Evaluating spatial variability of free-phase gas in peat using ground-penetrating radar and direct measurement

Several recent studies have estimated that peatlands may have substantial quantities (up to 15-20% by volume) of biogenic free-phase gas (FPG) below the water table. The presence, distribution, and release of this FPG play important roles in peatland hydrology and biogeochemistry. Despite its importance, investigation of FPG is difficult because most measurement techniques involve disturbance of the peat, potentially altering gas volume in the process. Ground-penetrating radar (GPR) provides a noninvasive approach to quantify gas content in peat; however, its use for investigating variability in gas content at the microform scale across a peatland and with depth has been limited to date. We investigated variability in subsurface CH4 stock and FPG content between peatland microforms using (1) GPR and the common midpoint method to determine subsurface electromagnetic wave velocity and (2) direct measurement in order to estimate FPG content at three ridges and three hollows in a peatland in northern Alberta, Canada. Higher volumes of FPG were associated with higher CH4 concentrations, suggesting that a reliable estimate of gas volume could also serve as an estimate of stored CH4. In general, both methods found higher volumes of gas under ridges than hollows. When electromagnetic wave velocity was considered, a significant correlation was present with direct measurement of FPG volume. Zones with FPG content up to 20% of peat volume were observed throughout the peat profile including within the upper 1.5 m of peat. Further research is needed to determine physical, chemical, and biological controls on the variability of FPG volume in peat.