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Regional groundwater flow in inter-till and buried-valley aquifers, southwestern Manitoba

Inter-till and buried-valley aquifers are among the main sources of groundwater in the Prairies. The Spiritwood buriedvalley
aquifer system extends from Manitoba and across North Dakota where it has been a significant source for
groundwater supply for decades yet its resource potential in Manitoba has not been evaluated. The Geological Survey of
Canada (GSC) has been studying the regional geology and hydrogeology of the Spiritwood buried-valley aquifer in
southwestern Manitoba. Helicopter time domain electromagnetic (HTEM) and high resolution seismic reflection (HRSR)
surveys permitted detailed mapping of inter-till and buried-valley aquifers within the study area. The main buried valley is
a broad 10-15 km wide and 60-70 m deep valley cut into Cretaceous Pierre shale. A series of nested channels of
different age, origin, dimensions and orientations are cut into bedrock and within the sand/silt till. Where these channels
are filled with coarse sediment, they form aquifers: deep buried-valley aquifers (60-100 mbgs) incised into shale below
the base of the broad buried valley, and inter-till aquifers within the till. A 3D numerical geological model of the regional
bedrock surface and sediments was developed, using Leapfrog Hydro geological modelling software. A significant
feature of the geological model is that the nested channels and fans that form inter-till aquifers together with the fractured
bedrock surface may provide more permeable pathways to deep buried-valley aquifers. Another significant finding is the
discovery that the buried-valley aquifer outcrops in incised surface valleys where sustained baseflow was measured
during a period of extended drought.
Regional groundwater flow generally follows topography but is perpendicular to the orientation of the main buried valley.
However, flow also occurs along the deep buried valley which acts as a regional drain towards the incised surface
valleys. Both hydraulic and HTEM results suggest that there are no transverse hydraulic boundaries within the deep
buried-valley aquifer. A steady state finite element numerical groundwater model has been developed from the
geological model to: i) assess the different flow pathways within the inter-till and deep buried-valley aquifers, and ii)
constrain the estimates of groundwater flow. The numerical flow model replicates the regional flow patterns, discharge
zones and the groundwater divide in the deep buried-valley aquifer. Results to date suggest that, despite the presence of
preferred hydraulic pathways, natural replenishment of the deep buried-valley aquifer is small. © Her Majesty the Queen
in right of Canada 2015.

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