Aquifer depressurization and water table lowering induces landscape scale subsidence and hydrophysical change in peatlands of the Hudson Bay Lowlands

Abstract

The depositional history of the Hudson Bay Lowlands (HBL) in Ontario, Canada has created a low relief, poorly drained landscape, favouring the formation of one of the largest peatland complexes in the world. High volume dewatering associated with resource extraction in this area, such as the De Beers Victor Diamond Mine, tests the ability of the underlying confining layer to limit water losses in the peatlands above. This research quantifies the deepening of water tables and increase in effective stress related to mine dewatering and the resulting changes to bog and fen peatland hydrophysical structure and function. Long-term implications of these impacts are discussed. One impacted and two unimpacted transects were instrumented for meteorological (precipitation and evapotranspiration) and hydrophysical (hydraulic head, hydraulic conductivity (K_sat), and surface elevation) monitoring over a 12-year period in the vicinity of the Victor Mine. Over this study period, the unimpacted peatlands operated within relative hydrological equilibrium, demonstrated through shallow water tables, negligible subsidence, and stable K_sat. Contrastingly, all impacted peatlands experienced deeper watertables, larger downwards gradients, and measurable long-term subsidence (4-15 cm). Hydrological impacts were highest in bogs with a thin underlying confining layer even if they were farther from the point of dewatering, highlighting the need for environmental monitoring programs which incorporate an assessment of aquitard thickness. Where subsidence occurred, associated decreases in K_sat deflected bog-fen-tributary flow-paths deeper, reducing the upwards transport of solute rich water to downgradient fens. The long-term effects of these landscape scale changes should be studied further, particularly since climate change in this region will potentially increase water deficits and further alter peatland connectivity. Peatland studies should be conducted in different landscapes experiencing water table lowering due to drought or depressurization in order to better understand the associated subsidence patterns and hydrophysical changes in varying geological and morphological regimes.

Keywords: Climate change; Dewatering/drainage; Flowpath analysis; Hydraulic conductivity; Mine impacts; Peatlands; Sub-arctic; Subsidence.