2. Sediment and Contaminant Processes

Contaminated sediments exhibit many features and challenges that differentiate its assessment and remediation from that of contaminated soil. Both soils and sediments tend to accumulate the hydrophobic organic and inorganic constituents that give rise to environmental contamination and risk. Sediments, however, are often found in dynamic environments that can lead to substantial contaminant migration. In general, contaminated sediment sites are a legacy of past contaminant discharge practices and the contaminants have accumulated in environments that are most conducive to such accumulation. Thus, a preponderance of contaminated sediment sites are in fine-grained, often organic-rich, sediments that are more likely to absorb hydrophobic contaminants and in environments where such sediments tend to accumulate, i.e., low energy depositional environments. Contaminated soils, however, often represent the source areas themselves and may exhibit a broader range of environmental and media properties. In addition, many of the processes that influence contaminant migration and fate in sediments (erosion, bioturbation, hyporheic exchange) are less pronounced or nonexistent at contaminated soil sites. Even when soils and sediments exhibit similar properties, there may be significant differences due to sediment characteristics. For example, the erosion characteristics of sediments are often controlled by the cohesive nature of fine-grained sediment depositions compared to the minimal cohesion of dry, wind-blown soils.

Moreover, sediments are often confined by one or more spatial dimensions, for example the containment of river sediments to the banks of a river and the adjacent floodplain, limiting the dilution often associated with migration. Thus, contaminated sediment sites may exhibit elevated concentrations and potential risks over large areas or distances compared to many contaminated soil sites. Contaminated sediment sites, by definition, are also associated with large amounts of water, which both complicates their assessment and management but also enhances the potential exposure and risk, for example to aquatic animals and organisms that depend upon them for food.