Northern St Lawrence Platform

Hydrogeological Context

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

Project
Richelieu

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Northern St Lawrence Platform unit description

Update:2014-02-17

Physiography
The context of the northern part of the platform of St. Lawrence (North Lowlands) is characterized by low relief and especially by relatively thick clayey sediments (> 20 m). These conditions imply a minimum recharge of the regional bedrock aquifer and a very low groundwater flow. This situation implies the presence of brackish water in the North Lowlands bedrock aquifer is due to partial leaching Champlain Sea water. This brackish water is not potable, resulting in a low aquifer potential for the bedrock, explaining the minimal use of groundwater and the use of surface water for water supply. The portion of the Yamaska between Montérégiennes and the St. Lawrence River is an important discharge. Other low diffuse dicharges are found where the clay is thinner, especially in rivers. The clay unit protects the bedrock aquifer, reducing the risk of contamination from the surface. Surficial sediment deposits such as sands overlying clay and the undifferentiated sediments in underlying clay form superficial granular aquifers.
Hydrogeological region	St. Lawrence Platform
Hydrogeological context
Relief	The north platform is a plain having elevations commonly below 60 m. The slopes are inferior at 2%, except in vicinity of the river where some slopes are 2-6%.
Area	2686.00 km2
Lithology
Description	The 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. They are covered by a clay unit with thickness of more than 10 m, followed by thin sandy littoral sediments unit and locally alluvial sediments.
Source
Aquifer
Aquifers	Liste des aquifères: surficial marginal aquifer (located in the thin sands) potential undifferentiated sediments under clay - granular aquifers fractured bedrock aquifer (regional aquifer)

Unit properties

Surficial aquifer media


Typical value:	porous
Description	Thick clay cover (> 10 m) covered by thin sandy coastal sediments, and locally thicker alluvial sediments (near Sorel). In some places, the coarse sediments under clay (including till) have more than 10 m. Stratigraphic sequence: Generally thin discontinuous sands (< 3 m) (except north of the area where they are 10-20 m thick); thick clay (10 to > 35 m, average 22 m) overlying rock or in some places till and / or thick coarse granular clusters well extended (> 10 m).

Bedrock aquifer media

Typical value:	Fractured
Description	Sedimentary Paleozoic rocks with low deformation, located at the west of Logan line. These rocks are mostly shales and sandstones of Queenston and Lorraine Groups.

Confinement

Typical value:	confined
Description	The regional rock aquifer is under absolute confined conditions, except in some local places, for example Contrecoeur, Varennes and some areas along Yamaska River.

Surficial sediment thickness
29.5 m Range: [0 to 85] m

Well depth
21 m Range: [6 to 63] m

Groundwater depth
3.8 m Range: [0 to 35] m

Bedrock hydraulic conductivity
0.00001 m/s
Description	Hydraulic conductivity for the hydrogeological unit The median value for hydraulic conductivity is 10^-6.0 m/s, but it ranges from 10^-7.3 to 10^-4.7 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 10^-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
1031.5 mm/y Range: [997.6 to 1295.3] mm/y

Regional evapotranspiration
581.3 mm/y Range: [369.4 to 648.9] mm/y

Regional runoff
458 mm/y Range: [76.9 to 980.1] mm/y

Regional recharge
13.8 mm/y Range: [0 to 312.2] mm/y
Description	low recharge rates

Regional discharge

Description	Significant emergence in the portion of the Yamaska between Montérégiennes and the St. Lawrence River. Diffuse emergence in different places and low discharge on major rivers where the clay is thinner. Little opportunity for the emergence river because of the thick layer of clay

Hydrogeological unit potential

Typical value:	low
Description	Groundwater isn't drinkable.

Aquifer vulnerability
90.3 Range: [61 to 184]
Description	Inside the North Platform, the presence of a thick layer of low permeability clay sediments causes low to very low vulnerability. However, locally the vulnerability can be intermediate or even high in areas where the thickness of the clay is less important (eg southeast of Contrecoeur and Varennes, some sections along Yamaska River).

Groundwater Quality

Typical value:	brackish
Description	Groundwater is generally not drinkable. Groundwater exceeds potability criteria Ba and F, and many aesthetic criteria, including Cl, Na, Fe and Mn and SO4 and S.