Oka Hills

Hydrogeological Context

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Version Metadata
1448b4e3-c71c-43e4-97d8-a4d2d6d99037
Project
Mirabel (AFSOQ)

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Oka Hills unit description

Update:2014-01-23
The topography of the sector is marked by the Oka hills and the fluvial terraces. The sector counts four types of aquifers: bedrock aquifer, outwash cones aquifer, perched fluvial/marine aquifer, superficial aquifer. The bedrock aquifer extends on the entire sector. It is covered by a thin discontinuous till layer. The bedrock outcrops in the Oka hills. The subglacial meltwater caused till erosion that formed the outwash cones aquifer. This aquifer is under confined conditions, thus its vulnerability to contamination is low. The aquifer can be connected to the underlying bedrock aquifer by till free windows. A discontinuous marine clay unit overlies the till deposit. At some places, the clay is covered by marine/fluvial sediments. Perched aquifer is formed where sedimentary deposit overlies the clay unit. Where the sedimentary deposit overlies a clay layer discontinuity, it forms a superficial aquifer. These are directly in contact with the bedrock aquifer. As the bedrock outcrops, the superficial aquifers and perched aquifer are under unconfined conditions and represent recharge areas. Those conditions can be applied to the bedrock aquifer because of many rock outcrops. Water percolates in the soil/rock and flows through the rock fractures to Outaouais River, Oak River or Two Mountains Lake. The recharge areas are more vulnerable to anthropogenic contamination. An important environmental problem in the Oka hills sector is the domestic exposure to radon, presents in alkaline rocks of Cretaceous intrusion.
Physiography
Hydrogeological regionSt. Lawrence Platform
Hydrogeological context
ReliefThe ground elevation increases from river and lake banks (0-13 m) to the Oka hills (204-249 m), located in the center of the sector. From the river, there are three successive fluvial terraces that raise the elevation of 30 m northbound. The terrain is characterized by rocky cliffs, where the altitude varies between 96-249 m atop Oka hills. Altitudes then decrease progressively to the north (44-73 m) apart for three rock outcrops that remain high elevations (125 m-140 m).
Area127.00 km2
Lithology
DescriptionOka sector is composed of Grenville bedrock, with a Cretaceous alkaline intrusion, composed of carbonatite rocks. These formations are covered by a thin discontinuous till layer. River banks and fluvial terraces show a marine clay unit that overlies the till layer. Fluvial and coastal sediments were deposited atop the clay unit. There are many bedrock outcrops in Oka hills.
Source
Aquifer
AquifersList of aquifers bedrock aquifer outwash cones aquifer perched/superficial aquifer

Unit properties

Surficial aquifer media
Typical value:porous - fractured
Description The fluvial and marine sediments form small discontinuous aquifers. These are perched when they are covered by clay. Oka hills are characterized by the rock outcrops.
Source Canadian Inventory of Groundwater Resources: Integrated regional hydrogeological characterization of the fractured aquifer system of southwestern Quebec
Bedrock aquifer media
Typical value:fractured
Source Canadian Inventory of Groundwater Resources: Integrated regional hydrogeological characterization of the fractured aquifer system of southwestern Quebec
Confinement
Typical value:confined - semi confined - unconfined
Description The sector is covered with thin continuous till layer and clay unit. Some fluvial and marine deposits overlie the clay unit, forming unconfined conditions.
Source Canadian Inventory of Groundwater Resources: Integrated regional hydrogeological characterization of the fractured aquifer system of southwestern Quebec
Surficial sediment thickness
13.32 m Range: [0 to 80] m
Description The superficial sediments include marine clay, till, fluvial and coastal sediments.
Source Canadian Inventory of Groundwater Resources: Integrated regional hydrogeological characterization of the fractured aquifer system of southwestern Quebec
Well depth
56.7 m Range: [0.9 to 224] m
Groundwater depth
3.6 m Range: [-0.53 to 19.87] m
Regional recharge
96 mm/y Range: [0 to 276] mm/y
Description The bedrock outcrops on Oka hills and the superficial aquifer design important regional recharge areas.
Source Canadian Inventory of Groundwater Resources: Integrated regional hydrogeological characterization of the fractured aquifer system of southwestern Quebec
Regional discharge
Description For the bedrock aquifer: Outaouais River and Two Mountains Lake
Source Canadian Inventory of Groundwater Resources: Integrated regional hydrogeological characterization of the fractured aquifer system of southwestern Quebec
Aquifer vulnerability
115.23 Range: [74.5 to 153]
Groundwater Quality
Typical value:fresh
Description The dominant water type is Na-HCO3. An important problem is the uranium/radon presence in the Cretaceous alkaline intrusions that can affect groundwater.
Source Canadian Inventory of Groundwater Resources: Integrated regional hydrogeological characterization of the fractured aquifer system of southwestern Quebec
Methode Water sampling and analysis