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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 11
Hydrol. Earth Syst. Sci., 14, 2289–2301, 2010
https://doi.org/10.5194/hess-14-2289-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.
Hydrol. Earth Syst. Sci., 14, 2289–2301, 2010
https://doi.org/10.5194/hess-14-2289-2010
© Author(s) 2010. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 18 Nov 2010

Research article | 18 Nov 2010

Dating of streamwater using tritium in a post nuclear bomb pulse world: continuous variation of mean transit time with streamflow

U. Morgenstern1, M. K. Stewart2, and R. Stenger3 U. Morgenstern et al.
  • 1GNS Science, P.O. Box 30 368, Lower Hutt 5040, New Zealand
  • 2Aquifer Dynamics & GNS Science, P.O. Box 30 368, Lower Hutt 5040, New Zealand
  • 3Lincoln Ventures Ltd. (LVL), Private Bag 3062, Hamilton 3214, New Zealand

Abstract. Tritium measurements of streamwater draining the Toenepi catchment, a small dairy farming area in Waikato, New Zealand, have shown that the mean transit time of the water varies with the flow rate of the stream. Mean transit times through the catchment are 2–5 years during high baseflow conditions in winter, increasing to 30–40 years as baseflow decreases in summer, and then dramatically older water during drought conditions with mean transit time of more than 100 years. Older water is gained in the lower reaches of the stream, compared to younger water in the headwater catchment. The groundwater store supplying baseflow was estimated from the mean transit time and average baseflow to be 15.4 × 106 m3 of water, about 1 m water equivalent over the catchment and 2.3 times total annual streamflow. Nitrate is relatively high at higher flow rates in winter, but is low at times of low flow with old water. This reflects both lower nitrate loading in the catchment several decades ago as compared to current intensive dairy farming, and denitrification processes occurring in the older groundwater. Silica, leached from the aquifer material and accumulating in the water in proportion to contact time, is high at times of low streamflow with old water. There was a good correlation between silica concentration and streamwater age, which potentially allows silica concentrations to be used as a proxy for age when calibrated by tritium measurements. This study shows that tritium dating of stream water is possible with single tritium measurements now that bomb-test tritium has effectively disappeared from hydrological systems in New Zealand, without the need for time-series data.

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