Effectively Managing Risks of Contaminated Sediments

Various research undertaken over the past decade has improved our ability to assess and effectively manage contaminated sediments. Key to this improvement is a better understanding of the risks, both in time and space. Sediments can pose risks if contaminants are present in the biologically active zone of a water body or if transport processes can move contaminants into that zone. Conversely, effective management of contaminants can be accomplished by physically separating contaminants from the biologically active zone or hindering the transport processes that can result in contaminant migration. Both of these are accomplished by sediment capping, either conventional capping with a passive barrier such as sand, or, when greater control over mobile contaminants is required, by active capping in which amendments are used to retard any applicable transport processes. In this paper, the key transport and exposure processes are assessed and the ability to manage these risks with both passive and active capping evaluated. In summary, conventional sand capping can be effective in reducing exposures, while active capping can provide greater control; options include activated carbon/coke (to sequester organic compounds), organo-clays (to control nonaqueous phase and dissolved liquids), and clay polymers (to control permeability). Field demonstrations are underway on both coasts (including Portland, Oregon, and the Anacostia) to assess the effectiveness of various in-situ options, including organo-clay mats and other active caps. In addition, a unique in-situ sampling approach is demonstrating that pore water concentrations can be determined and employed to assess baseline or post-remedial risks due to contaminated sediments. Additional capping research is addressing issues such as gas release, treatment for other contaminants, and geotechnical tests to evaluate mobility of nonaqueous phase liquids due to cap loading and consolidation. Science and technology are combining to support more realistic risk assessments and practical management measures for contaminated sediments, with particular emphasis on in situ approaches. (authors)