Dating of old groundwater: The Milk River aquifer

Full text: As part of a co-ordinated research programme, organized by the IAEA with the participation of several institutions, a field study' was performed on an aquifer with presumably very old groundwater to combine and compare several dating methods. For this study the Milk River aquifer in Alberta, Canada, was chosen because this aquifer has been studied extensively and hydrodynamic models are available. The aquifer underlies many thousands of square kilometres, is moderately uniform in composition and is fully confined except at the recharge area along the USA/Canada border. Water in the aquifer is under enough pressure to produce flowing wells over much of the area under consideration. Previous measurements have been done in some wells for stable isotopes, chemistry and chlorine-36. The water is supposed to range in age from less than a thousand years to more than a million years. In a sampling campaign in August 1985, most institutes participating in the coordinated research programme collected water for the analysis of 2H, 3H, 13C, 14C, 18O, 34S, 36Cl, 39Ar, 81Kr, 85Kr, 129I, 222Rn, noble gas contents, 234U/238U, general chemistry, methane, DOC, DIC and organic carbon. The results of the measurements allow the identification of three different water masses and their mixing in the aquifer, although this classification is not always supported by all methods. It is concluded from the results that considerable changes of concentrations of chemical components such as chlorine, iodine and others occur in the aquifer. In particular, large amounts of CH4 are added in certain parts of the aquifer, making radiocarbon dating very difficult. Large variations in He concentrations are also observed. The spatial variation observed in uranium concentrations and 234U/238U ratios accounts for different retardation factors in different parts of the aquifer. Flow patterns are found to be north oriented in the eastern part of the aquifer, have an east component in the western part, and have local characteristics in a discharge area in the south into the Milk River. Attempts to combine dating methods to date in the various parts of the aquifer have not been very successful. Only in the recharge area in the south could a component with an age younger than 1000 years be identified. Results of 40Ar/36Ar ratio and 4He content measurements indicate that water in the eastern pan of the aquifer is much older than 1000 years. (author)