Using hydrologic measurements to investigate free phase gas in a Maine peatland, USA

Northern Peatlands cover more than 350 million ha (~3 percent of Earth’s surface) 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. It is likely that gas production and groundwater flow are interconnected in peatlands: 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 and by creating excess pore fluid pressures. Water samples collected from three well sites at Caribou Bog, Maine, show substantial dissolved CH4 (5-16 mg/L) in peat waters below 2 m depth and an increase in concentrations with depth. This suggests significant production and storage of CH4 in deep peat that may be episodically released as FPG. Free phase gas was not collected in gas traps suggesting our monitoring wells do not influence the subsurface peatland pressure regimes and do not act as conduits for gas release. Two minute increment pressure transducer data reveal approximately 5 cm fluctuations in hydraulic head from both deep and shallow peat that are concluded to be indicative of FPG release. FPG release persists up to 24 hours during decreasing atmospheric pressure and a rising water table. Groundwater flow converges toward an area of relatively lower hydraulic head associated with the esker and pool system. Increased CH4 concentrations are also found at the depth of the esker crest suggesting that the high permeability esker is acting as a conduit for groundwater flow driving a downward transport of labile carbon and results in higher rates of CH4 production