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Natural attenuationNatural attenuation refers to the collection of processes that influence the contaminant levels to which organisms at-risk may be exposed. It may involve simple containment, for example, by the deposition of clean sediment that physically separates the contaminated sediment from benthic and pelagic organisms. It may also involve actual degradation or loss mechanisms that remove the contaminants from the sediment environment. Some natural loss processes may simply serve to move contaminants elsewhere, perhaps with sufficient dilution to reduce exposure and risk to acceptable levels. Natural attenuation requires identification, evaluation and quantification of the natural fate and transport processes in order to make rational decisions based upon the effectiveness of natural attenuation at any contaminated sediment site. In addition, continuous monitoring is necessary to ensure that the conceptual and quantitative models of the effectiveness of natural attenuation are confirmed and that natural attenuation of the contaminants to a level achieving risk based goals is ultimately realized. In the discussion below some of these processes and their effects are summarized.Natural fate and transport processes normally control the recovery of unremediated contaminated sediments, the effectiveness of in situ remedial processes, and the amount and fate of any residual contamination after disturbance of the sediment. For example, deposition and biological degradation processes control the natural recovery of sediment, contaminant desorption and migration generally controls the effectiveness of in situ containment or treatment processes, and contaminant desorption and deposition significantly impact the effectiveness of any removal action. Natural processes determine the exposure and risks resulting from any activity involving contaminated sediments. Ultimately, it is the portion of the chemical of concern (COC) that moves via natural processes into the water or food chain that is the source of exposure and potential risk even if human actions increase the amount of COC available for these processes. Natural processes include the physical, chemical, and biological processes that occur without human intervention or control. Among the more important processes are the natural release processes due to exposure, removal, or resuspension of sediment. These dynamic processes serve to relate COC concentration levels found in the overlying water body with that found in the sediment. Thus, natural fate and transport processes define the availability of COCs and the potential for exposure and risk to human and ecological receptors. Any attempt to summarize and compare natural attenuation processes in sediments must recognize the different environments in which contaminated sediments are found. The relative importance of these processes differ significantly between lacustrine, riverine, estuarine, and coastal environments. The range and significance of natural processes are influenced heavily by site-specific characteristics. This paper attempts to identify all of the potentially important natural attenuation processes and builds a matrix relating sediment and water body characteristics to these processes. The individual processes are discussed, including a means of assessing the importance of each process in particular field situations. The most important natural fate and transport processes at contaminated sediment sites are:
These processes and the reasons for their importance in the individual environments are discussed in more detail in subsequent sections. The most important factor in defining the fate and transport processes influencing COCs in sediment is the energy of the overlying flow. In high energy environments, the sediment tends to be coarse grained and noncohesive with little sorptive capacity and low depositional rates. These sediments pose little barrier to advective transport and allow oxygen transport deep within the sediment. In low energy environments, a significant deposition of fine-grained sediments exists, providing high sorptive capacity and significant slowing of advection and oxygen transport. Somewhat offsetting these differences is the fact that many organisms, especially head-down deposit feeders, prefer fine-grained sediments. Therefore, bioturbation (i.e., the mixing associated with the normal life-cycle activities of sediment-dwelling organisms) is often enhanced in areas of finer-grained sediments, which may be repositories of hydrophobic COCs such as polychlorinated biphenyls (PCBs) and some heavy metals.
Produced by the Hazardous Substance Research Centers/South & Southwest
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