SIMULATING GROUNDWATER REVERSALS IN THE RED LAKE PEATLAND

In 1991, vertical groundwater flow reversals were observed in the Glacial Lake Agassiz Peatlands (GLA). These reversals may play an important role in the carbon dynamics of boreal peatlands. Two hypotheses have been suggested for the flow reversals: 1) the interaction of regional groundwater flow with a weakened local-scale groundwater flow cell, and 2) the transient release of water from storage in a response to a changing water-table position. A three-dimensional groundwater model is being constructed to test the hypothesis that reversals in groundwater flow in the GLA can be present without the influence of large-scale flow. A transient groundwater flow model of the Red Lake Peatland, a component of GLA of northern Minnesota, was constructed using MODFLOW. The area was discretized into 129 rows, 72 columns, and 10 layers. The MapInfo GIS was used to create input files for MODFLOW. The model was constructed using the Rainy River as a northern no-flow boundary and Upper Red Lake as a southern constant head boundary. USGS digital elevation models provided topography and elevation information. Evapotranspiration and recharge rates were collated from Minnesota Geological Survey publications. Peat and mineral soil hydraulic conductivity values were compiled from previous studies and published data. Drains were used across the model surface to remove surface runoff and constrain simulated head in layer 1 to land surface. Transient simulations were run, altering evapotranspiration and recharge, to evaluate vertical groundwater reversals.