Groundwater flow patterns in a large peatland

Groundwater flow patterns and geochemistry were studied in the Mer Bleue bog, near Ottawa, Ontario, Canada. Groundwater flow patterns alternated between recharge, i.e. head gradients producing flow from the surface of the peatland to the deeper peat, and discharge, i.e. head gradients indicating flow from the deeper peat towards the surface of the peatland, during the summer of 1998. The patterns were controlled by changes in precipitation, evapotranspiration and the differential head response of catotelm peat (lower layer) to changes in water table elevation. Above-average rainfall in the spring created recharge patterns of groundwater flow in the peatland. Evapotranspiration exceeded precipitation for a three-week period in mid-summer decreasing head at the water table and reversing flow from recharge to discharge. A sustained moisture deficit maintained a flow reversal for 32 days, until a 46 mm rainfall raised the water-table, reversing the vertical hydraulic gradients and restoring recharge flow. The mixing of meteoric water and deeper groundwater controls geochemical profiles. Diffusion modeling shows that the peatland is a long-term recharge system. However, electrical conductivity and cation concentrations in peat pore-waters increase (up to 60%) during a flow reversal, and then decrease when recharge conditions are re-established. The redistribution of substrates from reversed flow is likely important to peatland biogeochemical function.