Groundwater hydrology of boreal peatlands above a bedrock tunnel – Drainage impacts and surface water groundwater interactions

Little attention has been given to the pattern of hydrological connectivity between peatlands and water bearing fracture zones in crystalline bedrock. The construction of the railway tunnel Romeriksporten provided an opportunity for studying impacts of bedrock tunnelling on peatland hydrology and the hydraulic connectivity between peat deposits and deeper layers in an area with fractured Precambrian gneisses, exposed bedrock, and surficial covers of thin till deposits and peatsoils. Above the tunnel the water level in peat wells fluctuated with maximum depths up to 3 m, and water that otherwise would have generated surface runoff infiltrated in the peat. Drawdowns of the groundwater table in peatlands were observed as far as 340 m away from the tunnel trace. The deep drainage base provided prolonged water table drawdowns in peatlands in dry periods, and differential settlements in drained peatsoils resulted in secondary changes in patterns of surface water storage and flow. The groundwater drawdowns were influenced by the balance between tunnel leakage and water supplies from catchments and the wetland position within the groundwater flow system. Deep and simultaneous lowering and fluctuations of hydraulic heads in wells in the peat, the subpeat sediments and the bedrock above the tunnel demonstrated hydraulic connectivity between the peat layers and the bedrock, and revealed vertical flow even through highly humified peat layers in the catotelm. This shows that in peatlands with a subsurface of a relatively thin till cover above fractured crystalline bedrock, favourable conditions for groundwater flow from peatsoils to rock are not always restricted to a few specific sites, and indicates that attention should be given to the influence of peat water–bedrock water connectivity on impacts of groundwater exploitation, droughts and climate changes in such areas.