Environmental remediation efforts that are underway at hundreds of contaminated sites in the United States will not be able to remediate large portions of those sites to conditions that would permit unrestricted access. Rather, large volumes of waste materials, contaminated soils and cleanup residuals will have to be isolated either in place or in new, often on-site, disposal cells with long term monitoring, maintenance and institutional control needs. The challenge continues to be to provide engineering systems and controls that can ensure the protection of public health and the environment over very long time horizons (hundreds to perhaps thousands of years) with minimal intervention. Effective long term management of legacy hazardous and nuclear waste requires an integrated approach that addresses both the engineered containment and control system itself and the institutional controls and other responsibilities that are needed. Decisions concerning system design, monitoring and maintenance, and the institutional controls that will be employed are best done through a "risk-nformed, performance-based" approach. Such an approach should incorporate an analysis of potential "failure" modes and consequences for all important system features, together with lessons learned from experience with systems already in place. The authors will present the preliminary results of a case study approach that included several sites where contamination isolation systems including institutional controls have been implemented. The results are being used together with failure trees and logic diagrams that have been developed for both the engineered barriers and the institutional controls. The use of these analytical tools to evaluate the potential for different levels of failure and associated consequences will be discussed. Of special interest is the robustness of different approaches to providing long-term protection through redundancy and defense in depth
