process includes a site investigation, comparison of remedial alternatives, and selection and implementation of a remedy. The process is affected by numerous scientific and technical issues. Resource-intensive surveys and analyses are required to document the distribution, depth, and concentration of contaminants in sediments and contaminant concentrations in the aquatic biota. Even if substantial resources are focused on a relatively small site, understanding the current and potential risks of contaminated sediments can be difficult and uncertain, relying heavily on surrogate measures and modeling of actual environmental effects. The process of estimating and comparing the modeled results of potential remedial actions (including no action) has substantial uncertainties that depend on a host of variables, including whether environmental conditions have been adequately characterized and the accuracy of near-term and long-term predictions of post-remediation contaminant behavior. The uncertainties are magnified by increasing duration of a remedial action and increased extent and complexity of a contaminated site.

Contaminated sediments exist in a variety of environments and can differ greatly in type and degree of contamination. Site conditions are important in determining which remediation techniques (and combinations thereof) are appropriate. The techniques include removing the sediments from the aquatic environment (for example, by dredging), capping or covering contaminated sediments with clean material, and relying on natural processes while monitoring the sediments to ensure that contaminant exposures are decreasing, or at least not increasing. Those approaches differ in complexity and cost; dredging is the most complex and expensive, and monitoring without active remediation is the least difficult and least expensive. Remedial approaches have tradeoffs with respect to the risks that are created during implementation and that remain after remediation. Dredging may create exposures (for example, through the resuspension of buried contaminants) during implementation, but it has the potential to remove persistent contaminants permanently from the aquatic environment. Monitoring without removal does not itself create risks, but it leaves contaminants in the aquatic environment. Remedial operations also vary in efficacy within and among the different approaches. The variability is driven by several factors, including site conditions and implementation of the remedial approach.

Decisions about whether to dredge at contaminated sediment sites have proved to be among the most controversial at Superfund mega-



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