EFFECT OF SUB-HYDROSTATIC CONDITIONS ON BASIN-SCALE GROUNDWATER FLOW IN THE SOUTHERN PORTION OF THE CANADIAN ROCKIES FORELAND BASIN

The foreland basin of the Canadian Rockies encompasses major parts of the province of Alberta and some parts of Saskatchewan and British Columbia. The present study explores the nature of basin-scale hydrodynamics in the major clastic bedrock aquifers of this foreland basin. Previous studies have noted normal and sub-hydrostatic conditions in these aquifers. Hydrostratigraphic modeling has resulted in delineation of seven units overlying the mid-Cretaceous aged, basin-scale aquitards of the Colorado group. Analyses of formation fluid pressure with depth confirmed the existence of a zone characterized by sub-hydrostatic pressure conditions. The magnitude of the difference between expected hydraulic head values under normal hydrostatic conditions and those determined by analysis of pressure values can be up to 1000 m in the south-west part of the study area in the vicinity of the city of Calgary. The geostatistically-determined transition zone between normal and sub-hydrostatic conditions can be found at depths as shallow as ~150 m below ground surface southeast of the city of Calgary, with the transition zone deepening substantially towards the north.

Analytical mapping and groundwater flow modeling indicate predominance of topography-driven, local to medium-scale flow systems in the upper hydrostratigraphic units (Drift, Paskapoo, Scollard), with recharge of these units occurring in the Canadian Rocky mountains front. Groundwater flow within these units is towards the northeast. In the lower aquifer units (Horseshoe Canyon and Belly River) the potential flow paths split into a northeast trending, gravity-driven system and a southwest trending, sub-hydrostatically driven flow-system. The sub-crop of the Bearpaw aquitard unit acts as a regional flow barrier. Estimation of flow time suggests that the recharging water can reach the base of the Horseshoe Canyon aquifer within a few hundred to a few thousand years. However, it would take >10,000 years for groundwater to cross the Bearpaw aquitard unit. In spite of there being a strong flow gradient across the Bearpaw unit, only a minor volume of groundwater is able to flow towards the sub-hydrostatic centre. This is a potential cause for the continued existence of the sub-hydrostatic zone, which is hypothesized to have been created during the last glacial maxima.