Pursuant to a congressional request, GAO reviewed the Department of Defense's (DOD) cost estimates to identify and contain or clean up hazardous waste sites, focusing on: (1) why DOD has had difficulty developing reliable estimates; and (2) DOD efforts to produce better estimates.

GAO found that: (1) factors that affect DOD ability to develop reliable estimates of how much it will cost to clean up its hazardous waste sites include unidentified clean-up sites, incomplete contamination studies, unanticipated clean-up costs, and undetermined cleanup times; (2) in 1985, DOD estimated that the costs for assessment, study, and potential cleanup of 400 to 800 sites ranged from $5 to $10 billion; (3) in 1989, DOD estimated that its potential clean-up costs for about 7,100 sites ranged from $4.2 billion to $42.2 billion, depending on the technology needed for remedial actions; (4) although DOD based its 1985 estimates on limited study and research, it produced its 1989 estimate to examine available clean-up options; (5) in the past, DOD either based its estimates on projected clean-up costs of individual sites or updated previous estimates; and (6) DOD used study and cleanup experiences, historical costs, and the military services' input in developing its most recent estimate of $24.2 billion.