A GLUE‐based uncertainty assessment framework for tritium‐inferred transit time estimations under baseflow conditions |
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| Authors: | Francesc Gallart Maria Roig‐Planasdemunt Michael K Stewart Pilar Llorens Uwe Morgenstern Willibald Stichler Laurent Pfister Jérôme Latron |
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| Affiliation: | 1. IDAEA (CSIC), Barcelona, Spain;2. Aquifer Dynamics and GNS Science, Lower Hutt, New Zealand;3. GNS Science, Lower Hutt, New Zealand;4. Helmholtz Zentrum Muenchen, Ingolst?dter, Neuherberg, Germany;5. Luxembourg Institute of Science and Technology, Belvaux, Luxembourg |
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| Abstract: | The last decade has seen major technical and scientific improvements in the study of water transfer time through catchments. Nevertheless, it has been argued that most of these developments used conservative tracers that may disregard the oldest component of water transfer, which often has transit times greater than 5 years. Indeed, although the analytical reproducibility of tracers limits the detection of the older flow components associated with the most dampened seasonal fluctuations, this is very rarely taken into account in modelling applications. Tritium is the only environmental tracer at hand to investigate transfer times in the 5‐ to 50‐year range in surface waters, as dissolved gases are not suitable due to the degassing process. Water dating with tritium has often been difficult because of the complex history of its atmospheric concentration, but its current stabilization together with recent analytical improvements open promising perspectives. In this context, the innovative contribution of this study lies in the development of a generalized likelihood uncertainty estimation‐based approach for analysing the uncertainties associated with the modelling of transit time due to both parameter identification and tracer analytical precision issues. A coupled resampling procedure allows assessment of the statistical significance of the transfer time differences found in diverse waters. This approach was developed for tritium and the exponential‐piston model but can be implemented for virtually any tracer and model. Stream baseflow, spring and shallow aquifer waters from the Vallcebre research catchments, analysed for tritium in different years with different analytical precisions, were investigated by using this approach and taking into account other sources of uncertainty. The results showed three groups of waters of different mean transit times, with all the stream baseflow and spring waters older than the 5‐year threshold needing tritium. Low sensitivity of the results to the model structure was also demonstrated. Dual solutions were found for the waters sampled in 2013, but these results may be disambiguated when additional analyses will be made in a few years. Copyright © 2016 John Wiley & Sons, Ltd. |
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| Keywords: | GLUE transit time water dating uncertainty tritium baseflow exponential‐piston model analytical errors |
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