MODELING HEAT TRANSPORT IN A GLACIAL LAKE AGASSIZ PEATLAND BOG: A KEY TO EVALUATING ADVECTIVE FLOW?

The Glacial Lake Agassiz Peatlands are a significant terrestrial source of atmospheric methane. At depth methane is produced by anaerobic bacteria in the middle of ~3-4 meter-deep peat soils. An important variable that controls bacterial growth and production of methane is heat. In 1998 the peat at the crest of the largest bog in the peatland, (Red Lake Bog) was instrumented to measure temperature at 12 depths at variable separation and hydraulic head measurements at 100 cm intervals at sub-daily time intervals. We created a vertical, 1-dimensional transport model for heat in the bog peat to better understand how heat moves within saturated peat soil, and how seasonal and longer term changes in air temperature may affect the subsurface zone where methane is produced. The model was created with SUTRA, a USGS finite-element variable density code that couples heat transport with hydraulic head based transport. We modified the code to account for the non-linear relationship between temperature and density at temperatures between 0 and ~8 ^OC. The boundary conditions were specified inflow rates at both the top and bottom of the model, including internal drains to simulate vertically converging porewater flow from local and regional groundwater flow systems. The inflow water temperature was obtained from measured daily averages. The model was run transiently at 1-hour time steps for a complete year.

Our calibrated model shows that a significant amount of heat transport through the peat can be accounted for by simple conduction where water is essentially at hydrostatic conditions (the absolute residual mean error is less than 0.3 ^OC for the measured versus modeled temperatures). Small, but systematic deviations between the actual and model results are probably caused by upward and downward advective flow from the mineral soils underlying the peat and the groundwater mound under the bog respectively. Models with the addition of this advective flow essentially remove the systematic errors. Future improvements to the model will include changes to adapt for the freezing and thawing of ice and evolution of multi-phase transport of heat in methane gas pockets.