REGIONAL HYDROGEOCHEMICAL COMPARISON OF NEAR-SURFACE TILL, CARBONATE, AND DEEPER SANDSTONE AQUIFERS, EAST OF THE RED RIVER SALINE FRONT IN SOUTHERN MANITOBA, CANADA

Groundwaters were sampled from domestic wells in southern Manitoba, Canada, as part of a large-scale regional hydrogeochemical comparison between the major aquifers of the area. The 6000 square km study area is on the eastern flank of the Williston Basin and is defined by the United States border to the south to 60 km north of the border, and the Red River in the west to the Canadian Shield in the east. There are three main aquifers present: (1) shallow sand and till, (2) deeper Ordovician to Silurian carbonates, and (3) basal Ordovician sandstone. Field measurements included pH, Eh, dissolved oxygen (DO), temperature, and conductivity, taken as quickly as possible directly from the source. Groundwaters were analyzed for a full suite of major and trace elements by Inductively Coupled Plasma (ICP) and for light stable isotopes (C, O, H, S) by mass spectrometry. Stable isotopes are being used to constrain groundwater flow, recharge and mixing between the aquifers. Groundwaters have variable conductivity (0.1 to 7.4 mS) and, in general, salinities increase from east to west towards the saline front near the Red River. Alkalinity ranges from <100 to >1000 mg/L. Wells with alkalinities >350 mg/L are uniformly shallow, whereas, deeper (40-140 m) wells have <250 mg/L alkalinity. Oxygen and hydrogen isotope data indicate that most waters represent modern recharge, although shallow wells (<10 m) are isotopically heavy, suggesting influence of summer precipitation.