Evidence that piezometers vent gas from peat soils and implications for pore-water pressure and hydraulic conductivity measurements

✍ Scribed by J. M. Waddington; S. J. Ketcheson; E. Kellner; M. Strack; A. J. Baird

• Book ID: 102266162
• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 155 KB
• Volume: 23
• Category: Article
• ISSN: 0885-6087
• DOI: 10.1002/hyp.7244

✦ Synopsis

Entrapped gas bubbles in peat can alter the buoyancy, storativity, void ratio and expansion/contraction properties of the peat. Moreover, when gas bubbles block water-conducting pores they can significantly reduce saturated hydraulic conductivity and create zones of over-pressuring, perhaps leading to an alteration in the magnitude and direction of groundwater flow and solute transport. Some previous researches have demonstrated that these zones of over-pressuring are not observed by the measurements of pore-water pressures using open-pipe piezometers in peat; rather, they are only observed with pressure transducers sealed in the peat. In has been hypothesized that open-pipe piezometers vent entrapped CH 4 to the atmosphere and thereby do not permit the natural development of zones of entrapped gas. Here we present findings of the study to investigate whether piezometers vent subsurface CH 4 to the atmosphere and whether the presence of piezometers alters the subsurface concentration of dissolved CH 4 . We measured the flux of methane venting from the piezometers and also determined changes in pore-water CH 4 concentration at a rich fen in southern Ontario and a poor fen in southern Quebec, in the summer of 2004. Seasonally averaged CH 4 flux from piezometers was 1450 and 37•8-mg CH 4 m -2 d -1 at the southern Ontario site and Quebec site, respectively. The flux at the Ontario site was two orders of magnitude greater than the diffusive flux at the site. CH 4 pore-water concentrations were significantly lower in open piezometers than in water taken from sealed samplers at both the Ontario and Quebec sites. The flux of CH 4 from piezometers decreased throughout the season suggesting that CH 4 venting through the piezometer exceeded the rate of methanogenesis in the peat. Consequently we conclude that piezometers may alter the gas dynamics of some peatlands. We suggest that less-invasive techniques (e.g. buried pressure transducers, tracer experiments) are needed for the accurate measurement of porewater pressures and hydraulic conductivity in peatlands with a large entrapped gas component. Furthermore, we argue that caution must be made in interpreting results from previous peatland hydrology studies that use these traditional methods.