Geochemical tracers of recharge-discharge functions in southern Quebec aquifer-peatland systems

Miryane Ferlatte, Marie Larocque, Vincent Cloutier

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Groundwater-Surface Water Interactions III

ABSTRACT: In areas where peatlands are abundant, they are likely to play a significant role in the hydrological and hydrogeological dynamics of a watershed. However, the processes behind aquifer-peatland interactions are not well understood. The objective of this study was to identify geochemical tracers to be used by catchment management authorities to assess the hydrogeological role of peatlands in southern Quebec, hence promoting sustainable management of groundwater resources. Twelve peatland profiles instrumented with piezometers were investigated in the Abitibi and Centre-du-Quebec regions of southern Quebec. The two regions have contrasted climate, geology and geomorphology. Abitibi peatlands are found on esker slopes while Centre-du-Quebec peatlands are depression wetlands. Variations of hydraulic heads, as well as variations of inorganic and organic composition of water from the superficial aquifer, peat pore water and groundwater from beneath the peatland suggest a variety of possible flow connections: groundwater inflow from the superficial aquifer, lateral outflow of peatlands water to the superficial aquifer and vertical peat pore water flow percolating down through the underlying mineral sediments. Only limited indications of vertical groundwater up flow from beneath the peatland have been observed. The first component (PC1) of a principal component analysis is characterized by elements typical of groundwater, and represents the primary control of the geochemical variation in peat pore water. Total dissolved solids (TDS) are highly correlated with PC1 scores and indicate probable zones of groundwater inflow through the peat at thresholds above ~14 mg/l. Diffusion calculation shows that the diffusion process alone cannot explain the composition of most peat pore water samples. This is a strong indication of a groundwater upward flow discharging from the mineral deposits beneath the peatland (when measured concentrations are higher than simulated concentrations) or of a peat pore water downward flow recharging the underlying mineral waters (when measured concentrations are lower than simulated concentrations). This study shows that a majority of the studied peatlands are long-term storage systems receiving lateral contributions from a superficial aquifer. Some peatlands provide water laterally to the superficial aquifer, some receive groundwater vertically and some lose water to the underlying deposits. All of these exchanges are important components in the sustainability of peatland hydrological functions. Keywords: peatlands, groundwater, geochemistry, hydrological functions, southern Quebec Technical theme: Groundwater-Surface Water Interactions and Ecohydrology 1 miryanef@gmail.com, (514-987-3000, poste 1515)

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