Consolidation of gyttja in a rewetted fen peatland: Potential implications for restoration

Abstract

Water availability is a major concern when restoring degraded vacuum harvested peatlands. The seasonal expression of water arising from the compression of underlying organic deposits may provide a source of water. A bog near Rimouski, Québec (Canada) was harvested using the vacuum extraction method and cut down to underlying sedge peat; restoration procedures were implemented in the autumn of 2009. The residual peat layer is 0.4–1 m thick, and overlies a 1–1.5 m thick gyttja deposit. Gyttja is a coprogenous lake bottom sediment on which peat sometimes forms. Little is known about the hydrophysical properties of gyttja and the implications it may have for peatland restoration. The upper 40 cm of gyttja had average bulk density 0.12 g cm-3, particle density 1.57 g cm-3 and porosity 92 %. The organic matter and fibre contents of the gyttja were > 45 %, and both decreased with depth. Oedometer test results showed 9 and 72 % strain at 3.5 and 200 kPa, respectively, demonstrating the potential for water release upon compression, which in the field is caused by water table lowering. However, during the year of measurement (2011) the changes in effective stress caused by water table lowering were < 1 kPa, corresponding to a water table decline of < 10 cm. Under these conditions the volumetric moisture content of the top 30 cm of gyttja decreased by only 0.4 % and we observed 0.5 cm of gyttja settlement; this represented 0.4 % and 1.7 % strain, respectively. On the basis of these strains, we conclude that the release of water due to the consolidation of gyttja in 2011 had little impact on rewetting of the overlying peat. However, it may be important under drier conditions with a larger seasonal water deficit, and may have been important prior to restoration when larger water table fluctuations were observed.

Keywords

• compression
• hydrology
• organic soil
• restoration