For the purpose of saving the waste disposal cost a self-governing community on the whole reclamates the wetland and uses it as a landfill site. During the operation period of site, the environmental impact by the leachate is usually neglected. Therefore, to predict the environmental impact and dispersion of leachate, the wetland adjacent to the South Han River in Chungju is selected as a site of case study. The main content of this article is to apply the simulation model CONMIG to leachate transfer in the ground water and to derive the impact by the leachate. A kind of non-reacting conservative material, chloride ion is used as a tracer to quantify impacts by the leachate.