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Groundwater age has become a fundamental concept in groundwater hydrology,but ages originating from isotopic analyses are still identified with a lack ofclarity and using models that occasionally are unrealistic. If the effect ofadvection and dispersion on water ages has already been extensively identified,very few studies address the reliability of using radiometric ages as derivedfrom isotopic data to estimate aquifer properties such as average velocities.Using simple one-dimensional and two-dimensional analytical solutions forsingle-site and two-sites mobile-immobile systems, we compare the radiometricages to the mean ages (or residence times) as deduced from a direct,physically-based simulation approach (using the mean age equation), and showthat the competition between isotope decay rate and dispersion coefficient cangenerate important discrepancies between the two types of ages. A correctionfor the average apparent velocity originating from apparent isotopic ages isadditionally provided. The particular case of the Tritium age dating method isalso addressed, and a numerical example is finally given for illustrating theanalysis considering a more complex and heterogeneous aquifer system. Ourresults suggest that age definitions based on the radioactivity of isotopes maynot be representative for the mean age of the sample or for the groundwatervelocity at given locations, and may not always be suitable for constrainingthe calibration of hydrogeological models.

On the biases affecting water ages inferred from isotopic data