ATTEMPTING TO DETERMINE GROUNDWATER INTERACTION WITH A REGULATED GLACIAL-FED RIVER AS IT FLOWS ONTO THE PRAIRIES

The North Saskatchewan River (NSR) originates in the Rocky Mountains and flows east to the prairies, passing through the City of Edmonton, Alberta where it is the sole water source for over 1 million people. As a glacial-fed river, nearly 90% of its water is generated in the mountains, which is regulated by two hydroelectric dams that provide more consistent flow as it travels to the semi-arid prairies. While the role of groundwater is generally acknowledged as the primary source of the NSR in the winter and during dry periods, the exact contribution of groundwater is unknown. To begin learning about groundwater interaction with the NSR, we use a combination of river sampling and groundwater modeling to investigate the area upstream of the City of Edmonton where the NSR emerges from the foothills onto the prairies. Water samples were collected from 13 river locations and 8 tributaries along a 200 km segment of the NSR several times during a 3-year period. Samples were collected during open water flow periods and spanned a range of seasons and river flow rates. Samples were analyzed for geochemistry, stable isotopes of water, and occasionally for additional environmental tracers (e.g., radon-222, strontium isotopes). A steady-state groundwater model (MODFLOW) developed to understand surface water – groundwater interactions in the Edmonton region was found to have a realistic representation of the complex hydrostratigraphy, including recently mapped pre-Laurentide buried valley systems that have incised into bedrock formations and form local aquifers. While the longitudinal concentrations in the river were generally consistent for each sampling event, subtle variations in elements associated with geological weathering (e.g., Sr, Ba) appeared to indicate areas of groundwater discharge. Radon-222 and strontium isotopes also show promise in learning about variability of groundwater interaction. The interpretation from geochemical results is consistent with the flow system represented in the groundwater model. Together, river sampling and groundwater modeling shows promise in determining patterns of groundwater interaction for a regulated river on a prairie landscape.