" The modeling of leachate generation from a municipal solid waste (MSW) landfill or landfill simulation should utilize a mechanistic approach which properly accounts for the microbially mediated processes of landfill stabilization. Previous models have been based on the solubility of waste constituents in the water percolating through a landfill. These models, called washout models, provided a reasonable approximation of leachate constituent concentrations after the landfill or landfill simulation had reached a period of relative dormancy, called maturation, but were deficient in predicting leachate constituent concentrations in the early stages of landfill stabilization, and gas production and quality after methane fermentation had been established. These early stages in the life of a landfill are extremely important, because, in most cases, the highest leachate strengths and the most extreme conditions a liner or the surrounding environment would be subjected to occur during this period. Similarly, the methane fermentation stage is important in predicting the potential for gas production, migration and possible utilization. A mechanistic three-step model, GTLEACH-I, was developed to simulate the microbially mediated processes of landfill stabilization in terms of hydrolysis of substrate, acid formation and methane fermentation. The model was applied to two sets of experimental data and provided a reasonable prediction of volatile acids and gas generation."