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

Abstract

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.