Seasonal Geophysical Monitoring of Biogenic Gases in a Northern Peatland: Implications for Temporal and Spatial Variability in Free Phase Gas Production Rates

A set of high resolution surface ground penetrating radar (GPR) surveys, combined with elevation rod ( to monitor surface deformation) and gas flux measurements, were used to investigate in situ biogenic gas dynamics within a northern peatland (Caribou Bog, Maine). Gas production rates were directly estimated from the time series of GPR measurements. Spatial variability in gas production was also investigated by comparing two sites with different geological and ecological attributes, showing differences and/or similarities depending on season. One site characterized by thick highly humified peat deposits (5-6 m), wooded heath vegetation and open pools showed large ebullition events during the summer season, with estimated emissions (based on an assumed range of CH(4) concentration) between 100 and 172 g CH(4) m(-2) during a single event. The other site characterized by thinner less humified peat deposits (2-3 m) and shrub vegetation showed much smaller ebullition events during the same season (between 13 and 23 g CH(4) m(-2)). A consistent period of free-phase gas (FPG) accumulation during the fall and winter, enhanced by the frozen surficial peat acting as a confining layer, was followed by a decrease in FPG after the snow/ice melt that released estimated fluxes between 100 and 200 g CH(4) m(-2) from both sites. Estimated FPG production rates during periods of biogenic gas accumulation ranged between 0.22 and 2.00 g CH(4) m(3) d(-1) and reflected strong seasonal and spatial variability associated with differences in temperature, peat soil properties, and/or depositional attributes (e. g., stratigraphy). Periods of decreased atmospheric pressure coincided with short-period increases in biogenic gas flux, including a very rapid decrease in FPG content associated with an ebullition event that released an estimated 39 and 67 g CH(4) m(-2) in less than 3.5 hours. These results provide insights into the spatial and seasonal variability in production and emission of biogenic gases from northern peatlands.

Geophysical and Hydrological Evaluation of Two Bog Complexes in a Northern Peatland: Implications for the Distribution of Biogenic Gases at the Basin Scale

Ground penetrating radar (GPR) was used to determine peat basin geometry and the spatial distribution of free-phase biogenic gasses in two separate units of a northern peatland (Central and Southern Unit of Caribou Bog, Maine). The Central Unit is characterized by a deep basin structure (15 m maximum depth) and a raised (eccentric) bog topographic profile (up to 2 m topographic variation). Here numerous regions of electromagnetic (EM) wave scattering are considered diagnostic of the presence of extensive free-phase biogenic gas. In contrast, the Southern Unit is shallower (8 m maximum depth) and has a slightly convex upwards bog profile (less than 1 m topographic variation), and areas of EM wave scattering are notably absent. The biogenic gas zones interpreted from GPR in the Central Unit are associated with: (1) wooded heath vegetation at the surface, (2) open pools at the surface, (3) high water table elevations near the center of the basin, and (4) a region of overpressure (at approximately 5 m depth) immediately below the zone of free-phase gas accumulation. The latter suggests (1) a transient pressure head associated with low hydraulic conductivity resulting from the biogenic gasses themselves or confining layers in the peat that restrict both gas release and groundwater flow and/or (2) overpressure in the peat column as a result of the gas buildup itself. In contrast, the Southern Unit, where zones of EM scattering are absent, is characterized by: (1) predominantly shrub vegetation, (2) a lack of open pools, (3) only minor variations (less than 1 m) in water table elevation throughout the entire unit; and (4) generally upward groundwater flow throughout the basin. The results illustrate the nonuniformity of free-phase biogenic gas distribution at the peat basin scale and provide insights into the processes and controls associated with CH4 and CO2 accumulation in peatlands.