Vertical migration of leachate pollutants in clayey soils beneath an uncontrolled landfill at Huainan, China: A field and theoretical investigation

To assess the extent of leachate migration, continuous samples of clayey soils (about 9 m) were obtained beneath a 17-year old uncontrolled landfill in southeastern China. The soil samples were sub sectioned and analyzed to determine the concentrations of chloride, sodium and COD in the pore water. Total nitrogen and soil organic matter content of the soil samples were also determined. Leachate-derived chloride was detected in the clayey soil to a maximum depth of 9 m. Sodium and COD were found to migrate into the soils to depths of 3-4 m due to the attenuation of solutes by the soil organic matter and clay minerals at the shallow soils. The estimated migration depths for the chloride are 3 m in the case of pure diffusion. Advection and mechanical dispersion were found to be more important than molecular diffusion for this site with an 8 m high leachate mound. By comparing the results obtained by the mathematical modeling for layered advection-dispersion problem with the measured concentration profiles, the ranges of the effective diffusion coefficient, retardation factor and dispersivity of the soils were estimated. Better fits are obtained by employing an artificial effective interface about 1 m above the observed interface. The clayey soils showed a relatively high attenuation capacity for COD with the estimated retardation factor of 5.