Fig. 12.3. Typical concentration profiles that develop in the gas phase of a landfill cover soil in the (a) absence and (b) presence of methane (CH4) oxidation.

response is typical for enzyme kinetics with both high-temperature and low-temperature inactivation as described by Sharpe and DeMichele (1977).

At 10-30°C the temperature response is approximately exponential with Q10 values ranging from 1.7 to 4.1 (Boeckx et al., 1996; Czepiel et al., 1996a; De Visscher et al., 2001; Börjesson et al., 2004; Park et al., 2005). King and Adamsen (1992) and De Visscher et al. (2001) found that Q10 is substantially larger at 1-3% CH4 than at 0.01-0.025% CH4.

Low-temperature inactivation is evidenced by increased Q10 values. This was observed by Christophersen et al. (2000), who found a Q10 value of ~5 at 2-15°C. High-temperature inactivation is evidenced by the existence of an optimum temperature for CH4 oxidation, which ranges from 22°C to 38°C (Boeckx and Van Cleemput, 1996; Boeckx et al., 1996; Czepiel et al., 1996a; Whalen and Reeburgh, 1996; Gebert et al.,

2003; Park et al., 2005). It appears that in landfill conditions type I methanotrophs tend to have a lower temperature optimum than type II methanotrophs (Gebert et al., 2003). Consequently, type I methanotrophs are more dominant at 10°C than at 20°C (Borjesson et al., 2004).

A 10°C temperature rise stimulates the diffusion coefficient in the gas phase by only 6-7%. Consequently, CH4 oxidation in landfill cover soils is increasingly limited by oxygen transfer as the temperature increases. The result is a less pronounced temperature dependence of CH4 oxidation on temperature than indicated by the Q10 value. This is illustrated with model calculations in Section 12.5.

Moisture content

Diffusion, advection and methanotrophy are strongly affected by soil moisture content. As a consequence, moisture content is the most important factor influencing CH4 oxidation in landfill cover soils.

Moisture content affects diffusion at two different scales, a complication that is not always fully appreciated. The two scales are associated with gas-phase diffusion (large scale: centimetres to decimetres) and liquid-phase diffusion (micro scale or pore scale: micrometres to millimetres).

Gas-phase diffusion is four orders of magnitude faster than liquid-phase diffusion. Consequently, large-scale diffusion can only occur in the gas phase. Vertical diffusive transport as indicated in Fig. 12.3 belongs to this category. At increasing moisture content, the gas-filled pore space becomes smaller and more tortuous, which hinders diffusion and limits the amount of oxygen that can penetrate the soil. Wet soils can only support a thin methanotrophic layer below the surface and do not oxidize CH4 efficiently. Empirical equations to describe the effect of soil moisture content on the effective diffusion coefficient of gases in soils have been developed by many researchers (e.g. Penman, 1940; Millington, 1959; Millington and Shearer, 1971; Moldrup et al., 1996, 2000a,b).

At the pore scale, CH4 usually has to diffuse through a water layer before it reaches the methanotrophic bacteria. This layer becomes thicker with increasing moisture content, which can lead to a substantial decrease of the CH4 oxidation rate, especially in aggregated soils. Mass transport effects at the pore scale cannot usually be distinguished from physiological effects within the bacteria. For that reason the physiological effect and the pore-scale effect of moisture content on CH4 oxidation are treated simultaneously.

Many researchers have found that CH4 oxidation rates pass through a maximum when plotted as a function of moisture content (Boeckx and Van Cleemput, 1996; Boeckx et al., 1996; Whalen and Reeburgh, 1996). Low CH4 oxidation rates at low moisture contents are associated with water stress of the methanotrophs (Striegl et al., 1992; Kruse et al., 1996). Schnell and King (1995) quantified this effect by inducing an osmotic stress in soils and methanotrophic cultures. In both cases they found that the methanotrophic activity decreases with osmotic potential (increasing stress), to become zero at an osmotic potential of -3 to -4 MPa.

Low CH4 oxidation rates at high moisture contents are associated with pore-scale diffusion limitations (Whalen et al., 1990; Boeckx and Van Cleemput, 1996; De Visscher et al., 2001).

Water stress in soils depends largely on the matric potential of the soil, whereas pore-scale diffusion limitation depends largely on the water-filled pore space. Consequently, the optimum moisture content for CH4 oxidation will depend on the soil type. Some researchers have tried to overcome this difficulty by normalizing their results to the water-holding capacity. Boeckx et al. (1996) found an optimum at 50% of water-holding capacity, whereas Whalen and Reeburgh (1996) found an optimum at 20-50% of water-holding capacity. De Visscher and Van Cleemput (2000) developed a model that incorporates pore-scale diffusion, water stress and the influence of temperature on CH4 oxidation.

The air permeability of soils decreases with increasing moisture content. This can potentially lead to a decrease of the advec-tive transport of CH4 through a landfill cover. However, the gas produced by the waste must somehow escape to the atmosphere, so high moisture contents in a cover soil will either lead to lateral transport of CH4 and emissions adjacent to the landfill or to a pressure build-up creating the necessary driving force for advective transport through the soil. A tragic instance of the former possibility was documented by Kjeldsen and Fischer (1995) at Skelligsted landfill, Denmark, where lateral landfill gas migration led to a fatal explosion in a nearby house. The latter possibility will not affect the CH4 transport observed in the cover soil if the soil is homogeneous. However, cracks and fissures usually occur in landfill cover soils as a result of waste settlement, so wet landfill cover soils lead to large CH4 emissions through these 'hot spots'. Czepiel et al. (1996b) found that about 5% of the cover area of a landfill in New Hampshire was responsible for 50% of the CH4 emissions. Bergamaschi et al. (1998) found that 70% of CH4 emissions through cover soils of landfills in Germany and the Netherlands happened through cracks and leakages in the soil. Mice burrows have also been found to increase CH4 flux variability in landfill cover soils (Giani et al., 2002).

Figure 12.4 shows a model fit to the data on the influence of moisture and temperature on CH4 oxidation from Boeckx et al. (1996) and De Visscher and Van Cleemput (2000).

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