MEASURING BIOGENIC GAS EBULLITION EVENTS USING HYDRAULIC HEAD TIME SERIES DATA IN CARIBIBOU BOG, MAINE

Northern peatlands are an important source and sink for biogenic gases (eg. methane, carbon dioxide) that influence the global climate. The episodic release of free-phase gas that accumulates within peat deposits is an important component of biogenic gas flux from peatlands. We hypothesize that the accumulation and rapid release of free-phase biogenic gas will effect the hydraulic head within the peat, and the temporal and spatial (depth) patterns associated with this process can be assessed by monitoring the hydraulic head within the peat column. To test this hypothesis, three clusters of 6 to 8 monitoring wells were installed in Caribou Bog, a 2200 ha peatland in central Maine. Wells fitted with 30 cm long screens were installed at about 1m intervals within each cluster from the peat surface to the interface with underlying mineral sediments (6.5 m deep). Data logging pressure transducers recorded the hydraulic head in each well at 2 to 4 minute intervals from 2011 through 2015 from the spring through early winter. Data was collected at 8 minute intervals from the winter through early spring when wells were frozen and loggers could not be downloaded. Visual assessment of the time series data reveal short, rapid hydrualic head fluctuations up to about 5 cm in amplitude. Observed fluctuations were recorded in both deep and shallow wells and persist for up 24 hours following drops in atmospheric pressure. These data suggest that free-phase biogenic gas is present in both the deep and shallow zones within the peatland and that release of this gas is triggered by drops in atmospheric pressure. To further assess the data, digital signal processing is ongoing in an effort to better quantify temporal patterns and identify the timing of anomolous hydraulic head fluctuations infered to be driven by biogenic gas movement within the peat column.