Hydrological and geochemical factors controlling the leaching of cemented MSWI air pollution control residues: A lysimeter field study

A pilot landfill (area, 16 m2; depth 1.5 m) containing cement-stabilized air pollution control (APC) residues from municipal solid waste incinerators (MSWI) was investigated in order to determine the hydrological and geochemical factors that control the leaching process. Between May and August 1996, 3 rain events were investigated. Rainfall and discharge were monitored and leachate samples taken for the analysis of major components. It was found that water from the rain events took 2–3 days to pass through the lysimeter. Leachate dilution at the beginning of rain events indicated that rainwater passed through the lysimeter as preferential flow. The change in the concentration of leachate components with time was found to be almost identical for most components. Conductivity, dominated by Na+ and OH−, also positively correlated with the concentrations of Al, Cs, K, Si, sulfate, chloride, carbonate and organic substances in the drainage discharge. The Ca concentrations were not influenced by the rain events, indicating that this component was leached by rainwater from the lysimeter cover layer. This was also found to apply to observed changes in Sr concentrations. Both conductivity and Sr were used as tracers for rainwater in the leachate and results agreed well. The solubility of the main constituents, Ca, Si, and Al could be explained by thermodynamic equilibrium with the solids calcium silicate hydrate, ettringite and possibly K-feldspar. The results indicate that the leaching process (within the first 5 years) is dominated by pore waters between the cement blocks that are in equilibrium with the cement-stabilized APC residues (thus explaining the constant composition of the leachate) and is flushed out by rain events.