in transit, which are highly non-linear and remain poorly understood.