Alluvial floodplains

Hydrogeological Context

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0ea68738-1bd5-4755-9ed6-ce341fd5f47c
Project
Portneuf

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Alluvial floodplains unit description

Update:2014-01-30
The rock formations underlying alluvial floodplains belong to the crystalline bedrock or sedimentary formations. The crystalline rocks are massive and little fractured. The Paleozoic rocks of the sedimentary platform are highly fractured rocks, very friable in the case of shale. The rock formations are covered by a till, overlain by thick marine silts and clays of the Champlain Sea followed by alluvial floodplains. The alluvium includes fine sands, silts and clays. Exposed at the surface, alluvial floodplains have unconfined water table conditions. The aquifers are vulnerable to surface contamination. This little thickness of the deposit limits the water quantity available, causing weak aquifer potential. The surficial aquifer is used locally as water supply. However the sedimentary and igneous rock formations, underlying marine silt and clay and till constitute the main aquifers.
Physiography
Hydrogeological regionSt. Lawrence Platform
Hydrogeological context
ReliefAlluvial floodplain has low altitudes. However, these are most important for the alluvium located north. Bordering the Sainte-Anne River alluvial flood plains are relatively flat.
Area20.00 km2
Lithology
DescriptionThe rock formations underlying alluvial floodplains belong to the crystalline bedrock or sedimentary formations. The sections of the crystalline bedrock considered are mainly composed of gneiss and gabbro. Sedimentary formations include the shales of the Utica Group. The rock formations are covered by a till, overlain by thick marine silts and clays of the Champlain Sea followed by high terraces sands.
Source
Aquifer
AquifersList of aquifers alluvial floodplains aquifer

Unit properties

Surficial aquifer media
Typical value:porous
Description The alluvium includes fin sands, silts and clays.
Bedrock aquifer media
Typical value:fractured
Description The crystalline rocks are massive and little fractured. The Paleozoic rocks of the sedimentary platform are highly fractured rocks, very friable in the case of shales.
Confinement
Typical value:unconfined
Surficial sediment thickness
Description The alluvial floodplains are thick.
Hydrogeological unit thickness
Description The alluvial floodplains are thin.
Well depth
20.13 m Range: [7 to 96] m
Surficial unit hydraulic conductivity
Description The hydraulic conductivity of the clays underlying the alluvial floddplains is 5.3E-8 m/s The hydraulic conductivity of the till underlying the clays is 9.0E-7 m/s
Methode Hazen formula, based on 8 samples for the clays and 10 for the till
Regional precipitation
1121.5 mm/y Range: [1063.1 to 1168.2] mm/y
Methode Precipitation data are from Environment Canada. The value represents the average of the climate normals (1981-2010) measured on the stations distributed in or near the hydrogeological unit. The stations were selected so the average precipitation is as representative as possible of the whole hydrogeological unit.
Hydrogeological unit potential
Description The little thickness of the alluvial floodplains limits the water quantity available, causing weak aquifer potential.
Groundwater usage
Typical value:domestic
Description This surficial aquifer is used locally as water supply.
Aquifer vulnerability
4 Range: [3 to 5]