Abstract:Anisotropy and heterogeneity of hydraulic conductivity (K) are suspected of greatly affecting rates and patterns of ground-water seepage in peats. A new laboratory method, termed here the modified cube method, was used to measure horizontal and vertical hydraulic conductivity (K h and K v ) of 400 samples of bog peat. The new method avoids many of the problems associated with existing field and laboratory methods, and is shown to give relatively precise measurements of K. In the majority of samples tested, K h was much greater than K v , indicating that the bog peat was strongly anisotropic. Log 10 K h , log 10 K v , and log 10 (K h /K v ) were found to vary significantly with depth, although none of the relationships was simple. We comment on the scale dependency of our measurements.

[1] To date, very little information has been available on the build-up and release of biogenic gas bubbles in poorly-decomposed bog peats near the peatland surface (upper 1 m). We investigated the importance of ebullition of biogenic gas bubbles as a mechanism for the transport of CH 4 to the atmosphere in eight cores (24 cm diameter, 22 cm depth) of poorly-decomposed, near-surface bog peat. Ebullition was recorded in all but one sample but varied greatly between samples. Maximum rates of CH 4 efflux via ebullition were also highly variable, ranging from 2.2 to 83.0 mg CH 4 m À2 day À1. These rates are similar to rates of diffusive CH 4 efflux. Our results also show that wetland methane models are likely to need revision because they assume that unrealistically high CH 4 porewater concentrations are required before bubbles can be produced and because, in part, they do not account for gas bubble build-up prior to ebullition.

Abstract. A laboratory investigation was used to determine whether biogenic gas bubbles accumulate and block water-conducting pores below the water table in poorly decomposed Sphagnum peat. We found that biogenic gas bubbles did accumulate under realistic incubation temperatures. At the end of incubations at 10.5øC, volumetric water contents in two peat samples decreased to between 0.8 and 0.85 (porosity of the samples ranged from 0.96 to 0.97), indicating that the peat was considerably undersaturated with respect to water. Methane was found to be an important constituent of the gas bubbles. The presence of gas bubbles appeared to have a major effect on hydraulic conductivity (K). In control incubations, prior to which the peat had been irradiated and dosed with a biocide, biogenic gas bubbles did not accumulate, and K was 5-8 times higher than at the end of the microbially active incubations. Our results suggest that biogenic gas bubbles have a potentially significant effect on seepage in peat soils.

Abstract:Anisotropy and heterogeneity of hydraulic conductivity (K ) are seldom considered in models of mire hydrology. We investigated the effect of anisotropy and heterogeneity on groundwater flow in bog peat using a steady-state groundwater model. In five model simulations, four sets of K data were used. The first set comprised measured K values from an anisotropic and heterogeneous bog peat. These data were aggregated to produce the following simplified data sets: an isotropic and heterogeneous distribution of K ; an isotropic and homogeneous distribution; and an anisotropic and homogeneous distribution. We demonstrate that, where anisotropy and heterogeneity exist, groundwater flow in bog peat is complex. Fine-scale variations in K have the potential to influence patterns and rates of groundwater flow. However, for our data at least, it is heterogeneity and not anisotropy that has the greater influence on producing complex patterns of groundwater flow. We also demonstrate that patterns and rates of groundwater flow are simplified and reduced when measured K values are aggregated to create a more uniform distribution of K. For example, when measured K values are aggregated to produce isotropy and homogeneity, the rate of modelled seepage is reduced by 28%. We also show that when measured K values are used, the presence of a drainage ditch can increase seepage through a modelled cross-section. Our work has implications for the accurate interpretation of hydraulic head data obtained from peat soils, and also the understanding of the effect of drainage ditches on patterns and rates of groundwater flow.