The results of field and laboratory investigations of a domestic waste landfill overlying 30 m of natural clay are presented. Four study phases extending over a ten-year period will be summarized.

Concentration profiles for several soluble constituents (sodium, potassium, calcium, magnesium, Cl⁻ and dissolved organic carbon) show migration to a maximum depth of ∼1.5 m in 15 years. this compares to an advective or seepage advance estimated to be only 3 to 5 cm, indicating migration primarily by diffusion. Concentration profiles for the heavy metals (iron, lead, zinc, and copper) indicate rapid field attenuation with migration to only 10 cm in the carbonate-rich clayey soil.

Graphed solutions to the coupled advection/diffusion equation using constant values for C₀₊ D, and V_s are only partially successful in predicting the observed chemical profiles because of apparent chemical partitioning at the waste/clay interface. A better fit is obtained by employing an artificial “effective” interface about 25 cm above the observed interface; however, this does not resolve the interface problems.

Recently, new analytical and numerical procedures have been developed for modelling this and other contaminant migration problems. These techniques automatically take into account time-dependent concentration variations within the landfill (for example, because of mass transport into the soil) and allow for a sharp drop in concentration because of surface effects (such as surface smear, drying, and interface partitioning). The numerical solution allows consideration of the effects of a thin but finite interface layer with properties different from those of the underlying soil. Both of these techniques are used in the field case resulting in much superior predictions than earlier models.

Comments are presented on the significance of this work relative to chemical fluxes through thin clay liners.