Effects of dry bulk density and particle size fraction on gas transport parameters in variably saturated landfill cover soil
Highlights
► The effects of soil physical properties on gas transport parameters were investigated. ► Higher values of Dp and ka exhibited in the ‘+gravel’ than the ‘−gravel’ fraction at same soil–air content (ε). ► Recent power law models for Dp (WLR) and ka (RPL) were modified. ► Model parameters were linearly related to easily measurable dry bulk density (ρb).
Introduction
The enhanced atmospheric concentrations of the major greenhouse gases (GHG) carbon dioxide, methane and nitrous oxide may potentially lead to significant regional and global climate shifts with inherent regional and global environmental problems. Methane in particular is a large potential contributor to climate change as its global warming potential (GWP100) is 25 times that of carbon dioxide (IPCC, 2007). Making up 28% of the total anthropogenic methane emissions, solid waste disposal on land constitutes the second-largest anthropogenic source of methane in Europe, after agriculture (Gebert et al., 2010). In terms of total anthropogenic greenhouse gas emissions, i.e. including carbon dioxide release from fossil fuels, the waste sector globally represents the fourth largest source with an annual release of 0.5 Tg CO2-equivalents (CO2e) in 2007 (UNFCCC, 2009). Landfill GHGs are produced mainly under oxygen-limited (anaerobic) conditions and can subsequently emit to the atmosphere through the landfill final cover soil (Hilger et al., 1999, De Gioannis et al., 2009). Therefore, the landfill final cover should be designed to promote oxygen exchange between the atmosphere and waste layer to maintain aerobic conditions and high methane oxidation in the final cover soil layer (Berger et al., 2005, Abichou et al., 2006, Moon et al., 2008), and at the same time secure a good hydraulic performance. The recommended design criteria for a landfill final cover system should also include measures to (i) minimize infiltration of precipitation into the waste, (ii) promote good surface drainage, and (iii) resist erosion (US EPA, 1993).
The soil gas diffusivity (Dp/Do, ratio of gas diffusion coefficients in soil and free air) is the governing transport parameter for gas diffusion under a concentration gradient, while air permeability, ka (μm2), is the governing parameter for advective gas transport under a pressure gradient. Soil physical properties such as dry bulk density (ρb) and particle size fraction, and soil pore structure parameters including soil–air content (ε), total porosity (f) and pore connectivity–tortuosity as inferred from gas diffusivity, all strongly affect the gas transport parameters (Moldrup et al., 2001). Recently, Hamamoto et al. (2011) investigated Dp/Do and ka in differently-compacted, sandy landfill final cover soils and observed an almost linear increase in measured Dp/Do values and a non-linear increase in measured ka values with increasing ε in highly compacted soil. Chamindu et al. (2011) found that soil compaction more than soil type was the major governor of Dp/Do and to some extent also of ka. Hamamoto et al. (2009) investigated the effect of particle size distribution on gas transport parameters in sandy soils and observed enhanced Dp/Do and ka values for the coarser sands (larger mean diameter d50), suggesting that larger pore diameters become available for gas transport with increasing d50.
Until recently, research on landfill final cover soil design has focused on the hydraulic performance and necessary criteria to minimize water infiltration and soil erosion (Jang et al., 2002, Kamon et al., 2003, Meer and Benson, 2007, Moon et al., 2008). Less research has been devoted to the implications of landfill cover soil gaseous phase performance for greenhouse gas emissions from landfill to atmosphere. The recent soil–gas transport studies cited above (Hamamoto et al., 2009, Hamamoto et al., 2011, Chamindu et al., 2011) all imply that soil compaction and particle size fraction are key parameters to understanding landfill cover soil gaseous phase performance. They suggest that further investigations are needed to understand and predict the combined effects of the basic soil physical characteristics on gas transport parameters in landfill final cover soils that typically are constructed of highly compacted loamy or even clayey soils.
This study therefore focused on the effects of particle size fraction and dry bulk density on Dp/Do and ka with the following objectives: (i) to measure the Dp/Do and ka as a function of soil–air content (ε) for differently compacted landfill cover soil; (ii) to modify recent models for Dp/Do and ka by considering the model parameters as a function of dry bulk density, ρb; and (iii) to develop a graph that illustrates the effects of dry bulk density and particle size fraction on both Dp and ka by model sensitivity analyses using the newly-developed predictive models.
Section snippets
Soil sampling and properties of soil
The considered waste landfill site is located in the Saitama prefecture in Japan. An approximately 2.5-m thick soil layer is currently used as a final cover above the waste layer. The final cover soil at the sampling site is highly compacted and the in situ dry bulk density was around 1.90–1.95 g cm−3. Disturbed soil samples were taken from the final cover and were sieved through a 35-mm mesh in the laboratory to eliminate larger fractions. Subsequently, part of the soils were further sieved
Models for gas diffusivity
Typically, it is assumed that the soil–gas diffusivity follows a power-law function of the soil–air content, as originally suggested by Buckingham (1904):where ε is the soil–air content (cm3 cm−3) and X is an exponent characterizing pore connectivity–tortuosity. Buckingham, 1904, Marshall, 1959 proposed values for X of, respectively, 2 and 1.5.
The next generation of gas diffusivity models started to incorporate the soil type and density effects through soil porosity. Among the commonly
Compaction curves and saturated hydraulic conductivity for the final cover soil
The results of the compaction tests are shown in Fig. 3a for the ‘+gravel’ (<35 mm) and the ‘−gravel’ (<2 mm) fractions. The field moisture content was approximately 9%, the optimum moisture content for Modified Proctor was approximately 9%, and for Standard Proctor was 10% for the ‘+gravel’ fraction, and for the ‘−gravel’ fraction the corresponding values were 10% and 12%, respectively. The ρb values were approximately 2.1, 1.90, and 1.85 g cm−3, respectively. The saturated hydraulic conductivity (
Conclusion
The study has presented measurements of the main soil–gas transport parameters (gas diffusivity and air permeability) for highly and extremely compacted sandy and gravelly porous media, used for landfill final cover soils in Japan, and has developed and tested predictive models that are taking account of dry bulk density and water blockage. We observed large effects of both particle size fraction and dry bulk density on soil–gas diffusivity (Dp/Do) and air permeability (ka) for
Acknowledgments
This work was supported by Research for Promoting Technological Seeds, JST, and by the grant from the Takahashi Industrial and Economic Research Foundation. Part of this work was also supported by the projects Gas Diffusivity in Intact Unsaturated Soil (“GADIUS”) and Soil Infrastructure, Interfaces, and Translocation Processes in Inner Space (“Soil-it-is”) from the Danish Research Council for Technology and Production Sciences, and by the research grant from the JST/JICA Science and Technology
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