Présentations Colloques

    Flash Poster
    Session 5.02: Groundwater and global change
    Laurencelle Marc
    Modeling of the palaeo-hydrogeological evolution of a fractured-rock aquifer following the Champlain Sea Transgression in the St. Lawrence Valley (Quebec)
    A quantitative reconstitution scenario has been developed for the evolution of groundwater salinity within a regional aquifer system following deglaciation. In our study area, Montérégie Est (~9 000 km2, in southern Quebec, Canada), brackish groundwater at shallow depths is related to large palaeoenvironmental changes that took place over North America during the last glacial-deglacial cycle and the ensuing postglacial period. The region was covered by the Laurentide Ice Sheet for ~20 ka, until about 13 ka BP, at which time it was invaded by an arm of the Atlantic Ocean, thus forming the Champlain Sea. The seawater salinity eventually decreased, due to sustained meltwater production and isostatic rebound, until it formed a shallow freshwater basin, Lake Lampsilis. Subsequently, the drainage system evolved towards its present-day configuration through continued isostatic adjustment. Along with these spatio-temporal variations in water level and salinity, silts and clays were deposited during the flooding episodes. These fine sediments formed thick low-permeability units that retarded the transfer of saline seawater into the underlying fractured rock aquifer, but also impeded the later flushing of brackish water from the aquifer system, which still contains brackish groundwater of marine origin over a 2 200 km2 area.**A fully coupled vertical 2D density-dependent flow and mass transport numerical model was set up to simulate the marine and post-marine migration of salt within the study area. The relative influence of various processes and parameters was then assessed. The model was then further constrained with present-day observed groundwater salinities. Results show that salinization of the rock aquifer was a density-driven convection process, and that the accumulation of fines had a profound influence on salt migration, leading to the currently incomplete and uneven desalinization of the regional aquifer system.**In practical terms, this study offers a better understanding of regional groundwater dynamics and quality changes, along with improved estimates of groundwater age and vulnerability, which are key to sustainable management of the resource amid conflicting uses.


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