Unsaturated Hydraulic Properties of Biochar and Biochar-Amended Soils for Landfill Covers

The high concentrations of methane emissions from landfill covers pose significant risks to the environment and global climate. Recent investigations have shown that landfill-cover soils amended with biochar have the potential to considerably reduce the unmitigated methane emissions by means of adsorption and oxidation. Biochar is a solid byproduct from gasification or pyrolysis of biomass such as waste wood, corn stove, and other organic wastes. Biochar, with its relatively large internal porosity and surface area, facilitates microbial activity, thereby enhancing the methane oxidation rates. Although biochar amended soils serve beneficially to alleviate fugitive emissions of methane from landfill covers, the hydraulic behavior of such soils is still unclear. The hydraulic properties of landfill-cover soils are significantly important as they determine the percolation and movement of water into the MSW. This in turn quantifies the amount of leachate generated within the landfills. Furthermore, it is crucial to understand the unsaturated hydraulic properties of landfill-cover soils as these soils experience varied climatic conditions throughout any given year. In this study, the saturated and unsaturated hydraulic properties of biochar, unamended cover soil, and biochar-amended cover soil are presented. The saturated hydraulic conductivity was measured by performing flexi-wall (triaxial) permeability tests and the unsaturated properties were calculated from soil-water retention tests performed using Tempe cells. The soil used classifies as low plasticity silty clay (CL) according to the USCS. The amount of waste-wood derived biochar varied from 2 to 10% for the biochar-amended soil samples tested. The soil-water characteristic curve (SWCC) obtained for each of the samples was best fitted to the van Genuchten (vG) model to obtain the vG model parameters. It was found that the saturated hydraulic conductivity of the soil increased due to biochar amendment, but it was still low enough to serve as landfill cover soil. It was also observed that the air-entry value of the biochar-amended soil increased with increasing amount of biochar, and it was found to be highest for biochar itself. The results strongly imply that an increase in biochar content in the soil increases the water holding capacity of the amended soil.