Aquifer System

Portneuf


Igneous/metamorphic rocks (outcropped)
The crystalline bedrock is mainly composed of gneisses, granites and migmatites with the presence of gabbro and crystalline limestones. These rock formations are massive, slightly fractured and have a terrain of locally rugged hills. The igneous/metamorphic rocks outcrop on an important part of the region, mainly in the northeast/north. Those outcrops create unconfined water table conditions. The aquifer is vulnerable to contamination from the surface. Water percolates and flows through fractures and voids in the rocks. Considering the weak fracturing, water flows slowly causing a low aquifer potential. Groundwater type is calcic bicarbonate and it shows low mineralization.
Sedimentary rocks (outcropped)
The sedimentary rocks include limestones of Trenton Group and shales of Utica Group. These are highly fractured rocks, very friable in the case of shales, and affected by the dissolution process that led to an early karstification in limestone of St. Casimir and Pont-Rouge. The sedimentary rocks outcrop on some sections along the St. Lawrence River, creating unconfined conditions. The aquifer is vulnerable to contamination from the surface. Surface water infiltrates and flows through fractures and voids in the rock. The aquifer potential is good and water calcium bicarbonate-type show low mineralization.
Sedimentary and igneous rocks overlain by marine clays
The bedrock includes two major geological units: crystalline bedrock and rocks from the sedimentary platform. 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. A thick marine silts and clays unit covers the bedrock. Granular formations such as till or glaciofluvial sediments may be between the rock and fine sediments, but current boreholes data do not allow defining clearly these units. The water in these geological formations flows very slowly and little or no involvement in the groundwater flow process is observed in permeable granular surface. It should be noted, however, that below the low permeability granular units may be units with high potential aquifer such as sand and gravel. Considering that the aquifer potential of surficial formations is minimal or zero, the rock aquifers are the main exploited aquifers in these areas. The rock formations have low aquifer potential. The clay cover confines the bedrock aquifers, and consequently helps to protect those aquifers from potential surface contamination.
Sedimentary rocks overlain by till
The sedimentary rocks include limestones of Trenton Group and shales of Utica Group. These are highly fractured rocks, very friable in the case of shales, and affected by the dissolution process that led to an early karstification in limestone of St. Casimir and Pont-Rouge. The rocks are covered by a continuous till unit. Depending on the extent and thickness of till that overcomes the aquifer, the water table conditions are highly variable. It is the same for the vulnerability of the aquifer, which is also function of the till deposits characteristics. Groundwater flows through fractures and voids in the rock. The aquifer potential is good and the water of calcic bicarbonate type show low mineralization.
High terraces sands
The rock formations underlying high terraces sands 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 high terraces sands. Exposed at the surface, high terraces sands have unconfined water table conditions. The aquifers are vulnerable to surface contamination. The thickness of the sand deposits varies between 1-2 m. This little thickness 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. The water type is calcic/sodium bicarbonate. It shows low mineralization.
High terraces sands on deltaic sands and gravels
The high terraces sands on deltaic sands and gravels are overlying Precambrian bedrock and sedimentary platform. The Precambrian bedrock is massive and little fractured, showing unproductive aquifer. Sedimentary formations are highly fractured and shales are very brittle, demonstrating good potential aquifer. These units are covered by marine silts and clays of the Champlain Sea followed by deltaic sands and gravels and finally a thin alluvium layer. These sediments represent the hydrogeological unit called high terraces sands on deltaic sands and gravels. This unit are partly located in the discharge areas, along rivers Saint-Anne and Jacques Cartier, mainly in the northeast section of Portneuf area. The predominant hydraulic connection between aquifer and rivers is indirect, showing that groundwater seeps along the river banks and that the river bed overlies a silt and clay unit or the bedrock. On the Jacques Cartier River, some sections of the river bed overlie sedimentary rocks or sands and gravels, suggesting connection between the two aquifers. The connection is the discharge area. The high terraces sands on deltaic sands and gravels have one of the best aquifer potential. The water table is under unconfined conditions, thus the aquifer is vulnerable to surface contamination. Groundwater types are sodium bicarbonate and calcium chloride and shows low mineralization.
Alluvial floodplains
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.
Fluvioglacial sands and gravels
The territory sections, where the glaciofluvial sands and gravels are present count three aquifers: Precambrian bedrock aquifer, sedimentary formations aquifer and glaciofluvial sands and gravels aquifer. The Precambrian bedrock is massive and little fractured, showing an unproductive aquifer. Sedimentary formations are highly fractured and shales are chattered, demonstrating good potential aquifer. These units are covered by discontinuous till layer. At some places, the glaciofluvial sands and gravels aquifer directly overlies the rock aquifers. The glaciofluvial deposits aquifer are located along Sainte-Anne and Jacques-Cartier rivers in the northeast part of Portneuf area. The glaciofluvial sands and gravels have a high permeability, giving a high aquifer potential. However, the extent of glaciofluvial deposits is often limited, thus limiting the amount of usable water. This type of aquifer is exploited for private wells, mostly shallow wells or wellpoint. The water table is under unconfined conditions, thus the aquifer is vulnerable to surface contamination. The groundwater type is calcic chloride.
Igneous/metamorphic rocks overlain by a continuous till cover
The crystalline bedrock is mainly composed of gneisses, granites and migmatites with the presence of gabbro and crystalline limestones. These rock formations are massive, slightly fractured and have a terrain of locally rugged hills. A continuous till unit overlies these rock formations. Depending on the extent and thickness of till that overcomes the aquifer, the water table conditions are highly variable. It is the same for the vulnerability of the aquifer, which is also function of the till deposits characteristics. The aquifer potential is weak and the water of calcic bicarbonate type show low mineralization.
St. Narcisse Moraine
The territory sections, where St. Narcisse moraine is present count three aquifers: Precambrian bedrock aquifer, sedimentary formations aquifer and St. Narcisse moraine aquifer. The Precambrian bedrock is massive and little fractured, showing an unproductive aquifer. Sedimentary formations are highly fractured and shales are very brittle, demonstrating good potential aquifer. These units are covered by till layer. The till is overlain by St. Narcisse morainic deposit and sand and gravel. The sand and gravel are just found on the flanks of the moraine. It is formed of a mixture of blocks of sand and gravel contained in a silty sandy matrix. Although the aquifer potential of Moraine has not yet been demonstrated, the presence of sand and gravel suggests that the aquifer has a good potential. Since the summit of the moraine is not covered by sand and gravel, the water table is unconfined allowing surface water to percolate. The aquifer is vulnerable to surface contamination. The water type is sodium bicarbonate.
Deltaic sands and gravels
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.
Littoral sands
The territory sections, where littoral sands are present count three aquifers: Precambrian bedrock aquifer, sedimentary formations aquifer and surficial littoral sands aquifer. The Precambrian bedrock is massive and little fractured, showing an unproductive aquifer. Sedimentary formations are highly fractured and shales are very brittle, demonstrating good potential aquifer. These units are covered by till layer. The till is overlain by a thick Sea Champlain marine silts and clays unit, followed by littoral sands. The littoral sands aquifer is located along the deltaic sand and gravel deposits. The littoral sands have a high permeability, giving a high aquifer potential. However, the thickness of the littoral sand deposits is thin, thus limiting the amount of usable water. This type of aquifer is exploited for private wells, mostly shallow wells or wellpoint. The water table is under unconfined conditions, thus the aquifer is vulnerable to surface contamination. The groundwater type is calcic bicarbonate/calcic sulfate, showing low mineralization.
Project Relevant Publications More info
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PortneufCartographie hydrogéologique régionale du piémont laurentien dans la MRC de Portneuf: géologie et stratigraphie des formations superficielles.
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