Intercomparison of tritium and noble gases analyses, 3H/3He ages and derived parameters excess air and recharge temperature

doi:10.1016/j.apgeochem.2014.03.005

Applied Geochemistry

Volume 50, November 2014, Pages 130-141

https://doi.org/10.1016/j.apgeochem.2014.03.005 Get rights and content

Highlights

•
Key results of the groundwater age-dating interlaboratory comparison exercise.
•
The reproducibility of the tritium measurements was 13.5%.
•
The noble gas reproducibility was <2% (R, He, Ne) and <3% (Ar, Kr, Xe).
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The measurement uncertainty meets the requirements for ³H/³He dating.
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Other sources of uncertainty are less well defined than the analytical uncertainty.

Abstract

Groundwater age dating with the tritium–helium (³H/³He) method has become a powerful tool for hydrogeologists. The uncertainty of the apparent ³H/³He age depends on the analytical precision of the ³H measurement and the uncertainty of the tritiogenic ³He component. The goal of this study, as part of the groundwater age-dating interlaboratory comparison exercise, was to quantify the analytical uncertainty of the ³H and noble gas measurements and to assess whether they meet the requirements for ³H/³He dating and noble gas paleotemperature reconstruction.

Samples for the groundwater dating intercomparison exercise were collected on 1 February, 2012, from three previously studied wells in the Paris Basin (France). Fourteen laboratories participated in the intercomparison for tritium analyses and ten laboratories participated in the noble gas intercomparison. Not all laboratories analyzed samples from every borehole.

The reproducibility of the tritium measurements was 13.5%. The reproducibility of the ³He/⁴He ratio and ⁴He, Ne, Ar, Kr and Xe concentrations was 1.4%, 1.8%, 1.5%, 2.2%, 2.9%, and 2.4% respectively.

The uncertainty of the tritium and noble gas measurements results in a typical ³H/³He age precision of better than 2.5 years in this case. However, the measurement uncertainties for the noble gas concentrations are insufficient to distinguish the appropriate excess air model if the measured helium concentration is not included. While the analytical uncertainty introduces an unavoidable source of uncertainty in the ³H/³He apparent age estimate, other sources of uncertainty are often much greater and less well defined than the analytical uncertainty.

Introduction

Groundwater age dating with the tritium–helium (³H/³He) method has become a powerful tool for hydrogeologists. The principle of ³H/³He dating (Tolstikhin and Kamenski, 1969) is the decay of radioactive ³H (half-life τ_½ = 12.32 a (Lucas and Unterweger, 2000)) to helium-3 (³He) and the accumulation of the decay product ³He in groundwater. Combined determination of the ³H and tritiogenic ³He (³He_trit) concentrations in groundwater allows for the calculation of the apparent ³H/³He age (τ) given the decay constant of ³H (λ = ln(2)/τ_½ = 0.05626 a⁻¹) (Eq. (1)). $τ = \ln (1 + [^{3} {He}_{trit}]/ [^{3} H]) λ^{- 1}$

The apparent ³H/³He age corresponds to the groundwater travel time under the assumption that ³He is confined in groundwater below the water table and both ³H and ³He are transported at the same rate as the groundwater flow. First applications of ³H/³He dating were published in 1987 (Takaoka and Mizutani, 1987) and 1988 (Poreda et al., 1988, Schlosser et al., 1988).

Tritium is naturally produced in the atmosphere by cosmic radiation resulting in a tritium concentration in precipitation of less than 10 tritium units (1 TU corresponds to a ³H/¹H ratio of 10⁻¹⁸). Large quantities of ³H were released into the stratosphere since 1953 by above ground testing of thermonuclear devices. ³H enters the groundwater system by infiltration of tritiated water (³H¹HO) in precipitation. The historical concentrations of tritium in precipitation have been monitored at several locations around the world by the Global Network of Isotopes in Precipitation (GNIP) network and are available online from the International Atomic Energy Agency (IAEA).

Groundwater contains ³He from five sources: equilibration with the atmosphere, excess air from bubble entrainment in the unsaturated zone, nuclear fission of ⁶Li (nucleogenic: ⁶Li(n,α) ³H → ³He) associated with the production of ⁴He by U–Th decay (radiogenic ⁴He) (Schlosser et al., 1989), mantle helium and tritium decay (tritiogenic). ³H/³He dating requires calculating the tritiogenic component by subtracting the atmospheric components (equilibrium and excess air) from the measured ³He concentration (if the nucleogenic and mantle helium components are negligible). If the recharge temperature is known, the excess air is assumed to be unfractionated with respect to the atmosphere and the ⁴He/Ne ratio indicates that no terrigenic helium (radiogenic or mantle helium) is present, the atmospheric component can be derived from the concentration of ⁴He. If terrigenic helium is present, the atmospheric helium component can be derived from the concentration of neon (Schlosser et al., 1989) assuming a known recharge temperature and unfractionated or no excess air. A more advanced method is to derive the atmospheric ³He component from inverse fitting the concentrations of the other noble gases to excess air fractionation models. The resulting “derived parameters” (noble gas recharge temperature, excess air amount and fractionation) are in itself useful proxies of past recharge conditions (Aeschbach-Hertig et al., 2000, Stute et al., 1995). Radiogenic helium is also a tracer for groundwater age, typically in the range of 10³ to 10⁶ years (Marine, 1979).

The uncertainty of the apparent ³H/³He age (σ_τ) depends on the analytical precision of the ³H measurement (σ_3H) and uncertainty of the tritiogenic ³He component (σ_3Hetrit) derived from the propagation of the noble gas measurement uncertainty (Solomon et al., 1993). σ_3Hetrit includes the uncertainty of the helium isotope ratio of a terrigenic component, if present. A linear approximation of the age uncertainty is given by Eq. (2). $σ_{τ} = λ^{- 1} {{([^{3} H] + [^{3} {He}_{trit}])}^{- 1} {(σ_{3 Hetrit}^{2} + ([^{3} {He}_{trit}]/ [^{3} H])}^{2} σ_{3 H}^{2})}^{1 / 2}$

The uncertainty of the tritiogenic ³He component includes the uncertainty in the determination of the recharge temperature and excess air fractionation (Ballentine and Hall, 1999). The purpose of this study, as part of the groundwater age-dating interlaboratory comparison exercise (Labasque et al., 2014) including also CFC and SF₆ dating techniques (Labasque, 2014), was to quantify the uncertainty related to field-sampling procedures as well as the analytical uncertainty of the ³H and noble gas measurements, and the resulting uncertainty of the apparent ³H/³He ages and derived parameters.

For low-level tritium activity measurements in water, the International Atomic Energy Agency (IAEA) regularly organizes interlaboratory comparison exercises (“TRIC”, Gröning, 2009). The last exercise, TRIC2008, included the results from 63 participating laboratories anonymously submitting the analysis results of five low level (<15 TU) tritium samples. The five samples are prepared by IAEA by gravimetric dilution of tritiated standard water with water of near-zero tritium concentration.

No such exercise exists for the analysis of noble gases in water samples. Interlaboratory comparison occurs occasionally when two laboratories with the same analytical capabilities participate in the same research project. Developments of new sampling or analytical techniques are often validated against accepted methods, often at the same laboratory (Roether et al., 2013). This is the first interlaboratory comparison exercise for noble gas analyses in water samples. The goal of this study was to assess how the demonstrated sampling and analytical uncertainty propagate into the ³H/³He groundwater age and noble gas paleotemperature reconstruction.

Section snippets

Methods

A detailed presentation of the experiment design, site geological and hydrogeological context and participants is given in (Labasque et al., 2014). Samples for the intercomparison exercise were collected from existing and previously studied wells. The observed variability is due to both analytical procedures and sample collection. The stability of the well was confirmed by repeat samples and field measurements during the time it took to collect all samples for the intercomparison exercise (

Measured tritium concentrations

Tritium was reported for 17 samples from the Albian borehole. Measured tritium concentrations are near zero in the Albian borehole (Fig. 1g, Table 3). Negative reported values for the Albian are expressed in the Fig. 1g boxplot. Two measured values were identified as outliers for the Albian samples. The tritium concentration was below the detection limit for nine of the 12 laboratories (14 of the 19 samples analyzed). Tritium was measured above the measurement uncertainty by two laboratories at

Measurement uncertainties

The uncertainty of a tritium measurement is less than 15% for the two samples containing 2.6 and 6.4 TU of tritium. Some laboratories provide measurements that appear to be biased from the borehole means of SLP4 and SLP5, pointing towards opportunities for improvement by recalibrating. Validating the results against a new tritium standard and participating in the IAEA TRIC exercises is recommended.

The single sample reproducibility was 1.3% for the helium isotope ratio, 1.5% for the helium and

Conclusions

This first intercomparison exercise for ³H/³He groundwater ages investigated the measurement uncertainty for the ³H/³He age dating community as a whole thanks to the participation of a large number of laboratories. Performing the exercise has not only provided an independent assessment of laboratory performance and a demonstration that such assessments are needed and are useful, but also with invaluable experience in organization and preparation for the joint sampling event.

A future ³H/³He

Acknowledgements

We gratefully acknowledge the participating laboratories for providing the GDAT team with tritium and noble gas isotope analyses. We thank “Source du Val Saint-Lambert” for permission to sample the boreholes. We thank Peter Cook, Daren Gooddy, Ed Busenberg, Niel Plummer, Axel Suckow, Kip Solomon for discussions on the design of the GDAT intercomparison exercise and László Palcsu for his comments on the manuscript. We thank three anonymous reviewers for their valuable comments.

Part of this work

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Intercomparison of tritium and noble gases analyses, 3H/3He ages and derived parameters excess air and recharge temperature

Highlights

Abstract

Introduction

Section snippets

Methods

Measured tritium concentrations

Measurement uncertainties

Conclusions

Acknowledgements

Geochim. Cosmochim. Acta

Int. J. Appl. Radiat. Isot.

J. Hydrol.

Appl. Geochem.

J. Hydrol.

C. R. l’Acad. Sci. – Ser. IIA: Earth Planet. Sci.

Deep Sea Res. Part I: Oceanogr. Res. Pap.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Interpretation of dissolved atmospheric noble gases in natural waters

Water Resour. Res.

Palaeotemperature reconstruction from noble gases in ground water taking into account equilibration with entrapped air

Nature

A mass spectrometric system for the analysis of noble gases and tritium from water samples

Environ. Sci. Technol.

A limited memory algorithm for bound constrained optimization

SIAM J. Sci. Comput.

Constraining the age distribution of highly mixed groundwater using 39Ar: a multiple environmental tracer (3H/3He, 85Kr, 39Ar, and 14C) study in the semiconfined Fontainebleau Sands Aquifer (France)

Water Resour. Res.

European climate variations over the past half-millennium reconstructed from groundwater

Geophys. Res. Lett.

Intercomparison of tritium and noble gases analyses, ³H/³He ages and derived parameters excess air and recharge temperature

Constraining the age distribution of highly mixed groundwater using ³⁹Ar: a multiple environmental tracer (³H/³He, ⁸⁵Kr, ³⁹Ar, and ¹⁴C) study in the semiconfined Fontainebleau Sands Aquifer (France)