How do hydrogeological setting and meteorological conditions influence water table depth and fluctuations in ombrotrophic peatlands?

Résumé 0

Peatlands are wetland ecosystems where net primary production exceeds organic matter decomposition. They are characterized by a near-surface water table controlled by a combination of internal and external processes, influenced by short-term meteorological and long-term climate variations among other factors. Site-specific conditions, such as peat hydrodynamic properties, surface vegetation patterns, and hydrogeological setting also substantially influence water table dynamics. The objective of this work was to characterize the influence of hydrogeological setting and meteorological conditions on water table depths (WTD) and on fluctuations therein in seven ombrotrophic peatlands in or near the St. Lawrence Lowlands (southern Quebec, Canada). Up-gradient, mid-gradient, and down-gradient locations were monitored in the seven peatlands, using dipwells with hourly WTD recordings. WTD was also monitored in the marginal minerotrophic zone found in three of the seven peatlands. Additionally, heads in the outflow (i.e., receiving diffused water from the peatland) and inflow (i.e., providing diffused water to the peatland) zones within the adjacent mineral deposits were monitored in seven and three peatlands respectively, using piezometers with hourly hydraulic head recordings. Hydraulic conductivities for the outflow zones ranged between 1.4*10-7 and 8.5*10-3 cm/s, whereas those of the inflow zones ranged between 5.6*10-7 and 3.9*10-6 cm/s. Evapotranspiration was shown to be the dominant factor controlling monthly cumulative water table decreases (MCD), while precipitation dominated the monthly cumulative water table increases (MCI). A strong correlation was found between mean peatland WTD and outflow zone hydraulic conductivity. Peatlands that were identified as being strongly connected with the adjacent mineral deposits in a diffuse underground outflow zone showed the greatest variations in water storage. This study highlights the importance of the connection between peatlands and adjacent mineral deposits in controlling WTD, as found for those located in the St. Lawrence Lowlands. The results show that water table fluctuations are strongly controlled by meteorological conditions, and that hydrogeological setting exerts a strong control on MCI and MCD. Moreover, this work shows that WTD in ombrotrophic peatlands is influenced by the hydraulic conductivity of the outflow zones, and confirms that aquifer – peatland connectivity influences peatland water storage variations, and therefore peatland vulnerability to disturbances in aquifer groundwater levels.