long are they likely to work effectively? The answers to these questions were derived from analysis of available data and case history information, input from federal agency managers and outside experts, information provided by waste containment facility operators and technical personnel, and the research and practical experience of members of the committee.

This report focuses on engineered barriers that were designed to contain municipal solid waste, other nonhazardous solid and liquid waste, hazardous and toxic wastes, and low-level radioactive wastes. Barriers constructed for both waste landfills and corrective action containment and cleanup of contaminated sites were considered. Barrier systems that were intended to treat rather than isolate waste (e.g., reactive barriers that transform contaminants into nonhazardous substances) were not considered.

Many barrier systems are intended to function for tens to thousands of years. Because most regulated sites have been in operation less than 30 years, it is not possible to make unequivocal conclusions about the actual long-term performance of these systems. Consequently, the focus of this report is on performance over periods of several tens of years—the operating, maintenance, and monitoring period for many waste collection systems (Box 1.2)—and on information that provides useful insights about future behavior. Partly because of the very long times (on the order of 1 million years) required to isolate high-level radioactive waste, and partly because no examples exist in the United

BOX 1.2 Terms Used in This Report Contaminant: Any solid, liquid, or gas resulting directly or indirectly from human activities that may cause an adverse effect on human health and/or the environment. Modes of contaminant transport: Advective transport of contaminants driven by hydraulic or pneumatic gradients Diffusive transport of contaminants driven by chemical gradients Coupled flows of contaminants driven by thermal and electrical gradients Mechanical transport of contaminants by processes such as wind or water erosion Flux: A measure of the amount of contaminant flow through the ground or a two-dimensional barrier. Flux is usually expressed as a volume or mass passing though a unit cross-section area per unit of time. Hydraulic conductivity: The rate at which a liquid flows through soil under a unit hydraulic gradient. The hydraulic gradient is the amount of fluid total head loss divided by the flow distance over which it is lost. Periods of performance (as used in this report): Short term: the period until completion of construction of the barrier component Medium term: the operating period of the waste unit Long term: the postclosure period