Southern St Lawrence Platform

Hydrogeological Context

Version Metadata
1e0430c8-9747-4e66-a879-4ffe78c0606f
Project
Richelieu

Datasets

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Southern St Lawrence Platform unit description

Update:2014-02-17
In the context of the southern area of the St. Lawrence Platform of (south Lowlands), the clay unit is generally not continuous or thick. The bedrock is rather covered by a till unit of at least 10 m thick which may allow significant bedrock aquifer recharge rates. This limited sedimentary cover suggests that there would be links between the bedrock aquifer and streams, particularly along some sections of the Richelieu River, which constitute discharge areas. The flow is oriented east-west, from the recharge areas to Richelieu River or others discharge areas. The surficial permeable sediments with significant thickness have small spatial extension, thus that the aquifer potential is mainly based on fractured bedrock aquifer. In the unit, there is a significant use of groundwater as water supply. The predominant semi-confined conditions involve a moderate vulnerability of the bedrock aquifer. Groundwater exceeds frequently some aesthetic criteria as Fe, Mn, S, Na, and F in the central area of the hydrogeological unit.
Physiography
Hydrogeological regionSt. Lawrence Platform
Hydrogeological context
ReliefThe south platform is a slightly undulating plain with elevations ranging from 60 to 110 m. The slopes are smaller than 2%.
Area2144.00 km2
Lithology
DescriptionThe rock formations of the St.Lawrence platform are Ordovician and sedimentary. In ascending order, the platform stratigraphic sequence shows: Potsdam sandstone, Beekmantown sandstone/dolomite/limestone, Chazy sandstone and limestone, Black River dolomite, Trenton limestone, undifferentiated flysch, Lorraine shale/sandstone/siltstone and limestone and Queenston sandstone. Those rock formations unconformably overlie igneous and metamorphic rock of the Grenville Province and are commonly arranged in horizontal layers, slightly deformed by the formation of the Appalachians. The rock groups of Beekmantown, Chazy, Black River, Trenton, Ste.Rosalie and Lorraine are the most common in the south platform. A till unit overlie the rock formations. This unit is followed by discontinuous sandy and clayey sediments which thickness is smaller than 5m.
Source
Aquifer
AquifersList of aquifers potential undifferentiated sediments under clay - granular aquifers fractured bedrock aquifer (regional aquifer)

Unit properties

Surficial aquifer media
Typical value:porous
Description Till plain covered with relatively discontinuous and generally thin argillaceous sediments (<5 m). sequence: Discontinuous thin clay (< 5 m, 2.6 m average), except along the Richelieu River, where it approaches 20 m locally. Clay generally covers the till and, locally, other coarse sediments or rock.
Bedrock aquifer media
Typical value:fractured
Description Sedimentary Paleozoic rocks with low deformation on the western side of the Logan line (mainly sandstones, dolomites, limestones, mudstones and shales of Beekmantown, Chazy, Black River, Trenton, Ste. Rosalie and Lorraines groups).
Confinement
Typical value:semi confined
Description For the regional rock aquifer: Semi-captive conditions predominate, with some places under confined and free conditions.
Surficial sediment thickness
8.4 m Range: [0 to 27.9] m
Well depth
12 m Range: [4 to 38] m
Groundwater depth
2.7 m Range: [0 to 41.4] m
Bedrock hydraulic conductivity
0.0001 m/s
Description Hydraulic conductivity for the hydrogeological unit The median value for hydraulic conductivity is 10^-5.0 m/s, but it ranges from 10^-6.5 to 10^-3.8 m/s. Hydraulic regional conductivity The decreasing trend of transmissivity with depth in the rock is observed in all contexts. The average hydraulic conductivity is 10E-3.9 m/s near the top of bedrock (z = 1 m) which gradually loses an order of magnitude at a depth of 10 m (10^-4.9 m/s), 25 m (10^-5.9 m/s), 60 m (10^-6.9 m/s) and 200 m (10^-7.9 m/s).
Regional precipitation
1064.5 mm/y Range: [965.1 to 1295.3] mm/y
Regional evapotranspiration
558.9 mm/y Range: [375.1 to 649] mm/y
Regional runoff
364.4 mm/y Range: [62.5 to 1117.3] mm/y
Regional recharge
100.1 mm/y Range: [0 to 500] mm/y
Description Moderate recharge rates through the semi-confined till areas
Regional discharge
Description Discharge predominantly occurs along the Richelieu River (mainly in the portion between the geological contact Trenton/Lorraine Formations) and the Chambly basin. There are local discharges at the edge of Piedmont and towards the Missisquoi Bay.
Aquifer vulnerability
137.5 Range: [64.8 to 191.5]
Typical value:medium regional
Description The vulnerability is generally moderate but there are some places where it's high.
Groundwater Quality
Typical value:fresh
Description Frequent exceeding of the aesthetic criteria for Fe, Mn, S, Na, and F in the central part of the area.