Measurements and interpretations of 36Cl in groundwater, Milk River aquifer, Alberta, Canada

The 36Cl/Cl ratios of 12 groundwater samples from the Milk River aquifer were determined by accelerator mass spectrometry. Using known Cl concentrations, 36Cl concentrations were deduced. Approximately linear relations were observed between the logarithm of the 36Cl concentration, the Cl concentration, and the distance from the recharge area along two flow paths. The results are discussed using two approaches: (1) in an interpretation of the linear relation between logarithm of the 36Cl concentration and Cl concentration excluding and including in situ production of 35Cl; (2) in a diffusion model the increase of the Cl concentration with the distance from the recharge area is considered to be due to diffusion of Cl from the underlying confining Colorado shale to the aquifer. Flow velocities ranging between 0.04 and 0.14 m/a, and ages of the groundwater between 0.6 and 2 Ma are obtained at a distance of 80 km from the recharge area. (author)