Deltaic sands and gravels

Hydrogeological Context




Deltaic sands and gravels unit description

The deltaic sands and gravels are mainly found along St-Anne and Jacques Cartier River. These deposits overlie Precambrian bedrock and sedimentary rock, in the Pont-Rouge sector. The Precambrian rock formations are massive and little fractured, showing unproductive aquifer. Sedimentary formations are highly fractured and shales are very brittle, demonstrating good potential aquifer. The rock formations are covered by a till layer. Marine silts and clays of the Champlain Sea followed by deltaic sands and gravels can overlie the till. The deltaic sediments are the most productive aquifer in the region. The high permeability, the extent and the large thickness of sediments gives them the best aquifer potential of the region. Since the permeability of the sediments and the flat topography, the annual recharge is highest in areas where the deltaic sediments outcrop. There are also units that supply the main groundwater municipal systems. Nevertheless, these aquifers are very vulnerable to surface contamination. A contamination introduced over these areas has the potential to penetrate and to spread vertically and horizontally across a large portion of the aquifer. Water types are calcium bicarbonate and calcium sulfate, showing good quality.
Hydrogeological regionSt. Lawrence Platform
Hydrogeological context
ReliefThe deltaic sands and gravels unit is located on flat terrain. Largely located along streams, this hydrogeological unit has low altitudes. Only sediments located north have higher elevations.
Area310.00 km2
DescriptionThe rock formations underlying the deltaic sands and gravels are from the Precambrian. From south to north, various groups of gneisses are observed: Bostonian Complex (othogneiss), Montauban Group (paragneiss) and granodiorite orthogneiss and gneiss. These are the most common under the deltaic sands and gravels. Limestone formations of the Trenton Group and shale of the Utica Group are found in the area of Pont-Rouge. The rock formations are covered by a till layer. Marine silts and clays of the Champlain Sea followed by deltaic sands and gravels can overlie the till.
AquifersList of aquifers surficial deltaic sand and gravel aquifer

Unit properties

Surficial aquifer media
Typical value:porous
Description The sediments (deltaic sand and gravel) have a high permeability.
Bedrock aquifer media
Typical value:karst - fractured
Description The igneous metamorphic rocks and the sedimentary rocks are the hydrogeological units underlying deltaic sands and gravels. The Precambrian rocks don't represent productive aquifer. The limestones of the sector of Pont-Rouge shows a beginning of karstification.
Typical value:unconfined
Surficial sediment thickness
Description The minimal and maximal surficial sediment thicknesses are 10-15 m and 45-50 m respectively. Those represent local thicknesses, generally mean surficial sediment thickness is 35-40 m.
Methode Interpolation of the bores has reached the rock
Well depth
21.3 m Range: [0 to 169.5] m
Groundwater depth
1.28 m Range: [0 to 14.7] m
Surficial unit hydraulic conductivity
0 m/s
Description represents the hydraulic conductivity of the deltaic sand, not the gravel.
Methode Hazen formula, based on 99 samples
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.
Regional recharge
Description Given the high permeability of deltaic sands and flat topography, the annual recharge is highest in areas where they are exposed.
Hydrogeological unit potential
Description This formation is the most productive aquifer formation of the area.
Groundwater usage
Typical value:porous
Description These units supply the main groundwater system of Portneuf area.
Aquifer vulnerability
5 Range: [0 to 6]
Description This aquifer is the most vulnerable to surface contamination. Contamination introduced over the outcrop areas of this formation has the potential to percolate and spread vertically and horizontally across a large part of the aquifer.
Groundwater Quality
Typical value:fresh
Description The water shows low mineralization. The two main types of water are calcium bicarbonate and calcium sulfate.