Waste Manage Res 2003: 21: 127–136 Printed in UK – all rights reserved Waste Management & Research Copyright © ISWA 2003 Waste Management & Research ISSN 0734–242X 127 Introduction Leachates are fluids generated by the release of excess water from solid waste, and by seepage of rain water through a strata of solid waste that is basically in a state of decay (Uguccioni 1997). Leachate is characterised by its high content of organic constituents, metals, acids, dis- solved salts, and micro-organisms. These characteristics constitute a flow highly aggressive to the environment, with a contamination potential exceeding that of several industrial-waste materials (Orta et al. 1999). Pollution of aquifers by leachates renders the aquifers unavailable for exploitation for a long time, because their rehabilitation can take several years. An estimate of leachate generation throughout the life of a landfill is valuable in order to min- imise the impact of this on the environment (Khanbilvardi et al. 1995, Orta et al. 1999). Several computer programmes for estimating leachate generation have been developed, for example: Hydrology Evaluation Leachate Performance (HELP) (U.S. EPA 1984), FULLFILL (Noble 1991), and SOILINER (Johnson 1986). All of them are based on the so-called Water Balance Method (WBM) developed by the U.S. Environment Protection Agency (Fenn et al. 1975). Other Ma. Teresa Orta de Velásquez Reynaldo Cruz-Rivera Neftalí Rojas-Valencia Ignacio Monje-Ramírez Institute of Engineering, Cub.214 Coordination of Environmental Engineering, National Autonomous University of Mexico, Post Box 70-472, Coyoacán 04510, Mexico D.F. Jorge Sánchez-Gómez SICA S.A de C.V. Nubia 52, Col. Clavería, Azcapotzalco, 02080 Mexico D.F. Keywords: Leachate, confining cells, serial water balance, macro- cell, wmr 407–1. Corresponding author: Ma. Teresa Orta de Velásquez, Instituto de Ingenieria, Cub. 213, Coordinación de Ingenieria Ambiental, Universidad Nacional Autónoma de Mexico, Coyoacan 04510, Mexico D.F. Tel: (52)(55) 622–3320 Extn. 17, Fax: (52)(55)616–2164 E-mail: tol@pumas.iingen.unam.mx Received 10 July 2000, accepted in revised form 20 January 2003. This paper presents the "Serial Water Balance” method for predicting leachate generation in landfills. This proce- dure makes it possible to calculate the total leachate like- ly to be generated, by estimating an individual cell by cell water balance. This new development considers the inter- action effects between cells, through the execution of sim- ple field capacity tests on solid waste samples under different loading conditions. The procedure described in this paper simulates the effect induced by fluid percolating from a cell at an upper layer to cells in the immediately subjacent layer. The method described here was applied to the landfill in Nuevo Laredo, Tamaulipas (Mexico), after first ascer- taining information regarding: duration of construction time, exposure time, surface area, the quantity of waste, number of confining cells, and local weather conditions. In a full-scale test case the suggested method has pre- dicted 67% of the leachate produced in a period of 4 years. Further testing and more detailed analysis of the composi- tion of the waste used in the calculations, may improve the accuracy in predicting leachate production. Even though this SWB methodology was applied to a landfill subject to extreme climatological conditions (high daytime temperatures), it is possible to adapt the method- ology to solid waste disposal sites in regions with more humid or moderate climates. Serial water balance method for predicting leachate generation in landfills