Articles | Volume 21, issue 9
Hydrol. Earth Syst. Sci., 21, 4615–4627, 2017
Hydrol. Earth Syst. Sci., 21, 4615–4627, 2017

Research article 13 Sep 2017

Research article | 13 Sep 2017

Aggregation effects on tritium-based mean transit times and young water fractions in spatially heterogeneous catchments and groundwater systems

Michael K. Stewart1, Uwe Morgenstern2, Maksym A. Gusyev3,4, and Piotr Małoszewski5,† Michael K. Stewart et al.
  • 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
  • deceased

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.

Short summary
This paper presents for the first time the effects of aggregation errors on mean transit times and young fractions estimated using tritium concentrations. Such errors, due to heterogeneity in catchments, had previously been demonstrated for seasonal tracer cycles by Kirchner (2016a). We found that mean transit times derived from tritium are just as susceptible to aggregation bias as those from seasonal tracer cycles. Young fractions were found to be almost immune to aggregation bias.