Danny Reible, PhD, PE
Chevron Professor of Chemical Engineering
Director, Hazardous Substance Research Center/S&SW
Management of contaminated sediments poses some of the most difficult site remediation issues that we face today. The term management is used here as a series of actions designed to reduce or eliminate the risk of contaminated sediments to human or ecological health. Can we manage contaminated sediments? Contaminated sediments typically reside in spatially variable and dynamic systems subject to seasonal flow variations and episodic storm events. The volume of sediments that must be managed often exceeds a million cubic yards, dwarfing many contaminated soil sites. These sediments are also associated with equally daunting volumes of water and efforts to remove the contamination typically entrains even more water. All of these problems makes remediating contaminated sediments a difficult and costly process and leaves one wondering whether effective low-cost management of contaminated sediments is even possible.
The history of contaminated sediment management is one that has many similarities to the management of contaminated soil and groundwater sites. Our initial faith in technological solutions to soil and groundwater problems have evolved such that we now recognize that sometimes natural attenuation processes, especially for dense nonaqueous phase liquids, are as effective as technological solutions. Similarly, the default approach to managing contaminated sediment sites has been to remove the contaminants from the body of water in the search for permanence. Large, complicated sites where this seemed to difficult or expensive have essentially been deferred. Unfortunately, removal actions that have been attempted have often not been as effective nor as permanent as desired.
Generally, removal has been focused on a portion of the contaminated sites where contaminant levels are the highest and potential success is limited to the extent that the "hot-spot" contributes to overall risk of the site. In addition, contaminant resuspension during removal leads to mobilization and residual contamination on surficial sediments. Because much of the sediment contamination now observed is due to past practices, deposition of relatively clean sediments has often buried the highest levels of contamination. If these areas are underlain by "hardpan" or bedrock, the inability to reach clean sediment causes incomplete removal and may actual increase surficial sediment concentrations in the short term.
As a result of these problems there is a growing recognition for the need for in-situ approaches, such as natural attenuation or in-situ capping, for managing contaminated sediments in situations where removal options are not appropriate or are less effective. A valid conceptual model of the system and well-defined objectives are a prerequisite for any successful remediation, but this foundation may be even more important for effective application of in-situ approaches. To successfully implement an in-situ sediment management option requires an understanding sufficient to predict contaminant exposure and risk far into the future and extensive monitoring to develop and continuously test this understanding. In most cases, this requires development of an extensive database and a sophisticated mathematical model of the system and its interaction with the potential in-situ management options.
A model capable of supporting in-situ management options must be able to describe the key physics, chemistry and biology that controls the mobility and fate of contaminants in sediments. These processes define exposure and ultimately risk to human and ecological receptors. From the perceptive of a human or ecological receptor, three criteria must be satisfied before contaminants in sediments result in exposure and risk.
In a similar manner, knowledge transfer at a university requires that a student have access to that university, access to an instructor that provides information in an available form, and the student must have the assimilative capacity to absorb that information.
Accessibility is largely defined by either erosion and resuspension of sediments or, in stable sediments, the presence or absence of contaminants in the biologically active zone, the upper layer of sediments that is effectively mixed by benthic organisms. Since most contaminants are strongly associated with the solid fraction, particle movement processes are potentially much more effective at moving contaminants than porewater processes such as diffusion and advection. Even in stable sediments, bioturbation, the burrowing and sediment ingestion/defecation of benthic organisms, will likely dominate contaminant mobility and fate processes. Bioturbation is typically limited to the upper 10-15 cm of stable sediments, however, suggesting that contaminants buried below that depth are largely immobile unless storm or other events expose these underlying sediments.
Availability is largely defined by the fraction of the contaminants that can partition into the porewater within the accessible zone. Sorbed contaminants must generally desorb before they are available to microbes, macrobenthos or higher animals. For metals the presence of significant quantities of acid volatile sulfides (AVS) in amounts exceeding that of simultaneous extractable metals (SEM) generally means that divalent metal ions are tied up as insoluble metal sulfides and not available to organisms. For organics, a number of researchers have found that desorption is often biphasic, with a portion that is readily desorbable and a portion for which the rate and ultimate extent of desorption may be less.
Assimilative capacity depends upon the potentially available contaminant being absorbed or otherwise assimilated by a receptor organism. For benthic organisms that live within the biologically active zone of sediments, their lipids can sorb organic contaminants in a manner similar to organic matter in the sediment. That is, the lipid normalized accumulation within such organisms typically equals the organic carbon normalized accumulation in the sediment. Preliminary data in our lab suggests that this is reduced proportionately by any sequestered or non-bioavailable fraction. For organisms that are not in direct contact with the sediment, the relationship of normalized accumulation to the available fraction in the sediment is much more complicated and dynamic.
Although these processes are complicated, only through their characterization can the influence of in-situ management options on risks be assessed. And it is only by evaluating the risks of contaminated sediments and the various management options that appropriate decisions can be made. As our knowledge and understanding of these natural processes grow, so too does our ability to effectively manage contaminated sediments. In answer to the question posed at the start, managing contaminated sediments is rapidly becoming less and less of an oxymoron.
Dr. Reible graduated with a BS in Chemical Engineering from Lamar University (1977) and a PhD from Caltech (1982). As Director of the Hazardous Substance Research Center/South & Southwest, he works collaboratively with researchers at the Georgia Institute of Technology in Atlanta and Rice University, in Houston, to address critical hazardous substance problems, especially as they relate to contaminated sediments. His professional activities has focused on the fate and transport processes influencing environmental exposure to contaminants.