Application of isotopic methods to dating of very old groundwaters: Milk River aquifer, Alberta, Canada

The aim of this joint project was to evaluate the usefulness of available geochemical and isotope techniques for dating very old groundwater. This paper represents a synthesis of an IAEA sponsored study for which purpose the Milk River aquifer groundwaters were sampled from 16 wells during 1985 and subsequent years for the following measurements: ²H, ³H, ¹³C, ¹⁸O, ¹⁴C, ³⁹Ar, ⁸¹Kr, ⁸⁵Kr, noble gases including ²²²Rn, ³⁶Cl, ¹²⁹I and U isotopes. The Milk River aquifer was selected for this study because several preceding investigations had established that this groundwater system contains waters whose ages range from recent to > 1 Ma. The present study has established that the Milk River aquifer system is very complex both in terms of groundwater origin and in terms of the evolution of its chemical and isotopic contents. However, this apparent complexity proved to be an interesting challenge with respect to the use of different and complementary approaches to interpretation of the geochemical and isotope data in terms of groundwater residence time. Thus, the groundwater ages based on the hydrodynamic model should be considered as a lower limit of the average groundwater age in the Milk River aquifer (∼0.25 Ma) while those based on the ³⁶Cl/Cl data uncorrected for any dilution by dead Cl should be considered as an upper limit (<2 Ma). The Milk River Aquifer International Project is an excellent illustration of the variety of insight and increased confidence possible in flow-system analysis when multiple, independent dating methods are combined with detailed hydrogeological studies. The conclusion of the present study is that the potential for dating of very old groundwaters by a variety of isotope techniques is very high, provided a combination of methods is applied (never a simple method by itself).