| Glaciofluvial sand and gravel aquifer (esker) |
| Eskers trend across the Ottawa valley from north to south and have been mapped at the surface and inferred at depth within the Champlain basin part of the valley by Gorrell (1991). Extensive sedimentological work has been completed on the subaqueous fan portions of the eskers (see refs in Cummings and Russell, 2007). Detailed investigations of the Vars-Winchester and Crysler-Finch eskers have been completed to support Source Water Protection Studies (Cummings and Russell, 2007). The Vars-Winchester esker is an elongate, 40-km long, partially-buried ridge of sand and gravel extends north-south across the study area. It consists of two key elements, a gravely central-ridge and a sandy-fan carapace. The gravely central-ridge of the Vars-Winchester (VW) esker is 2 to 20 metres high (average 15 m). Since the VW esker is mostly buried, its lateral and longitudinal continuity is uncertain. Conceptually, the esker is assumed to be continuous; however, potential gaps have been identified. Since the esker underlies Champlain Sea mud unit, the aquifer is confined except where the esker outcrops at surface. The esker aquifer is recharged from precipitation that falls directly on the exposed esker and possibly from streams that intercept the esker. The gravelly core of the VW esker has a higher permeability than its surroundings, offering a preferential pathway for groundwater flow. Hence flow within the system may occur preferentially along the length of the esker where the regional gradient allows. The longitudinal groundwater flow within the esker may not be unidirectional. Groundwater flow within the esker, converge towards the Castor River and diverge at the local topographic high of Maple Ridge in the south. Based on bedrock topography, another groundwater divide seems to occur in the north near Vars. Groundwater discharges into streams. Esker aquifers are prolific and have production rates of up to 31 L/s. The eskers are excavated by a number of sand, gravel and aggregate companies. |
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| Till aquitard |
| The Vars-Winchester esker area consists of four main geological units. In ascending order, these units are: bedrock, till, glaciofluvial sand and gravel in eskers and Champlain Sea mud and sand. The latter may be overlain by alluvial sediments and organic deposits. The till is an extensive sheet that may be discontinuous. It is typically massive and considered to be 'over-consolidated'. The top surface of the regional till, where exposed, is sculpted into elongate, parallel, streamlined ridges (drumlins) that are oriented north-south. A thin (below 2 m) gravel sheet is located above the regional till and beneath Champlain Sea mud in the north part of the study area. The gravel sheet is considered as part of the contact zone aquifer with moderate hydraulic conductivity; however the till has low hydraulic conductivity. This unit is thin and confined by the Champlain Sea mud, except where till outcrops. |
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| Fractured bedrock aquifer of South Nation River Basin |
| The Ottawa River Valley occupies the Ottawa-Bonnechere Graben, which is a 175 million years old Mesozoic rift valley. In the Ottawa valley portion of The St. Lawrence Lowlands it underlain by down-faulted, nearly-horizontal Paleozoic carbonate and silici-clastic rocks that are extensively covered by till and Champlain Sea mud. The South Nation River watershed is underlain by 68% carbonate rocks, 31% clastic rocks and 1% metasedimentary rocks and is thickly covered by glacial till and glacio-marine clays with partially buried eskers (Telmer, 1996). In the study area, with the exception of a single well drilled to bedrock along Route 300, which intersected shale, non-fissile carbonate-mudstone was recovered at the base of newly drilled wells (Cummings, 2007). From a hydrogeological perspective, the bedrock is typically fractured, metre-spaced vertical fractures are ubiquitous in nearby quarries, and invariably extend beneath the quarry floor. Visible weathering (e.g., pervasive rusty colour) is typically absent below the bedrock surface. Hydrogeological data suggest that the upper several metres of bedrock have an order-of-magnitude higher permeability than the bedrock below. Wells in fractured-bedrock typically yield <0.6 L/s (Charron, 1978). |
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| Champlain Sea mud aquitard |
| From bedrock upward the Quaternary sedimentary succession consists of diamicton, esker sand and gravel and Champlain Sea mud and littoral sand. Locally alluvial sediment and organic deposits are present at the surface. Champlain Sea deposit includes three subunits, in ascending order: rhythmically laminated mud and sand, massive mud and stratified mud, locally with sand layers near or at the top. The deposit is considered as an aquitard, showing low hydraulic conductivity. It protects the underlying aquifers from surface contamination, but it also limits the recharge of the aquifer system. |
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