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Importance of tritium‐based transit times in hydrological systems

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Disappearance of bomb tritium from the atmospheres of both hemispheres in recent years means that tritium is becoming more effective for determining transit times in hydrological systems. In particular, increasing use of tritium is being made to determine transit times of water through catchments from rainfall to rivers in order to derive vital information for the protection and management of river systems. Tritium reveals the transit times of the older water in streamflow, transit times which are too long to be seen by stable isotope or major chemical variation methods (this has been characterized as ‘hidden streamflow’). Documenting these longer timescales is vital for understanding the flow and water quality responses of rivers to changes in land use, diffuse or point‐source chemical pollution, ecological degradation, and climate change, responses which have proven to be considerably longer than previously expected. Including the longer timescales in the models allows more realistic approaches to management of catchments and emphasizes interdisciplinary differences between surface water and groundwater hydrologists’ views of river basins. Contrasting research agendas are proposed for effective use of tritium in the Northern and Southern Hemispheres based on their different tritium input histories since the advent of nuclear testing in the atmosphere. WIREs Water 2016, 3:145–154. doi: 10.1002/wat2.1134 This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality
Tritium concentration in precipitation at Luxembourg and Oregon, USA (Northern Hemisphere), and Kaitoke, New Zealand (Southern Hemisphere). Tritium concentrations are expressed as tritium units (TUs) where 1 TU corresponds to a tritium/total hydrogen ratio of 10−18. The straight lines show the effects of radioactive decay of tritium in groundwater recharged in 1980. The inset shows the transit time distribution model applied to calculate the tritium concentrations in Figures and , expressed as normalized probability density function (Pdf times mean transit time) versus normalized transit time (transit time/mean transit time).
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(a, c). Tritium concentrations predicted in Luxembourg and Oregon USA streamflows after applying an exponential piston flow model (EPM) with f = 80% to their tritium inputs for sampling at present (2015 curve) and in the future (2030 curve). Multiple mean transit times are indicated for single tritium values when the curves rise, or are level, e.g., tritium values of 7.5 and 3 TU intersect the 2015 curves three times as shown. (b, d). Mean transit times determined with those sample tritium values in 2015 for the full range of EPM models (parameter f between 0 and 100%). Ambiguous results (three mean transit times) are obtained for all values of f. Dotted lines show the results for f = 80%.
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(a). Tritium concentrations predicted in Kaitoke, New Zealand streamflow after applying an exponential piston flow model (EPM) with exponential to total volume fraction f = 80% to the tritium input for Kaitoke shown in Figure for sampling at present (2015 curve) and in the future (2030 curve). The monotonic declines of the curves means that sample tritium values (e.g., 1 TU) intersect the curves only once indicating a unique mean transit time. (b) Mean transit times determined with sample tritium value 1 TU in 2015 for the full range of EPM models (parameter f between 0 and 100%). The dotted line shows the unique result for f = 80%, while ambiguous results (three mean transit times) are obtained with f < 25%.
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