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Hydrogeology and hydrogeochemistry of the Chaudière River aquifers, Québec, Canada

A comprehensive hydrogeological study has been conducted in the Chaudière River watershed (6695 km2) located south of the St. Lawrence River, near Québec City. Updated maps of simplified bedrock geology and surficial sediments are presented together with the newly generated maps
of the regional potentiometric surface, spatial distribution of the annual infiltration rate and spatial distribution of encountered groundwater types. Detailed analyses of the hydraulic conductivity, groundwater use, salinity sources and isotopic signatures are conducted. At the end, a conceptual
model describing the hydrogeological setting is proposed for the regional groundwater flow system. In the watershed, about 150,000 inhabitants or roughly 80% of the total population depend almost entirely on groundwater as a source of potable water. Fine-grained sedimentary rocks of the Appalachian
geological province form the major regional aquifer unit. The aquifer is generally unconfined to semi-confined in the Appalachian Uplands, and semi-confined to confined in the Appalachian Piedmont. Local granular aquifers are composed of coarse river valley sediments, but their extent and thickness
vary throughout the watershed. The hydraulic connection between the granular units and bedrock is strongly influenced by the nature of the ubiquitous glacial sediments. The overall recharge rate to the bedrock aquifers is estimated at about 27 mm/y, or roughly 180 Mm3/y, compared to the average
groundwater use of 17.4 Mm3/y. The low hydraulic conductivity, K~8×10?7 m/s, constrains groundwater flow to the top portion of the fractured strata. Groundwater flow in the Appalachian Uplands occurs at local scale and is dominated by recent recharge. Further along the flowpath in the Appalachian
Piedmont, the concentrations of the major chemical constituents gradually increase along with the radiocarbon age of groundwater. Still, independent of their location along the flowpath, groundwater samples always contain a fraction of relatively young water (< 50 years).

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