TRANSIENT NUMERICAL SIMULATION OF FLOW PATHS AND WEATHER-DRIVEN FLOW REVERSALS IN THE GLACIAL LAKE AGASSIZ PEATLAND

Vertical reversals in hydraulic gradients have been observed in the Glacial Lake Agassiz Peatland and several other peatland systems on time scales of months to years. Vertical flow reversals have important implication for solute transport processes that influence nutrient cycling and carbon dynamics in peatland systems. Changes in recharge and evapotranspiration have been suggested as driving mechanisms behind these shifts in vertial hydraulic head gradients. To further evaluate flow reversals in the Red Lake area of the Glacial Lake Agassiz Peatland, a 3-D numerical model was constructed using a modified version of MODFLOW. This 10 layer numerical model incorporated seasonal changes in evapotranspiration and precipitation, and used MODFLOW's drain package to remove excess runoff across the model domain. Flow paths generated by MODPATH are primarily lateral in the model, despite seasonal shifts in vertical hydraulic gradients driven by changes evapotranpiration and recharge. Oscillatory (centimeter-scale) vertical displacement in simulated flowpaths occur the deeper, less permeable peat. While the vertical flow induced by seasons shifts in weather are small, these oscillations may be an important mixing process within the peat.