Variations in free-phase gases in peat landforms determined by ground-penetrating radar

The spatial variability of biogenic gas produced by methanogenic archaea is difficult to assess within saturated peat soils and is often poorly quantified. This study uses ground-penetrating radar to noninvasively estimate the vertical distribution of biogenic free-phase gas (FPG) in two distinct peat landform types in the Glacial Lake Agassiz Peatland, Minnesota: a near-crest bog and a midslope lawn (i.e., both as defined by surface vegetation communities). Ground-penetrating radar velocities retrieved from common midpoint surveys were modeled using the Complex Refractive Index Model to estimate free-phase gas volumes along one-dimensional vertical profiles. Near-crest bog landforms are characterized by vertical variability in gas content with accumulations along the peat column up to 24% by volume of FPG localized within the deep peat (e.g., 2-4 m depth). Midslope lawn sites show lower total volumes (up to 12% free-phase gas) and less variability that result in a more even free-phase gas distribution throughout the vertical profile. High-resolution data also suggests the presence of thinner layers (<1 m) containing up to 40% free-phase gas at a near-crest site. Our results suggest that spatial distribution of free-phase gas along the peat column may vary in the Glacial Lake Agassiz Peatlands depending on peat landform. Statistical analysis supports vertical variability of ground-penetrating radar velocity data with a 95% confidence limit. Our results could potentially be upscaled using surface vegetation communities to estimate landscape-scale gas storage potential in peat landforms.