Simulating groundwater-peatland interactions in southern Quebec

Anne Quillet, Marie Larocque, Claudio Paniconi

Dans les comptes rendus d’articles de la conférence: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Groundwater-Surface Water Interactions III

ABSTRACT: Few studies have developed conceptual models of aquifer-peatland exchanges. It is important to understand interactions between aquifer and peatland in different contexts in order to grasp regional hydrogeological processes as well as the influence of aquifers in peatland evolution. Eight peatlands have been instrumented in the Bécancour (Centre-du-Québec) and Amos (Abitibi-Témiscamingue) regions in southern Quebec. At each site, either one or two transects of six piezometer nests (at 1.20 m depth in the organic deposits and in the mineral deposits below the peat) have been installed. The stratigraphy and geometry of the two compartments, as well as the hydrodynamic properties of the organic and mineral deposits have been measured. Groundwater levels have been recorded from autumn 2010 to summer 2012. Precipitation averages are 1114 and 912 mm/an in the Bécancour and Amos regions. The maximum peat thickness measured in the Bécancour peatlands is 6.4 m while it is 4.5 m in the Amos region. In both regions, peatlands are fringed by sandy deposits that extend at least partly under the organic deposits. The thickness of these underlying deposits is not well defined, but available data suggests a metric scale thickness in areas close to the adjacent superficial aquifer. The Bécancour peatlands have developed in depressions while the Amos peatlands have developed through the paludification of esker slopes. The main purpose of the study was to use groundwater flow modeling to understand exchanges between aquifer and peatland in two different geo-climatic contexts of southern Quebec. Field data is used to create 2D numerical models with GMS-Modflow to simulate flow between the shallow groundwater and the peatland on the eight peatland sites. The models are first calibrated to reproduce measured heads, head gradients and temporal variations. In order to assess typical flow patterns and exchanges, a sensitivity analysis of the model is then performed. Results show that the geometry of the peatland-aquifer complex, the hydrogeological parameters and the recharge conditions control flow exchanges between the peatland and aquifer compartments. Based on the influence of each parameter on the flow, a graphical tool is proposed to help estimate the exchanges between groundwater and peatlands when limited data is available.

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