The simulation of leachate generation from a municipal solid waste landfill or landfill simulation should utilize a mechanistic model which properly accounts for the microbially mediated processes of landfill stabilization. In the past, several leachate generation models have been developed based on the solubility of the waste constitutents in water percolating through a landfill. These models, called 'washout models' provided a reasonable approximation of leachate constituent concentrations after the landfill or landfill simulation reached a period of relative dormancy, called maturation, but were deficient in predicting leachate constitutent concentrations in the early stages of the landfill. These early stages in the life of a landfill are extremely important because, in most cases, the highest leachate concentrations and the most extreme conditions a liner or the surrounding environment would be subjected to occur very early in the life of the landfill.