WATER RESOURCES BULLETIN VOL. 24, NO. 1 AMERICAN WATER RESOURCES ASSOCIATION FEBRUARY 1988 OVERVIEW OF LANDFILL BOTTOM LINER HYDRAULICS' Alexander C. Demetracopoulos2 h =k /i!ii m 2 /Kd Paper No. 86159 of the Water Resources Bulletin. Discussions are open until October 1, 1988. 2Assistant Professor, Department of Civil Engineering, University of Patra, Patra, Greece. 49 WATER RESOURCES BULLETIN ABSTRACT: Landfill bottom liners are important elements in the analysis and design of solid waste disposal facilities. This study pre- sents an overview of several models, describing the flow over and through liners. The examined models are compared against data from a laboratory investigation. A quasi-steady state (QSS) and a transient (TS) model yield the most complete description of linear hydraulics, and allow for estimation of the system's collection efficiency. The TS model performed slightly better than the QSS model, although the latter has the advantage of simplicity. The hydraulic conductivities of the liner and the drainage layer and the leachate accretion rates are parameters whose magnitudes must be known with relative accuracy for good evaluation of system performance. (KEY TERMS: analysis; collection system; design; liner; mathematical models; landfills; leachate; simulation.) INTRODUCTION The extensive use of solid waste landfills for disposal of municipal and industrial wastes has prompted increased atten- tion to ground water pollution caused by leachates generated in such landfills. The potential for ground water contamination by leachates has necessitated sound engineering designs for landfills; a very important aspect in this process is the analysis and design of the bottom liner and associated leachate collection sys- tem. A typical landfill bottom liner consists of a series of contiguous, alternating-direction sloping layers, constructed from material of low permeability (compacted clay), and overlaid by a layer of higher permeability (gravel or sand). The liner is equipped with perforated drainage pipes, along its lines of lowest elevation, which facilitate collection of the leachate. In recent applications, the use of synthetic membranes underneath the clay layer is becoming common. An understanding of the hydraulics of liners is essential in the correct analysis and design of such systems. In a very general sense, the problem of leachate drainage and leakage, over and through a liner, can be tackled in a manner similar to the classic infiltration problem, i.e., overland flow (corres- ponding to the flow in the drainage layer), coupled with flow through a partially saturated porous medium (corres- ponding to leakage through the clay layer). Limited utilization of this concept has appeared in the literature (Bureau, 1981; Moore, 1983). However, applications of this approach appear to be more suitable for the analysis of surface liners, where precipitation patterns can be such that partially saturated flow conditions may occur. On the other hand, the retentive effect of the landfill refuse on the infiltrating water (Kor- fiatis, et al., 1984) creates conditions of virtually continuous leachate accretion on the bottom liner system. Thus, with the exception of an initial period, the liner is considered fully saturated. The present study reviews three different types of models (steady, quasi-steady, and transient state) for the analysis of leachate movement over and through bottom liners of solid waste landfills. METHODS OF ANALYSIS Steady State Models The simplest liner configuration consists of a horizontal clay layer, overlaid by a drainage layer. Drainage pipes are located transversely for leachate collection (Figure la). For a constant leachate supply to the drainage layer and no leak- age through the liner, a series of steady state mounds will eventually be formed and the input rate will equal the drain- age rate. The maximum head of the steady state mound is given by (Harr, 1962): (1) in which L = distance between adjacent drains, N = rate of leachate input onto the drainage layer, and Kd = horizontal hydraulic conductivity of the drainage layer. Equation (1) provides the means for estimation of the drainage layer thick- ness; it is important that the mound does not extend into the landfill refuse where additional dissolution of contami- nants will take place.