Evaluation of isotopic methods for the dating of very old groundwaters: A case study of the Milk River Aquifer

The objective of the work presented is to evaluate the use of isotopic methods for the dating of very old groundwaters. It represents an IAEA sponsored study designed to test various methods available for that purpose. The Milk River Aquifer, in Alberta, Canada, was chosen for this study because it fits the definition of an idealized artesian system that dips gently from the recharge area and is confined by aquitards above and below. This feature, coupled with the fact that the groundwater in the Milk River Aquifer becomes 'old' (beyond 14C dating range) a short distance downflow from the dominant recharge area, makes this aquifer potentially useful in evaluating various dating techniques for old groundwaters. Numerous wells have been sampled in the course of many studies of the aquifer. Sampling for the IAEA sponsored campaign in 1985 took place in 16 wells. Samples for the following measurements were taken: major and minor ions (Alberta Agriculture and the IAEA), the stable isotopes δ18O, δ2H and δ13C, and 14C (IAEA, Universities of Waterloo and Arizona), 39Ar, 81Kr and 85Kr (University of Bern), noble gases, including Rn (University of Bath), 36Cl (University of Arizona and Technical University of Munich), 129I (University of Arizona) and uranium isotopes (Harwell Laboratory). The study concluded that groundwater ages based on the hydrodynamic model should be considered as being a lower limit of the average groundwater age in the Milk River Aquifer (< or approx. 0.500 Ma), while those based on 36Cl/Cl data uncorrected for any dilution by dead Cl should be considered as being an upper limit (<2 Ma). The Milk River Aquifer project is an excellent illustration of a successful 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 the dating of very old groundwaters by a variety of isotope techniques is very high provided a combination of methods is applied (i.e. never a single method by itself). (author). 18 refs, 5 figs