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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness
degree of contamination can be severe in some areas with nearly unadulterated original products, such as PCB-containing oils, pesticides, or coal-tar residues. In other areas, contaminants occur at low concentrations in sediments among functioning ecosystems of fish, plants, and benthic invertebrates. The thickness of the contaminated sediment is highly variable and often poorly characterized but can range from a few inches to many feet thick with marked differences over small spatial scales. In addition, the nature of the sediments and particularly of the underlying substrate can vary widely on the basis of local geology, hydrology, and human activities that have altered the watershed characteristics.
Because of the highly variable nature of sediments, the environments in which they occur, and the type and degree of contamination, there are many approaches to their remediation. The techniques, which can be employed in combination, 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 (known as monitored natural recovery [MNR]). In-situ treatments that, for example, reduce the bioavailability of contaminants can also be used. The techniques, which are examined in greater detail in Chapter 2, differ in complexity, cost, efficacy, and time frame. That variability is driven by several factors, including site conditions (for example, variations in water flow and depth), underlying substrate characteristics, and implementation of the remedial approach. Regardless, achieving expected reduction in risk is of primary importance to regulators who require cleanup of a contaminated sediment site, parties responsible for funding the cleanup, and communities and user groups that are directly affected by the contamination and the remediation process.
Managing the risks associated with contaminated sediments has been an issue at the federal level since at least the middle 1970s (Johanson and Johnson 1976, as cited in EPA 1987), although it received substantially greater attention in the 1980s when the U.S. Environmental Protection Agency (EPA) sought to document the nature and extent of sediment contamination (Bolton et al. 1985; EPA 1987). The 1989 National Research Council report Contaminated Marine Sediments: Assessmentand Remediation (NRC 1989) examined the extent of and corresponding