||"Nine hazardous waste surface impoundments (Si's) were assessed in a case study to compare actual and projected performances. The goal was to produce data that can significantly improve the design, construction, and operation of these facilities. The nine facilities represent a range of industries, waste types and volumes, ages, environmental settings, linear types and designs, and systems for leak detection and groundwater monitoring. In addition to the case studies, nine interviews were held with technical experts in four consulting engineering firms, one waste management company, one liner fabricator and installer, and regulatory agencies in three states. Recommendations for research and development are presented based on the case studies and the professional opinions collected. The poor performances of several Si's were attributed to factors such as lack of good project planning during design and construction, lack of quality assurance and control, deviations from liner specifications, inadequate waste-liner compatibility studies, and lack of proper site investigations before design and construction. The successful performances of Si's at two facilities are attributable to the use of a very impermeable clay liner after extensive compatibility studies; use of competent contractors; close scrutiny of all phases of design, construction, and inspection by the owner/operator; excellent quality assurance and control record keeping; and good communication among all parties involved. Technical experts consider the following factors essential to good site performance: Siting in good geologic formation, continuous geotechnical support throughout all project phases, supervised construction to ensure adherence to specifications, compaction of clay liner wet of optimum to eliminate air spaces, consideration of liner-waste compatibility, rigorous quality assurance and control in designing and installing liners, and provision and maintenance of protective covers for liners. Research and development areas should include documenting and disseminating design and performance data from operating sites, evaluating wasteliner compatibility under actual conditions, developing reliable techniques for early detection of site failure, establishing criteria for groundwater monitoring systems, and studying the causes and cures for plugged leachate collection systems in landfills."