INVESTIGATING FLOW PATTERNS AND MECHANISMS FOR FREE PHASE GAS VARIABILITY IN A MAINE PEATLAND

Northern Peatlands cover more than 350 million ha and are an important source of methane and other biogenic gases contributing to climate change. Free phase gas (FPG) accumulation and episodic release has recently been recognized as an important mechanism for biogenic gas flux from peatlands. It is likely that gas production and groundwater are interconnected: groundwater flow influences gas production by regulating geochemical conditions and nutrient supply available for methanogenesis while FPG influences groundwater flow through a reduction in peat permeability. In nine locations in Caribou Bog, Orono, ME, we installed well clusters (6-8 wells at 1 to 2 m increments to mineral soil, bottom 30 cm screened) to investigate the hydrological influence of a buried esker (crests approximately 2 m below the peat surface) and pool system. A groundwater map, created from GPS data and hand measurements of well cluster hydraulic head, shows preferential flow towards an area of relatively lower hydraulic head associated with the esker and pool system. The higher permeability esker may be acting as a drain driving higher rates of methane production. Three well clusters were equipped with data logging pressure transducers to monitor long-term head and atmospheric pressure at 2 min. increments. Data reveal approximately 5 cm fluctuations in pressure data lasting up to 10 hours and suggest FPG emission from both deep and shallow peat. Fluctuations occur much more frequently from deep peat and occur during or just after sharp decreases in atmospheric pressure. Readings from gas traps installed on one well cluster suggest fluxes from all wells were minor (< 5 ml/week). Well gas samples analyzed via gas chromatography indicate methane concentrations of 635 to 3369 ppm in deep wells and an absence of methane concentrations in shallow wells. This study suggests that short-term increases in pressure gradients between pore fluids and the atmosphere episodically trigger FPG release from deep peat (> 2 m) and less frequently from shallow peat (< 2 m).