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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness
conditions are always influenced by long-term natural attenuation processes and ongoing sources of contaminations if they exist, so long-term monitoring—over decades—may be needed to establish effectiveness; few of the sites reviewed by the committee have reached that level of maturity. Not counting the 5 pilot studies or hot-spot removal actions, about one half of the sites apparently did not achieve remedial action objectives or had inadequate monitoring to judge performance relative to remedial action objectives. Insufficient time has elapsed to judge achievement of remedial action objectives in approximately one quarter of the sites. The remaining sites apparently met remedial action objectives although the extent to which those remedial action objectives achieve long-term risk reduction may not be known.
There were often sufficient data to evaluate performance relative to cleanup levels or short-term implementation goals, but the relationship of these measures to long-term risk reduction was often not clear. An examination of Appendix C shows that many sites achieved cleanup levels; however, many were operational goals (mass removal or dredging to elevation) rather than contaminant-specific goals.
Natural processes are always modifying conditions at a site; their influence can be difficult or impossible to separate from the remedial action, particularly when control or reference sites are not monitored before and after remediation. Conditions also are often influenced by the implementation of combined remedies, such as dredging and capping, which complicate the assessment of the performance of dredging alone. Thus, the committee was unable to evaluate the effectiveness of dredging alone at most sites.
Experiences at the sites can nevertheless inform remedial project managers as to what may be achievable with dredging and what site and operational factors may limit dredging effectiveness or contribute to its success. Experience is especially useful in identifying factors that contribute to success or failure of dredging to meet short-term cleanup levels because monitoring has been conducted at most sites to judge performance relative to these standards. The ability to meet chemical-specific cleanup levels, however, does not in itself mean the ability to meet long-term risk-reduction targets or indicate the time frame over which any such targets might be met. This chapter discusses the lessons learned from sites where dredging was conducted and uses specific examples to