Retention of inorganic pollutants in clayey base sealings of municipal landfills

Abstract

The 10-year-old base sealing of a municipal waste deposit near Horn, located in the Austrian part of the Bohemian Massif, was examined by mineralogical and geochemical methods. The objective of this study was to measure its retention capability with respect to selected inorganic pollutants, especially heavy metals. The mineralogical results show a homogeneous composition of the base sealing, which consists of clay dominated by kaolinite. In the Bohemian Massif, this pattern is typical for products of weathered rocks, which were originally rich in feldspar. The hydraulic conductivity is very low ( k f≈1×10 −11 m s −1). Geochemical analyses show that heavy metals like Pb, Cu, Zn and Cd are concentrated in the uppermost parts of the sealing. Only a few centimeters below, the concentrations of these elements are at the geochemical background level. Anionic contaminants, however, are found in greater depth due to an almost nonexistent adsorption on the clay. This is the reason why they were used as tracers for further research on diffusion processes (apparent diffusion coefficient of chloride D a,Cl=2.6×10 −10 m 2 s −1). Detailed studies of the behavior of Cu, Pb, Zn and Cd were performed using sorption tests (diffusion and batch tests) and sequential extraction methods. From the diffusion experiments, breakthrough times of some heavy metal ions were evaluated for the actual thickness of the sealing. There were Zn >200 years and Pb >700 years. These times can be considered as “worst case” examples which give good indications of the retention capability of the sealing. The results clearly demonstrate that even after a period of 10 years of contact with contaminated landfill leachates, the clayey liner had fulfilled the requirements of retaining heavy metals. Furthermore, the base sealing was far from reaching the limits of its retention capacity.