A COMPARISON OF THE CONVECTIVE AND DIFFUSIVE FLUX OF ORGANIC CONTAMINANTS THROUGH LANDFILL LINER SYSTEMS

In an effort to protect groundwater, waste disposal regulations now re quire the use of composite liners consisting oi compacted soil overlai n with a flexible plastic membrane liner in both hazardous and municip al solid waste landfills in the USA. When properly installed, composit e liners decrease the convective flux of leachate flowing through flaw s in the plastic; however even ii all flaws could be eliminated, organ ic chemicals will diffuse through the liner system. Although actual fi eld measurements are not: available on the flux of contaminants throug h landfill liners, field measurements of leachate concentrations at so me landfills, leak rates at other landfills and laboratory data on dif fusion coefficients, allow estimation of the flux so that convective a nd diffusive transport can be calculated. Calculations were made to es timate the flux of five organic chemicals through. liners of various d esigns. For the flexible membrane liner alone, the flux of organic che micals is dominated by diffusion, and transport by convective flux thr ough flaws is one or two orders of magnitude less. For a 100 cm thick clay liner alone, the flux of organic chemicals is dominated by convec tive flow since the long path length limits diffusive flux, For a comp osite liner including a flexible membrane over a 100 cm thick clay lin er, the flux of organic chemicals is dominated by convective transport since the diffusion path length through the composite liner is much g reater than that through the flexible membrane finer (FML) alone. For a typical composite liner in which the FML contains 30 holes/ha(-1), t he convective flux of organic chemicals is typically two orders of mag nitude greater than the diffusive flute. A liner consisting of an FML, or an FML combined with a thin layer oi low permeability soil, will a llow a much greater flux of contaminants than a composite liner consis ting of an FML and a thick layer of low permeability soil.