Comment on “A comparison of catchment travel times and storage deduced from deuterium and tritium tracers using StorAge Selection functions” by Rodriguez et al. (2021)

Stewart, Michael K.; Morgenstern, Uwe; Cartwright, Ian

doi:10.5194/hess-25-6333-2021

Cited by 6 publications

(9 citation statements)

References 22 publications

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“…The specific magnitude of such a water age threshold remains difficult to quantify with the available data. However, given the results in the Neckar study basin, the question raised by Stewart et al (2021), if δ 18 O allows to see "the full range of travel times", can to some extent be answered: it can be assumed that, when used with a suitable model, δ 18 O contains sufficient information for a meaningful characterization of water ages in systems characterized by MTTs of at least ~15 -20 years, which encompasses the vast majority of river basins so far analyzed in literature (see Stewart et al, 2010 and references therein). As a step forward, the original hypothesis above can, for future research, be reformulated into: δ 18 O-inferred water age estimates are subject to increasing uncertainty and bias when compared to 3 H-inferred estimates when stream water MTTs of ~ 15 -20 years are exceeded in systems characterized by increasingly old water.…”

Section: The Individual Roles Of the Choices Of Tracers And Models Fo...mentioning

confidence: 99%

See 1 more Smart Citation

Stable water isotopes and tritium tracers tell the same tale: No evidence for underestimation of catchment transit times inferred by stable isotopes in SAS function models

Wang

Hrachowitz²,

Schoups³

et al. 2022

Preprint

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Abstract. Stable isotopes (δ18O) and tritium (3H) are frequently used as tracers in environmental sciences to estimate the age distributions of water. However, it has previously been argued that seasonally variable tracers, such as δ18O, generally and systematically fail to detect the tails of water age distributions and therefore substantially underestimate water ages as compared to radioactive tracers, such as 3H. In this study for the Neckar river basin in central Europe and based on a >20-year record of hydrological, δ18O, and 3H data, we systematically scrutinized the above postulate together with the potential role of spatial aggregation effects to exacerbate the underestimation of water ages. This was done by comparing water age distributions inferred from δ18O and 3H with a total of 12 different model implementations, including lumped parameter sine-wave (SW) and convolution integral models (CO) as well as integrated hydrological models in combination with SAS-functions (IM-SAS). We found that, indeed, water ages inferred from δ18O with commonly used SW and CO models are with mean transit times (MTT) ~ 1–2 years substantially lower than those obtained from 3H with the same models, reaching MTTs ~ 10 years. In contrast, several implementations of IM-SAS models did not only allow simultaneous representations of stream flow as well as δ18O and 3H stream signals, but water ages inferred from δ18O with these models were with MTTs ~ 16 years much higher than those from SW and CO models and similar to those inferred from 3H, which suggested MTTs ~ 15 years. Characterized by similar parameter posterior distributions, in particular for parameters that control water age, IM-SAS model implementations individually constrained with δ18O or 3H observations, exhibited only limited differences in the magnitudes of water ages in different parts of the models as well as in the temporal variability of TTDs in response to changing wetness conditions. This suggests that both tracers lead to comparable descriptions of how water is routed through the system. These findings provide evidence that allowed us to reject the hypothesis that δ18O as a tracer generally and systematically “cannot see water older than about 4 years” and that it truncates the corresponding tails in water age distributions, leading to underestimations of water ages. Instead, our results provide evidence for a broad equivalence of δ18O and 3H as age tracers for systems characterized by MTTs of at least 15–20 years. The question to which degree aggregation of spatial heterogeneity can further adversely affect estimates of water ages remains unresolved as the lumped and distributed implementations of the IM-SAS model provided inconclusive results. Overall, this study demonstrates that previously reported underestimations of water ages are most likely not a result of the use of δ18O or other seasonally variable tracers per se. Rather, these underestimations can be largely attributed to choices of model approaches and complexity not considering hydrological next to tracer aspects. Given the additional vulnerability of SW and CO model approaches in combination with δ18O to substantially underestimate water ages due to spatial aggregation and potentially other, still unknown effects, we, therefore, advocate to avoid the use of this model type in combination with seasonally variable tracers if possible, and to instead adopt SAS-based or comparable model formulations.

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Section: The Individual Roles Of the Choices Of Tracers And Models Fo...mentioning

confidence: 99%

“…The authors concluded that these results cast some doubt on "[…] the perception that stable isotopes systematically truncate the tails of TTDs" (Rodriguez et al, 2021). However, their interpretation was questioned by Stewart et al (2021), who pointed out that simply no older water may be present in their study catchment.…”

Section: Introductionmentioning

confidence: 98%

Stable water isotopes and tritium tracers tell the same tale: No evidence for underestimation of catchment transit times inferred by stable isotopes in SAS function models

Wang

Hrachowitz²,

Schoups³

et al. 2022

Preprint

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“…Present-day tritium activities in precipitation have recovered natural cosmogenic levels, i.e., similar to levels noted before the nuclear tests (Cauquoin et al, 2015;Terzer-Wassmuth et al, 2022). However, bomb tritium remains present in the hydrological cycle of the earth (Stewart et al, 2021). The interest in tritium water dating has recently gained new momentum (Morgenstern et al, 2010;Stewart et al, 2012).…”

mentioning

confidence: 57%

“…allowed first to carry out explicit water transit time analyses for several years past the mid-1960's bomb peak, the latter acting as a clear time marker. With the bomb tritium pulse decay observed after the 1970's, tritium dating of hydrological systems had to increasingly rely on time series with several measurements to deliver unambiguous results -eventually leading to a fading interest in tritium as a tracer of choice for water age dating (Stewart and Morgenstern, 2016;Stewart et al, 2021).…”

mentioning

confidence: 99%

Short high-accuracy tritium data time series for assessing groundwater mean transit times in the vadose and saturated zones of the Luxembourg Sandstone aquifer

Laurent

Stewart

Morgenstern

et al. 2023

Preprint

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Abstract. While in the southern Hemisphere a single water sample can be sufficient for tritium-based young groundwater dating, several tritium measurements spanning over multiple years are still needed in the northern Hemisphere. Although it is advised to focus tritium-based groundwater age dating on sites where long chronicles of tritium data are available, we tested in this study the potential for short high-accuracy tritium data series (~4 years) to date groundwater from 35 springs draining the Luxembourg Sandstone aquifer (Central Western Europe). We determined groundwater mean transit times by using the lumped parameter model approach in a Monte Carlo uncertainty estimation framework to provide uncertainty ranges inherent to the low number of tritium data at hand and their related analytical errors. Our results show that unambiguous groundwater mean transit time assessments cannot be determined solely based on such recent short tritium time series, given that several ranges of mean transit times appeared theoretically possible. Nonetheless we succeeded in discriminating groundwater mean transit times in the vadose and saturated zones of the aquifer through a stepwise decision process guided with several supplementary data. Our findings are consistent with both the tritium measurements of individual springs and the hydrogeological context of the study area. We have been able to improve our understanding of the water transit times in the Luxembourg Sandstone aquifer. We particularly highlighted a horizontal-vertical water velocity anisotropy in the Luxembourg Sandstone - a key feature that deserves to be explored further.

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“…We point out that, in New Zealand and the Southern Hemisphere in general, bomb tritium has now fully dispersed, removing any ambiguity from fitting LPMs and increasing the reliability of tritium-based age interpretations. This is not yet the case for the 225 Northern Hemisphere, where bomb tritium is still present in significant amounts within the groundwater systems still causing ambiguity in the age interpretations (Stewart et al, 2021).…”

Section: Datamentioning

confidence: 99%

Estimation of groundwater age distributions from hydrochemistry: Comparison of two metamodelling algorithms in the Heretaunga Plains aquifer system, New Zealand

Tschritter

Daughney

Karalliyadda

et al. 2022

Preprint

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Abstract. Groundwater age or residence time is important for identifying flow and contaminant pathways through groundwater systems. Typically, groundwater age and age distributions are inferred via lumped parameter models based on measured age tracer concentrations. However, due to cost and time constraints, age tracers are usually only sampled at a small percentage of the wells in a catchment. This paper describes and compares two methods to increase the number of groundwater age data points and assist with validating age distributions inferred from lumped parameter models. Two machine learning techniques with different strengths were applied to develop two independent metamodels that each aim to establish relationships between the hydrochemical parameters and the modelled groundwater age distributions in one test catchment. Ensemble medians from the best model realisations per age distribution percentile were used for comparison with the results from traditional lumped parameter models based on age tracers. Results show that both metamodelling techniques generally work well for predicting groundwater age distributions from hydrochemistry. Therefore, these techniques can be used to assist with the interpretation of lumped parameter models where age tracers have been sampled, and they can also be applied to predict groundwater age distributions for wells that have hydrochemistry data available, but no age tracer data.

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Comment on “A comparison of catchment travel times and storage deduced from deuterium and tritium tracers using StorAge Selection functions” by Rodriguez et al. (2021)

Cited by 6 publications

References 22 publications

Stable water isotopes and tritium tracers tell the same tale: No evidence for underestimation of catchment transit times inferred by stable isotopes in SAS function models

Stable water isotopes and tritium tracers tell the same tale: No evidence for underestimation of catchment transit times inferred by stable isotopes in SAS function models

Short high-accuracy tritium data time series for assessing groundwater mean transit times in the vadose and saturated zones of the Luxembourg Sandstone aquifer

Estimation of groundwater age distributions from hydrochemistry: Comparison of two metamodelling algorithms in the Heretaunga Plains aquifer system, New Zealand

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