Cost estimates have generally declined as more information has become available. This ISO study summarizes the cost estimates of a number of other analysts. Most estimates of ultimate NPL cleanup costs start with assumptions or estimates of the number of sites eventually subject to cleanup as NPL sites. Table 1 lists the estimates from four important studies. Therefore, the Academy's study concludes that the ultimate number of nonfederal NPL sites is likely to be about 2,000.
Some studies provide assumptions or estimates of the average cleanup cost per site.
ISO found that existing analyses of these average costs are difficult to compare because of inconsistent and unclear assumptions and calculations. Since few cleanups are now complete, the studies have little actual experience on which to base projections. The result is substantial variation in the studies' estimate operation and maintenance costs. One study predicts that private-sector transaction costs will rise several years but should start to fall during the next decade. The studies ISO considered for this summary generally estimate cleanup costs separately from transaction costs.