DIELECTRIC PROPERTIES OF PEAT WITH BIOGENIC FREE PHASE GAS

Carbon-rich biogenic free-phase gas impacts the water content (θ) of organic peat soil below the water table. Methane and carbon dioxide present as bubbles within the peat are released to the atmosphere and therefore peat-filled wetlands have an important role in the global carbon cycle. In this study we investigated the dielectric properties of peat monoliths extracted from three peatlands in response to small changes in near saturation θ using geophysical methods. The experimental design utilizes a novel method for controlling θ in the sample using pressure – similar to the natural mechanism by which gas bubbles change volume during variable atmospheric pressure conditions. This approach aims to simulate a somewhat natural distribution of water throughout the sample across the tested range of water content, and carefully investigate a range of water contents relevant to nearly saturated field conditions. We validated this method using direct volumetric measurements, time-lapse photographic tracking and ground penetrating radar observations. Our study examined samples of peat with <5% gas content as is regularly found in the field, particularly in the near surface. We used empirical equations and multiphase dielectric mixing models to fit data acquired on four peat samples from Minnesota, Maine and New Jersey, USA each with contrasting levels of decomposition as estimated on the von Post scale. Evidence is present indicating variability in the dielectric relationships might be due to peat structure and particle orientation particular to each sample. We present these results in terms of volumetric gas content that could be used to improve the precision of future investigations of methane and carbon dioxide content and flux in peat.