Hydraulic Tomography Using Detailed Pumping Test Data from the Glacial Lake Agassiz Peatlands, Northern Minnesota

Hydrology is a primary control that influences and is influenced by carbon-based gas sequestration in peatland ecosystems. The hydraulic conductivity of the peat in these important carbon reservoirs affects flow patterns and rates that influence biogenic gas production and release from these ecosystems. Biogenic gasses, such as methane, trapped within the peat complicate this relationship by lowering hydraulic conductivity of the peat and occluding pore space. Whereas traditional analysis of aquifer tests often do not reflect the small-scale variations in hydraulic conductivity that influence water and solute transport in the subsurface, hydraulic tomography provides better resolution of the spatial and directional variability in hydraulic properties. This is particularly important when considering the potential role of entrapped free-phase biogenic gas within the peat. To evaluate the hydraulic conductivity of the peat, pumping tests have been completed within the Glacial Lake Agassiz Peatland in northern Minnesota. Three-dimensional ground-water flow simulations were calibrated to field data to characterize the variablility of hydraulic conductivity within the peat. Computer models were constructed using FiPy, a finite-volume modeling library under development at the National Institute of Standards and Technology. PEST, a program that facilitates the calibration process, was used with the model to automatically determine the distribution of hydraulic conductivity. Preliminary simulations indicate not only a general decrease in hydraulic conductivity with depth (7E-5 m/s to 5E-7 m/s from top to bottom) but also zones of low hydraulic conductivity, 1E-5 m/s and 4E-5 m/s at depths of 0.5 and 2.5 meters respectively. These low hydraulic conductivity zones are hypothesized to be caused by biogenic gasses occluding pore space and inhibiting the vertical movement of water and solutes in the peat.