The elaboration of 10 years of monitoring of leachate quality and quantity, leachate treatment and degree of contamination of soil and surface waters at the Tre Monti site--an active, 4-million-m(3) landfill in Northern Italy--is presented in this study. A hydrological model of leachate production is applied, with a good match of the experimental data. The concentrations of all leachate components except sulfate are characterized by fluctuations over a constant or increasing value. Different ways of interpreting leachate quality data are discussed; the elaboration indicates that the pollutant load on the leachate treatment facility will remain basically constant as long as waste will be added to the landfill. The analysis of the data relative to 10 years of leachate pre-treatment in the adjoining, non-aerated lagoon system indicates that a significant removal is achieved for most leachate components; the operational conditions of the plant are described, and the removal mechanisms are discussed. Finally, the potential for contamination of soil and surface waters is examined by analyzing long-term quality trends of the sub-superficial waters sampled near the lagoons and by means of an analytical campaign conducted on clay cores sampled near and underneath the treatment ponds. The experimental values indicate that the clay layer located under the entire site offers an effective barrier to the migration of leachate contaminants.