Evaluating the Efficacy of a Low-Impact Delivery System for In Situ Treatment of Sediments Contaminated with Methylmercury and Other Hydrophobic Chemicals
Many Department of Defense (DoD) contaminated sediment sites contain elevated concentrations of mercury and hydrophobic organic compounds, such as methylmercury, PCBs, DDT and other pesticides, and PAHs. These sites often encompass large areas, some of which still provide important habitat function despite the presence of contaminants, or are difficult to dredge, such as in areas around piers. Most conventional approaches to in situ treatment of contaminated sediments have involved mechanical mixing of the material into the sediment. While these approaches offer a way to place material into the sediment, they disturb the habitat and create a potential for resuspension or release of contaminants. This project will field test the efficacy of an alternative approach to in situ treatment without invasive mechanical methods. The objective of the proposed work is to conduct a field demonstration of SediMite at two DoD sites and provide sufficient performance information to design SediMite applications at other sites. Bench-scale treatability studies conducted prior to the field demonstration will provide important information on the application rates of SediMite for the sites selected for field demonstration.
SediMite is a granular material developed to deliver treatment materials to sediments contaminated by organic chemicals and metals. The material can be applied at the water surface and is designed to sink to the bottom, where it slowly releases the treatment material. The material is designed to enable native benthic organisms to perform the mixing into the biologically active zone, thus obviating the need for mechanical mixing. Thus, the approach limits the potential for sediment resuspension and habitat impacts. Unlike many capping technologies, the approach also has minimal impact on the existing habitat. Because it is not designed to physically cover the sediments, there are no issues associated with trapped gases. SediMite amendments with activated carbon and a mercury sorbent with incorporated thiol groups have performed well to date in bench-scale testing, both in terms of treatment efficacy and in mixing to the desired treatment depths by benthic invertebrates. Exposures to methylmercury and PCBs are reduced by 70% to 90% in the bench-scale demonstrations, as measured by sediment chemistry and bioaccumulation.
The technology is expected to cost approximately $90,000 to $200,000 per acre (depending on the type of amendment), which compares favorably with capping (approximately $115,000 to $290,000 per acre) and dredging (approximately $1,000,000 per acre). In addition, use of SediMite would likely obviate the need for substantial habitat restoration, because application would enhance rather than disturb the functioning sediment ecosystem. Because of the nature of contamination at DoD sites, and because of the sensitivity of habitats at many of these sites, there will be many sites where the SediMite technology will provide benefits in the future. We believe the delivery system will be especially beneficial at DoD sites where there is a desire to limit physical impact on the environment (e.g., flooded wetland systems) and where there are hard-to-reach areas such as in and around piers. One of the advantages of SediMite over traditional capping is preservation of habitat, reducing the need for compensatory restoration. The costs of such restoration can range from $2,000 to $100,000 per acre and can be viewed as a potential additional cost for capping remedies. We estimate that SediMite will have lower ecological costs than capping and could be used in locations where the capping alternative is much more expensive. (Anticipated Project Completion - 2011)
Points of Contact
Dr. Charles Menzie
SERDP and ESTCP
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