178 Int. J. Environment and Pollution, Vol. 31, Nos. 1/2, 2007 Copyright © 2007 Inderscience Enterprises Ltd. Evaluation of the hazardous waste landfill cap system design and clay layer thickness criteria of the Turkish Regulation on the Control of Hazardous Waste (RCHW) using the Hydrological Evaluation of Landfill Performance (HELP) model F. Yalçın Pişkin General Directorate of Highways, Konya District, Konya, Turkey E-mail: fpiskin@kgm.gov.tr G.N. Demirer* Department of Environmental Engineering, Middle East Technical University, Inonu Bulvari, 06531, Ankara, Turkey E-mail: goksel@metu.edu.tr *Corresponding author Abstract: The hazardous waste landfill design criteria of the Turkish Regulation on the Control of Hazardous Waste (RCHW) was evaluated in this study. In the first part of the study, Hydrologic Evaluation of Landfill Performance (HELP) model was used to determine the significance of different components of the hazardous waste landfill cap system as required by the Turkish RCHW. In the second part of the study, the top and bottom clay layer thickness requirement of the Turkish RCHW was evaluated by running the HELP model for different top/bottom clay different layer thicknesses and comparing the corresponding leachate amounts produced. Keywords: hydrological evaluation of landfill performance model; HELP; landfill caps; clay layer; Turkish regulation on the control of hazardous waste; RCHW; İzmit hazardous waste lot; İzmit HWL. Reference to this paper should be made as follows: Pişkin, F.Y. and Demirer, G.N. (2007) ‘Evaluation of the hazardous waste landfill cap system design and clay layer thickness criteria of the Turkish Regulation on the Control of Hazardous Waste (RCHW) using the Hydrological Evaluation of Landfill Performance (HELP) model’, Int. J. Environment and Pollution, Vol. 31, Nos. 1/2, pp.178–192. Biographical notes: F. Yalçın Pişkin received her BS and MS in Environmental Engineering from the Middle East Technical University, Ankara, Turkey and was a graduate student at the time of the study reported in this paper. Presently, she is with the General Directorate of Highways, Konya District, Konya, Turkey.