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 (CH₄) 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 and is an integral near surface, critical zone (CZ), process that is not fully understood. It is likely that gas production and groundwater are interconnected in the CZ: 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 3 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 CH₄ production due to downward transport of labile carbon. Three well clusters were equipped with data logging pressure transducers to monitor long-term hydraulic head and atmospheric pressure at 2 minute increments. Data reveal approximately 5 cm fluctuations in hydraulic head lasting up to 10 hours during low atmospheric pressure and suggest FPG emission from both deep and shallow peat. Fluctuations occur much more frequently from deep peat and occur during sharp decreases in atmospheric pressure. Water samples collected from three well sites show significant dissolved CH₄ (5-16 mg/L) in peat waters below 2 m depth and raised concentrations at the esker crest. This study suggests significant production and storage of CH₄ in deep peat that may be episodically released as FPG due to atmospheric pressure fluctuations.