Geophysical estimation of free-phase gas content and distribution in peatlands

Emissions of methane and carbon dioxide gases from organic soils contribute to the global carbon cycle and may influence climate change. Revealing where these gases accumulate in the subsurface of saturated peat soils is a critical step towards understanding carbon cycling through peatlands. This dissertation presents field and laboratory geophysical investigations of free-phase gas contained within peatlands. Ground-penetrating radar (GPR) was used to estimate the vertical distribution of free- phase gas (FPG) in sites within the Glacial Lake Agassiz Peatland, MN. Bog-crest landforms were characterized by vertical variability in FPG and accumulations up to 24% FPG within the intervals 2-4 m deep. Lawn sites show lower volumes up to 12% FPG and a more even vertical gas distribution. These results demonstrate that spatial distribution of FPG varies vertically in the peat of Glacial Lake Agassiz peatlands depending on landform type. A second study was designed to elucidate spatial variability in FPG two-dimensionally along a transect crossing a bog crest, mid-slope lawn, and fen peat landforms. Variability in gas content and distribution was observed using GPR as a function of peat landform type. Estimated gas content up to 25% was observed in landforms dominated by woody surface vegetation. In the lawn, estimated gas content was > 15%, while estimated gas contents between 0% - 7% were found in the fen. Changes in gas content of up to 20% were observed over a transition between a stand of 10 m tall trees and a fen. These results support conceptual models based on accumulation and storage of FPG. Models relating geophysical measurements to pore water content limit the accuracy of FPG estimates. To improve these models, the dielectric properties of peat samples were measured in response to changes in water content. Dielectric permittivity was measured using GPR at water contents between 0.87 and 0.95 m3 m-3 on four samples with varying levels of humification. The resulting relationships indicate that there are differences in the permittivity-water content relationships between peat samples. Behavior of parameters in the dielectric mixing model indicates that variability in dielectric relationships may be attributed to peat structure.Ph.D.Includes bibliographical referencesIncludes vitaby Andrew D Parsekia