Demonstration of In Situ Treatment with Reactive Amendments for Contaminated Sediments in Active DoD Harbors
The objective of this project is to demonstrate and validate the placement, stability, and performance of reactive amendments for treatment of contaminated sediments in active Department of Defense (DoD) harbor settings. This project extends current pilot-scale testing of the application of activated carbon (AC) to decrease the bioavailability of polychlorinated biphenyls (PCBs) in contaminated sediment to near full-scale demonstration under realistic conditions at an active DoD harbor site. Because AC and the clay mineral associated with the amendment may also sorb methylmercury (MeHg), thereby reducing its bioavailability, a subset of the data collected to meet biological and chemical performance objectives will also focus on effectiveness of this amendment for MeHg-related endpoints. Performance objectives are specifically designed to assess physical endpoints (placement, distribution, mixing, and stability), chemical endpoints (changes in PCB partitioning/sorption in the presence of the amendment), and biological endpoints (tissue concentrations of contaminants and assessment of benthic community effects following placement). This range of monitoring endpoints will enable multiple facets of the amendment performance to be examined under an active harbor setting, including the feasibility of deep water material placement, the stability of material placement, the extent to which material placement reduces tissue residue concentrations of PCBs and mercury, together with the observable impact or enhancement of the structure, diversity, or density of the benthic community.
Reactive amendment technology for contaminated sediments is designed to enhance system recovery by introducing a chemical sorbent to impacted surface sediment. The chemical composition of the sorbent is selected based on the nature of sediment contamination and the extent to which amendment properties require specific implementation strategies. Among the amendments tested, AC has shown promising results at pilot scale for reducing the bioavailability of hydrophobic organic contaminants such as PCBs. However, to date most applications have been pilot-scale and used granulated AC, which may not be suitable for delivery and stability in deep water active harbors due to its low density.
This project will demonstrate an AC amendment over a relatively large-scale footprint (approximately 20,000 ft2) in an active DoD harbor area using a conventional deep water capping technology (AquaBlock). The AC will be combined with a clay and aggregate substrate to form a composite particle that will readily fall through the water. The placement process followed by natural (bioturbation) and ship-driven mixing will be relied on to incorporate the material into the surface sediment layer and the subsequent degree of transport, mixing, and reduction in bioavailability will be monitored over time. The initial layer is expected to be mixed vertically into the sediment by natural bioturbation and physical mixing by propeller wash. During this mixing period, the AC and clay mineral components will separate from the aggregate, distribute into the sediment, and bind the target contaminants. The extent to which the amendment redistributes laterally and is mixed vertically after placement and the extent to which this results in the anticipated reduction in bioavailability will be assessed at multiple postplacement sampling intervals and compared to conditions immediately following placement. The bentonite-based clay minerals are also known to have a high cation exchange and binding capacity for metals. Therefore, the composite amendment may also be effective for the co-occurring mercury contamination at the site.
Reactive amendments have the potential for widespread application at DoD contaminated sediment sites. Demonstration of these amendments with bioavailability-based monitoring techniques will form the basis for an integrated remedy strategy. Implementation is expected to result in (1) significantly improved remedy performance compared to conventional isolation capping, reduced construction costs compared to dredging, and reduced long-term monitoring costs compared to monitored natural recovery, and (2) reduced carbon footprint and improved sustainability as compared to removal, transport and disposal associated with dredging, minimization of impacts to benthic communities as compared to conventional isolation capping and dredging approaches, and the potential to treat isolated areas such as beneath piers that are currently difficult to remediate and constitute high potential risks for recontamination of areas remediated through other more expensive approaches, such as conventional dredging. Cost savings will be substantial if sediment amendments such as AC can be easily delivered in composite particle form, remain stable, and perform effectively to reduce PCB and mercury bioavailability in active DoD harbors. (Anticipated Project Completion - 2015)