DISSOLVED-GASES IN THE MILK RIVER AQUIFER, ALBERTA, CANADA

Geochemical relations between the noble gases, Ar-40/Ar-36 ratio, N2 and CH4 contents of groundwaters from the Milk River aquifer, Alberta, Canada, have been investigated. The CH4 contents for many of the groundwaters are several times their N2 contents and cause degassing to occur as the hydrostatic pressure decreases during their ascent in the wells. Recharge temperatures, derived for those groundwaters that were not degassed, indicate that the youngest groundwaters were recharged under cooler climatic conditions than prevailed during earlier recharge. This conclusion is supported by the stable isotope compositions of the groundwater. The He-4 concentration in the groundwater varies with depth according to the theoretical concentration/depth profile for accumulated radiogenic He-4 in the aquifer and its adjacent shales. The corresponding flux of He-4 to the atmosphere is < 2% of the total crustal He-4 production rate. Excess radiogenic Ar-40 is associated with CH4 and both gases originated within the adjacent shales. The CH4 is biogenic and was produced in an aqueous environment which ranged from marine to freshwater as the connate fluids in these marine shales were replaced. The degassing process has been modelled to explain the residual dissolved gas contents of the groundwater and to show that the CH4/N2 ratio in situ was approximately 12.