Gas transport parameters for compacted reddish - brown soil in Sri Lankan landfill final cover

Abstract

Gas exchange through (he compacted final cover soil at landfill sites plays a vital role for emission, fate, and transport of toxic landfill f,ases. This study involved measuring the soil-gas diffusivity tDp/Du, the ratio of gas diffusion coefficients in soil and free air) and air permeability (k,J for differently compacted soil samples (reddish-brown soil) from the final cover at the Maharagama landfill in Sri Lanka. The samples were prepared by either standard Proctor compaction or hand compaction to dry bulk densities of 1.60-1.94 g cm='. Existing and modified models for predicting Dp/ Do and k" were tested against the measured data. The simple, single-parameter Buckingham model predicted measured Dp/ Do values across compaction levels equally well or better than a dry bulk density (DBD) dependent model and a soilwater retention (SWR) dependent model. The measured ku values for differently compacted samples were highly affected by the compactionlevel and the sample moisture preparation method. Also, for air permeability, a single-parameter Buckingham-type k" model was most accurate in predicting k" in the differently compacted soil samples. Equivalent air-filled pore diameters <the effective diameter of the drained pores active in leading air through the sample) for gas flow, (/"q' were calculated from the measured DI'/ Do and k" values. The deq increased with compaction level, suggesting that a very high compaction level creates well-connected macropores in the reduced total pore space of the cover soil. This is an important consideration when designing cover soils for optimally low water and high oxygen exchange while minimizing climate and toxic gas emissions from the waste layer to the atmosphere.