Chlorine 36 dating of very old groundwater: 2. Milk River Aquifer, Alberta, Canada

The Milk River aquifer in southern Alberta, Canada, consists of sandstone interbedded between thick shale units. The groundwater is confined and discharges by both upward and downward leakage through the shales. The concentration of Cl − increases by about 2 orders of magnitude downgradient through the aquifer. This Cl − increase complicates 36 Cl dating of the system. Climatic changes, flushing of connate water, introduction of Cl − from older water below the aquifer, and ion filtration have been proposed as possible explanations for the increase. The 36 Cl data show a consistent decrease of the 36 Cl/Cl ratio downgradient, but an increase in the 36 Cl concentration downgradient in certain parts of the aquifer. Dates calculated from the 36 Cl/Cl ratio show a reasonable distribution in light of the hydraulic controls on the system, whereas dates calculated from the 36 Cl concentration give negative ages. This suggests that ion filtration is responsible for the Cl − increase and that the 36 Cl/Cl dates are to be preferred. We estimate water ages to be more than 2 m.y. near the distal end of the aquifer.