Regional Shallow Stratigraphy and Hydrogeology of the Grande Prairie–Valleyview Area, Northwestern Alberta

Basal coarse-grained sediments (sand and gravel deposits overlying bedrock and underlying fine-grained glacial and glaciolacustrine sediments) constitute important regional aquifers and aggregate resources in the Peace River valley and adjacent plains and uplands. Previous studies have indicated that similar deposits are present in the Grande Prairie–Valleyview area, and may serve as aquifers there.

In this study, the three-dimensional geometry of a basal coarse-grained sediment unit in the Grande Prairie–Valleyview area was rendered from water well drill log reports, subsurface geological investigations, and field investigation. The hydrogeological conditions of the basal coarse-grained sediment unit, as well as adjacent units, including the overlying fine-grained sediments and the underlying uppermost bedrock, were then characterized. Although the basal coarse-grained sediment unit is widely distributed in the Grande Prairie–Valleyview area, it is generally thin and discontinuous. The vast majority of water wells completely penetrate the unit and are completed in underlying bedrock. Within buried bedrock valleys where basal coarse-grained sediment thickness exceeds 5 m, some wells are completed within the sediment unit. However, even in these areas, most wells are completed in bedrock or well completions include screens that bridge bedrock and basal coarse-grained sediments.

Groundwater chemistry in the Grand Prairie-Valleyview area extends from fresh Ca-Mg-HCO3–type waters to older Na-SO4–type waters, which are more prevalent. Groundwater chemistry samples show no clear distinction between wells completed in bedrock and those completed in basal coarse-grained sediments, possibly indicating that basal coarse-grained sediments are recharged by groundwater from underlying bedrock units and not by precipitation/snowmelt from above. Due to potential hydraulic connectivity between bedrock aquifer systems and basal coarse-grained sediments in the Grande Prairie–Valleyview area, it may be useful to consider bodies of basal coarse-grained sediments as extensions of bedrock aquifers rather than as discrete aquifers. Where the overlying fine-grained sediment unit is thin, basal coarse-grained sediments may serve as a hydraulic conduit between bedrock aquifers and the surface.