Abstract:Although early studies of streamwater residence time included the use of stable isotopes (deuterium, oxygen-18) and tritium, work in the last decades has largely relied on stable isotopes (or chloride) alone for residence time determination, and derived scaling relations at the headwater and mesoscale watershed scale. Here, we review critically this trend and point out a significant issue in our field: truncation of stream residence time distributions because of only using stable isotopes. When tritium is used, the age distributions generally have long tails showing that groundwater contributes strongly to many streams, and consequently that the streams access considerably larger volumes of water in their catchments than would be expected from stable isotope data use alone. This shows contaminants can have long retention times in catchments, and has implications for process conceptualization and scale issues of streamflow generation. We review current and past studies of tritium use in watersheds and show how groundwater contributions reflect bedrock geology (using New Zealand as an example). We then discuss implications for watershed hydrology and offer a possible roadmap for future work that includes tritium in a dual isotope framework.

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 × 10 6 m 3 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.

We describe an advanced methodology for low-level tritium measurement in regard to calibration, electrolytic tritium enrichment, liquid scintillation counting (LSC) measurement, and prevention of sample contamination. Details are given on enrichment parameters and electrode processes for optimisation of enrichment reproducibility and on optimisation of LSC stability. Intercomparison results demonstrate high accuracy of the tritium measurement system. The use of accurate tritium data for groundwater dating in the southern hemisphere is demonstrated with data from several groundwater systems of New Zealand.

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