of fly ash and sludge from treatment of the wastewater from the wet scrubbing process. The maximum concentrations occur in the initial leachate for most parameters, and most of the concentrations have been observed in samples of leachate collected at or below L/S = 0.5 L/kg. For a particular disposal site S will be constant and L will increase as the leachate is formed; an L/S scale may therefore be transformed to a time scale if the rate of percolation or flow through the site is known. Such data may subsequently be used in conjunction with additional information (i.e. pH and redox conditions) to predict leachate quality as a function of time at a disposal site which contains waste/residues with similar properties and for which the rate of percolation of water is known. At low L/S values, the leaching of several contaminants, particularly trace elements, is solubility controlled and strongly influenced by the pH of the leachate (which in turn is governed by the major constituents of the MSWI residues and local conditions).