ASSESSING ENVIRONMENTAL ISOTOPES AS TRACERS OF GROUNDWATER ORIGIN AND FLOW IN GLACIAL AQUIFERS IN SASKATCHEWAN

A significant proportion of Saskatchewan’s population relies on groundwater to fulfill their domestic needs. This is especially true for rural communities, where over 80% of Canada’s rural population is reliant on groundwater. Much of this groundwater in Saskatchewan is produced from a series of shallow glacial aquifers located between the modern-day ground surface and the surface of the Cretaceous bedrock in sediments deposited during numerous glaciations during the Pleistocene Epoch. Many of these systems remain poorly understood, including in terms of their sources of recharge, and how they fit into regional groundwater flow regimes. Environmental isotopes, such as δ18O, δ2H, and 3H, have often been used as tracers in identifying the source, flow, and mixing of groundwater, as well as inferring its age. The research goals of this study include establishing if the environmental isotopes are effective at delineating the origins of groundwater in the intertill and buried valley aquifers of Saskatchewan, and to integrate the environmental isotopes into conceptual models for groundwater flow. Groundwater samples were taken from intertill and buried valley aquifers in west-central Saskatchewan. These groundwater samples were analysed for stable water isotopes (δ18O and δ2H), radioactive isotopes (3H), as well as for water chemistry. Along with historic environmental and water chemistry measurements taken from the provincial observation well network, the environmental isotope and water chemistry results were integrated to develop a conceptual model of regional groundwater flow in the study area. The groundwater sample’s stable water isotope values tended to plot along the trend of the meteoric water lines, with no significance fractionation processes affecting the isotope levels post-recharge. Despite the surficial, intertill, and bedrock aquifers occupying separate stratigraphic positions, they largely share similar ranges of δ18O and δ2H values with no notable patterns related to aquifer or geographic position of the groundwater sample. Detectable levels of 3H were only found in several intertill aquifers, indicating the presence of modern or sub-modern precipitation in these aquifers. The groundwater samples tended to be more isotopically similar to winter precipitation in Saskatchewan, though there is little consistency in the aquifer or aquifer type’s stable water isotope levels similarity to either summer or winter precipitation. Stable water isotope values from the Battleford Till tended to have lower δ18O and δ2H values than the majority of aquifer groundwater samples, and was isotopically more similar to expected stable water isotope values for the Pleistocene Epoch. [GF1.1] The conceptual model calculation for groundwater flow estimates that infiltrating waters from surface would take 10’s to 100’s of years to infiltrate vertically to the intertill aquifers, while taking 1000’s to 10,000’s of years for this water to reach the buried valley aquifers. The stable water isotope values provided poor constraints on the conceptual model calculations, while other data, such as 3H or other groundwater age estimates and groundwater chemistry were more effective metrics to validate the calculated groundwater travel times. Stable water isotopes were insufficient at determining the origins of the groundwater in the intertill and buried valley aquifers due to overlapping values in aquifers of different stratigraphic positions. Stable water isotope values tended to decrease with greater extents of water-rock interaction as shown by the water chemistry results. The chemistry observed in the surficial aquifers was commonly Ca-HCO3 type with low TDS, while concentrations of SO4 and TDS tended to increase with groundwater age. The groundwater chemistry is often reflective of the groundwater’s age and degree of confinement from overlying low permeability units. Any additional groundwater sampling should include further analysis of stable water isotopes, to improve spatial coverage of the aquifers, as well as radioactive isotopes for further age dating of the groundwater in the intertill and buried valley aquifers.

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