Two-year evaluation of hydraulic properties of biochar-amended vegetated soil for application in landfill cover system.

Landfill cover should ideally have a medium with high water retention ability and low hydraulic conductivity to prevent rainfall infiltrating into the hazardous waste layer. Even though biochar amended soil (BAS) is advocated as cover medium, the interactions between biochar and plant, as well as the effects of biochar aging and plant growth on soil hydraulic properties are still not clear. This study aims to investigate the effects of grass (Cynodon dactylon) growth in BAS on soil water retention and saturated hydraulic conductivity (k) over a two-year period. Four ground conditions were tested, namely bare silty sand with and without biochar, vegetated silty sand with and without biochar. The biochar content was kept at 10% (v/v). During the first 6 months, soil water content corresponding to field capacity (FC) and permanent wilting point (PWP) in grassed soil increased by 17% and 27%, respectively. With biochar inclusion, 43% and 57% additional increases in FC and PWP respectively were observed. Moreover, k in biochar-amended grassed soil decreased by 50%. Furthermore, grass growth from 6 to 24 months reduced FC by 32%, PWP by 40% but caused 20 times increase in k of grassed soil. With the presence of biochar, FC and PWP decreased by only 6% and 8%, respectively, and k increased by 200% due to the enhanced plant growth (specifically root growth) by biochar. After two years, k of grassed soil with biochar was 16 times smaller than that without biochar. This study demonstrated the effectiveness of biochar in maintaining the enhanced soil water retention ability and reduced k in vegetated soil over a two-year study period.