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

Abstract

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.

Peat deformation and biogenic gas bubbles control seasonal variations in peat hydraulic conductivity

“…At site C, neither Δ γ nor strain is significantly correlated with log ( K ) within layer 0.2–0.3 m ( p > 0.05; data not shown). This insignificant relationship between log ( K ) and the two primary controls on its temporal variation is uncertain and may result from difficulties in obtaining accurate K measurements in the near‐surface poorly decomposed peat (Surridge et al ., ), from spatial variability in the entrapped gas content (Strack and Waddington, ) and/or from venting of gas through piezometers (Waddington et al ., ). Multiple regression relationships indicate that log ( K ) at site C is significantly correlated ( p < 0.05) with Δ γ but not strain within layers 0.3–0.5 and 0.5–0.7 m (Figure ).…”

In‐Well Degassing Issues for Measurements of Dissolved Gases in Groundwater

“…For example, the need to purge a well prior to sampling for volatile contaminants has been ascribed, in part, to degassing losses (Robin and Gillham 1987; Barcelona et al 1994). Also, Waddington et al (2009) proved recently that gas venting can occur through piezometers placed in peatlands, where gas bubble release has been shown to be a significant source of methane flux to the atmosphere (Rosenberry et al 2003). Meanwhile, dissolved noble gas data (e.g., Andrews et al 1991) have provided evidence for degassing of groundwater, although whether it is related to sampling or to the formation of gas bubbles in the aquifer under natural conditions (see Aeschbach‐Hertig et al 2008, and references therein) is still undetermined.…”

Trench infiltration for managed aquifer recharge to permeable bedrock

“…Bubbles from trapped air associated with the wetting front or incorporated in otherwise saturated porous media have been shown to cause up to a tenfold decrease in permeability (Christiansen, 1944; Constantz et al , 1988; Faybishenko, 1995; Heilweil et al , 2004). Biogenic gases produced during bacterial/microbial respiration, decay, and denitrification (Oberdorfer and Peterson, 1985; Beckwith and Baird, 2001; Waddington et al , 2009) can clog pore spaces and have been linked with seasonal decreases in infiltration during warmer summer months (Heilweil et al , 2009b). The increased dynamic viscosity associated with cooler water temperature has been shown to reduce effective hydraulic conductivity (Cho et al , 1999) and reduce spreading‐basin infiltration during cooler winter months at higher latitudes (Schuh, 1990).…”