T17 Gas Bubbles and Dissolved Gases in Groundwater

Sunday, 9 October 2011
8:00 AM - 12:00 PM
Room M101AB

IMPROVING UNDERSTANDING OF BIOGENIC GAS DYNAMICS IN NORTHERN PEATLANDS USING GROUND PENETRATING RADAR

Paper number: T17-10
Program: Technical Sessions
Time: 10:25 AM – 10:45 AM

Authors

Abstract

Improved quantification of biogenic gas production, storage and release in peatlands is critically needed in order to better constrain the role of peatlands in contributing to the atmospheric methane burden. Furthermore, free phase gas bubbles in peat can exert a strong control on peatland hydrology e.g. by locally reducing hydraulic conductivity. Over the last eight years we have pioneered the application of ground penetrating radar (GPR) as a non (or minimally) invasive technology for investigating in situ free phase gas dynamics in northern peatlands. To further advance this approach, recent laboratory measurements on peat blocks from three geographically distinct peatlands have been performed in a way to simulate volumetric changes in gas content that occur in peatlands as a result of bubble compression/expansion by atmospheric pressure variations. These experiments demonstrate the high precision of biogenic gas content estimation obtainable from dielectric permittivity measurements. However, the peat blocks showed significant differences in the linear best-fit relation between moisture content and permittivity, suggesting a role of peat structure on this critical relationship.

New field experiments conducted at a field site in Caribou Bog, ME, demonstrate how a time-series of common midpoint (CMP) measurements (or common offset measurements when specific reflectors are identified at a known depth) can be used to determine changes in the vertical distribution of free phase gas within a ~ 6 m peat column in response to atmospheric pressure variations. These measurements reveal evidence of gas volume changes due to pressure driven expansion/contraction in shallow soils (as simulated in our laboratory experiments), as well as ebullition events from deeper over-pressurized layers apparently driven by rupturing of a confining peat fabric during low pressure events. Our most recent work at Caribou Bog is turning to automation of GPR data acquisition in order to better capture short-duration ebullition events that remain poorly quantified despite their recognized importance in contributing to the total flux of methane from peatlands to the atmosphere.