Stable isotope contents of a major prairie aquifer in Central Manitoba, Canada

The regional and seasonal variations of the 18O concentrations in several groundwater flow systems in a single confined carbonate aquifer are described. The regional distribution of the δ18O values is related to these flows and varies between about -11 and -21 per mille. Variations within individual flow segments are considerably smaller and rarely exceed 3 per mille. The isotopic composition of these groundwaters is apparently influenced by the physical characteristics (vegetation cover, gravel pits, etc.) of the different recharge areas. Each area produces groundwater with a unique isotopic composition which may vary by as much as 3 per mille between different areas with identical climates. Within flow segments which contain water not older than a few thousand years, a decrease in 18O is observed from the recharge to the discharge area. This is possibly due to a change in the recharge pattern after the native vegetation cover has been removed by agricultural activities. These differences in δ18O values within individual flow segments makes it impossible to compare the 18O contents measured in the discharge area with those of the present-day recharge areas. Preliminary 14C data indicate that in the southern portion of this aquifer groundwater older than 10 000 years is present. There the lowest δ18O values are found. These groundwaters have been recharged in a considerably colder climate than today before a major climatic improvement occurred about 10 000 years ago. A comparison of 18O and salinity data enabled the different flow systems to be delineated and the importance of the seasonally varying pumping rates in Winnipeg on the quality of waters from wells in zones of confluence of different systems to be demonstrated. (author)