Aggregation effects on tritium-based mean transit times and young water fractions in spatially heterogeneous catchments and groundwater systems
- 1Aquifer Dynamics & GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand
- 2GNS Science, Tritium & Water Dating Laboratory, Avalon, Lower Hutt 5040, New Zealand
- 3International Centre for Water Hazard and Risk Management (ICHARM), Public Works Research Institute (PWRI), Tsukuba, Japan
- 4National Graduate Institute for Policy Studies (GRIPS), Tokyo, Japan
- 5Department of Hydrogeology and Engineering Geology, AGH University of Science and Technology Cracow, Al. Mickiewicza 30, 30-059 Cracow, Poland
Abstract. Kirchner (2016a) demonstrated that aggregation errors due to spatial heterogeneity, represented by two homogeneous subcatchments, could cause severe underestimation of the mean transit times (MTTs) of water travelling through catchments when simple lumped parameter models were applied to interpret seasonal tracer cycle data. Here we examine the effects of such errors on the MTTs and young water fractions estimated using tritium concentrations in two-part hydrological systems. We find that MTTs derived from tritium concentrations in streamflow are just as susceptible to aggregation bias as those from seasonal tracer cycles. Likewise, groundwater wells or springs fed by two or more water sources with different MTTs will also have aggregation bias. However, the transit times over which the biases are manifested are different because the two methods are applicable over different time ranges, up to 5 years for seasonal tracer cycles and up to 200 years for tritium concentrations. Our virtual experiments with two water components show that the aggregation errors are larger when the MTT differences between the components are larger and the amounts of the components are each close to 50 % of the mixture. We also find that young water fractions derived from tritium (based on a young water threshold of 18 years) are almost immune to aggregation errors as were those derived from seasonal tracer cycles with a threshold of about 2 months.