The objective of this research was to study the fate of toluene, acetone and 1,2- dichloroethane (DCA) in refuse excavated from a municipal solid waste landfill contaminated with organic solvents. Refuse excavated from the landfill was used to fill multiple 8-1 simulated landfill reactors that were operated with and without moisture addition to simulate the absence and presence of an engineered cover, respectively. An average of 31.3% of the added C-14-toluene was converted to (CH4)-C-14 and (CO2)-C-14 in reactors that received water additions, while only 12% was mineralized in the absence of water addition. Up to 30% of the added C-14 was recovered in humic and fulvic acid fractions and up to 12% of the C-14 was not extractable with dichloromethane (DCM) followed by 0.5 N NaOH. The non-extractable fraction may represent strongly sorbed toluene or toluene metabolites covalently bound to humins. From 21.6 to 40.3% of the added C-14-acetone was converted to (CH4)-C-14 and (CO2)-C-14 although there was no consistent effect associated with water. This is likely explained by the high solubility of acetone as considerably more acetone was present in the reactor leachate relative to toluene. Reductive dehalogenation of 1,2-DCA to ethylene was measured in all reactors. This study suggests that remediation activities designed to reduce moisture infiltration could adversely affect contaminant biodegradation at the source and that moisture infiltration must be balanced against the potential increase in contaminant release due to leaching. Further, the association with humic matter in decomposed refuse may represent an alternative mechanism for contaminant sequestration. (C) 2000 Elsevier Science Ltd. All rights reserved.