Stable isotope and chemical compositions of surface and subsurface waters in Saskatchewan

Abstract

The stable isotope and chemical compositions of water samples from several Upper Cretaceous and Quaternary aquifers in Saskatchewan were measured to determine the source of recharge within the individual aquifers and the various factors that have affected the waters in these aquifers since their formation. δD and δ18O values of waters from near-surface aquifers in Saskatchewan plot near the global Meteoric Water Line indicating that the waters present are meteoric in origin and generally have not been involved in appreciable evaporation or water-rock exchange reactions, as both processes enrich the samples differentially in D and 18O. Isotopic compositions in conjunction with water level records for the aquifers indicate that the predominant recharge occurs in the spring, coinciding with melting of winter snow accumulations in the province. Minor recharge is also expected from high intensity summer rain storms, however, the δD and δ18O values suggest that this input is minimal. Variations in the isotopic compositions of water samples from individual aquifers studied suggest that little or no interaction occurs between aquifers, even those located within the same stratigraphic zones. The dissolved constituents, which range from 200 to 3700 mg/1, are derived from the interaction of the recharging waters with sediments during infiltration to, and residence in, the aquifers, so that the chemical compositions of subsurface waters are controlled by the local geology. Due to the low temperatures and pressures near the earth's surface and the relatively rapid rate of infiltration, only the most soluble ions available within the sediments will dissolve. Aquifers within Quaternary and Upper Cretaceous sediments in Saskatchewan are saturated or nearly saturated with respect to several mineral phases including calcite, dolomite, anhydrite and gypsum, all of which are present in the local sediments. The isotopic compositions of surface waters in Saskatchewan vary substantially. Variations in the δD and δ18O values result from differences in the source of recharge (i.e. summer versus winter precipitation), surface temperature, size of the water body, and degree of evaporation that occurs in each body. More pronounced enrichments of D and 18O are recorded in the smaller lakes and creeks relative to the larger river systems as a result of surface evaportion. Because the amount of evaporation that occurs is dependent on temperature and degree of saturation of the local air masses, enrichments in D and 18O can be considerable during the summer months in Saskatchewan when temperatures are high and relative humidities low.