CHARACTERIZING THE DISTRIBUTION OF HYDRAULIC PROPERTIES IN THE GLACIAL LAKE AGASSIZ PEATLANDS USING A THREE-DIMENSIONAL NUMERICAL MODEL

The potential significance and susceptibility of peatlands in future climate change has urged a further understanding of the hydrologic processes that determine their development and distribution. The hydraulic conductivity (K) of the peat in these important carbon reservoirs affects flow patterns and rates that determine their spatial distribution and influence biogenic gas production and release from these ecosystems. Biogenic gasses (e.g., methane) within the peat complicate this relationship by occluding pore space thereby lowering K of the peat. Since traditional methods of determining the spatial and directional hydraulic properties of peat may not reflect the small-scale variations in K that influence water and solute transport in the subsurface, a three-dimensional model was constructed using FiPy, a finite-volume modeling library under development at the National Institute of Standards and Technology. To characterize the distribution of K, a methodology of spatial parameter definition and estimation based on pilot points and accompanied by the use of regularization was used to solve the inverse problem. Automated parameter estimation was undertaken with PEST, a program that facilitates the calibration process, using data collected during pumping tests conducted in the Glacial Lake Agassiz Peatland in northern Minnesota. One test was conducted on a raised bog and the other on a Sphagnum lawn.

Dependency of K with depth was found at both sites; however the relationship is not monotonic as assumed in several studies. Modeled K values between 7.71×10^-5 and 4.65×10^-6 m/s were found at the bog site with localized areas of lower K material. Modeled K values between 2.66×10^-3 and 3.52×10^-5 m/s were found at the lawn site with the presence of lower K material located over higher K layers. The differences in K between the two sites reflect the hydraulic distinction between the different types of peat landforms and the modeled K values for deeper peat indicate its potential significance in the transmission of water for regional models. Furthermore, the presence of low K zones found at both sites provide a unique look into the subsurface hydraulic structure and may provide further evidence to the development and existence of mobile free-phase gas within peat.