HYDRAULIC HEAD DATA IN A CENTRAL MAINE PEATLAND INDICATE METHANE EBULLITION EVENTS

Caribou Bog, a 2500 hectare peatland in central Maine, has been studied over the past two decades to understand the hydrology, stratigraphy, and biogenic gas (e.g. methane, carbon dioxide) emissions from this system. Clusters of 6 to 8 wells were installed at depths ranging from the water table to the base of the peat column. Hydraulic head was measured with pressure transducers at 2 to 8 minute intervals from 2012 through 2018. These data were collected to identify anomalies hypothesize to be related to biogenic gas movement in, or emission from, the peat column. Gas traps installed near these well clusters confirm episodic movement of biogenic gas through the peat column. Hydraulic head time series plots were visually inspected and subsequently transformed using wavelet analysis. Unusual high frequency oscillations are apparent in these data that are hypothesized to be associated with biogenic gas movement. Stationary wavelet analysis indicates that high frequency anomalies occur more frequently shortly after well installation and during other research activities, indicating that early data is impacted by disturbance. Data anomalies are synchronous with water pressure changes, with centimeter-scale high frequency oscillations occurring shortly after or during changes in water levels and atmospheric pressure associated with rain events, daily evapotranspiration cycles, and snow melt. Increased water pressure measured during winter months suggests that ice may trap gas, or snow loading increases hydraulic head within the peat column. Hydraulic head anomalies measured in the deep peat are less frequent than those measured in the shallow peat. Often, deep peat head anomalies occur with anomalies in shallower wells, suggesting these events are larger or gas is rapidly migrating through the entire peat column. Similar to work reported by previous researchers, we measured over-pressuring in deep wells possibly caused by gas pressure that accumulated when there is little human disturbance in the peatland. Our data suggests that gas is generated in both shallow and deep peat, with more frequent ebullition events originating in shallow peat and less frequent (but potentially larger) events originating in deep peat. The timing of these events is influenced by hydraulic forcing within the peat column.