Estimation pratique de l'âge des eaux souterraines par le tritium Tritium Dating of Groundwaters

Résumé

Le tritium, isotope radioactif de l'hydrogène, est considéré comme un élément dateur des eaux souterraines. Il permet à l'hydrogéologue d'estimer le temps de séjour moyen des eaux d'un aquifère.

Produit naturellement dans l'atmosphère, les teneurs dans les précipitations ont tout d'abord augmenté, suite aux essais thermonucléaires aériens de 1952 à 1963, pour ensuite diminuer et se stabiliser des industries nucléaires, supérieure à la teneur naturelle de 5 UT. Actuellement, la teneur moyenne se situe entre 10 et 30 UT.

Grâce au suivi des teneurs en 3H des précipitations, il est possible de déterminer le temps de séjour moyen des eaux souterraines d'un aquifère à l'aide de modèles mathématiques qui, à partir d'un signal d'entrée (teneurs en 3H dans les précipitations) et d'un signal de sortie (teneurs en 3H dans les eaux souterraines d'un aquifère), calculent le meilleur ajustement possible entre les sorties calculées et les valeurs mesurées. De cet ajustement, on en déduit le temps de séjour moyen.

Ces études isotopiques utilisent classiquement trois types de modèles suivant la façon dont les eaux se mélangent en traversant un milieu poreux: le modèle mélange ou exponentiel, le modèle piston et le modèle dispersif. Cet ensemble de modèle peut être généralisé en différenciant temps de séjour moyen et age d'une molécule d'eau.

Ces modèles ont été utilisés pour étudier les systèmes des sources minérales de la Versoie et d'Evian (Haute-Savoie, France) en appliquant un modèle mélange à la source de la Versoie et un modèle dispersif à la source d'Evian, ce qui a permis d'estimer leur temps de séjour.

Mots-clés:

• Tritium,
• eaux souterraines,
• datation,
• temps de séjour moyen,
• modèles mathématiques

Abstract

Tritium, the radioactive isotope of hydrogen, is considered as a dating element for ground-water and allows the hydrogeologist to evaluate the aquifer waters average transit time T. This parameter is important for questions about the evaluation, exploitation and protection of water resources.

Naturally produced in the atmosphere by the action of the neutronic component of cosmic radiation on nitrogen, 3H is present in precipitation at levels of about 5 UT. As a result of thermonuclear aerial tests from 1952 to 1980, the 3H content of precipitation increased markedly, reaching a maximum in 1963 of about 2500 UT and decreasing thereafter to about 50 UT in 1980. Since 1980, thermonuclear aerial tests have stopped but civil thermonuclear stations continue to influenced the 3H content of precipitation, which currently lies between 10 and 30 UT.

Thanks to the regular analysis of the tritium content of precipitation, it is possible to evaluate aquifer ground-water transit times, T, with mathematical models. This approach uses an "input signal" (3H contents in precipitation) and an "output signal" (3H contents in aquifer ground-water) and calculate the best fit between the calculated output and the measured values. This best fit yields an estimate of the transit time T.

According to the manner in which the waters mix as they pass through a porous media, three kinds of models are used: the well-mixed or exponential model, which supposes that water introduced into the system is completely and uniformly mixed with the aquifer water, such that the water output from the system is representative of the aquifer waters; the piston-flow model, which supposes that the incoming water traverses the aquifer with a constant velocity; and the dispersion model, which supposes a dispersion phenomenon due to the heterogeneity of the aquifer material. The piston-flow model and the exponential model represent the two possible extreme cases of this dispersion model: in effect, as dispersion increases the calculated output will tend towards the exponential model output. Similarly, for low dispersion systems, the calculated output will tend towards the piston-flow model output.

To generalise these models, a distinction can be introduced between the age of a water molecule, equivalent to the residence time, and the transit time. The residence time is the time elapsed since any element has entered into the system, as opposed to the transit time which is the time spent by an element between entry and outflow from the system. This distinction implies three cases:

- the residence time is smaller than the transit time, so depending on whether or not there is mixing, we have the dispersion or piston-flow model;

- the residence time is equal to the transit time, yielding the exponential model;

- the residence time is greater than the transit time, which is an unutilized case in classical modeling and which may be qualified as a short-circuit model.

These models have been applied to the Versoie and Evian mineral water systems (Haute-Savoie, France), for which we have long time-series of 3H measurements. The Evian mineral water system uses a dispersion model, for which the average transit time is between 40 and 80 years. The Versoie mineral water system uses a mixing model, for which the average transit time is between 2 and 9 years.

In conclusion, the long series of 3H measurements realized in France, Switzerland, Italy and Spain ground-waters from 1990 to 1993 allow us to distinguish four categories of results: 3H contents less than 2 UT, which characterizes an old ground-water with an average transit time greater than 2000 years; 3H contents between 2 and 10 UT, which characterizes a ground-water with an average transit time between 200 and 300 years, or a mixing between an old and a recent water; 3H contents between 10 and 40 UT, which characterizes a recent ground-water; 3H contents greater than 40 UT, which characterizes a groundwater near a nuclear station.

Keywords:

• Tritium,
• groundwater,
• dating,
• average transit time,
• mathematical models