AbstractThis article presents a design approach that has been developed and used on actual projects for a landfill closure plan, including a slurry wall, a leachate collection system, and cap design. A relationship exists between the cost of each of these landfill closure constituents and their impact on improving the site environmental condition. The slurry wall is constructed to prevent leachate migration off‐site. The leachate collection system reduces water mounding and conveys the on‐site leachate to a treatment plant. The cap decreases the infiltration rate of stormwater into the landfill, dependent on the cap shape, area, and vertical constituents reducing permeability. Consequently, the leachate disposal cost decreases and the maximum elevation of the leachate mound inside the landfill is lowered; however, the cap cost increases. This article presents a cost comparison analysis of the cap cost and the leachate disposal cost versus the cap area. The purpose of the analysis is to optimize the project construction cost and get the best environmental results possible by cleaning up the site. The HELP model (U.S. Environmental Protection Agency) is used to evaluate different cap alternatives including the existing condition (no‐cap scenario, only a topsoil cover). MODFLOW (U.S. Geological Survey) is used to develop a groundwater model. The model is used as a design tool, to demonstrate the establishment of inward hydraulic horizontal and vertical gradients into the site resulting from the proposed design and to study the effect of different cap areas on the leachate mound. The site environmental and economic balance is then summarized in a graph showing cap cost, leachate disposal cost, and maximum elevation of the leachate mound versus the cap area. Based on the graph, the optimum cap design is selected and the most feasible action for environmental remediation is recommended. © 2002 Wiley Periodicals, Inc.