Sediment caps can be a cost-effective method for managing contaminated sediments or post-dredging residuals in situ. Adding "active" layers within a cap to promote contaminant sequestration or degradation within the cap makes this approach very attractive. The objectives of "active" capping are to 1) stabilize contaminated sediments, 2) isolate sediment contaminants and prevent them from contacting benthic organisms or entering the water column, 3) encourage degradation or sequestration of sediment contaminants, and 4) promote habitat restoration with select benthic and aquatic species. Many environmental factors, e.g. groundwater seepage, NAPL seepage, and gas ebullition can affect the ability of sediment caps to contain and isolate sediment contaminants from the benthos and aquatic species. An "active" capping demonstration was recently conducted in the Anacostia River (Washington DC, USA). "active" caps evaluated included AquablokTM for hydraulic containment/control, coke for sequestering hydrophobic contaminants, and apatite for sequestering heavy metals. Methods to place each of these caps were evaluated, as well as the ability of their ability to sequester contaminants and mitigate contaminant transport through the cap. The ability of other types of cap amendments to degrade contaminants within the cap was also evaluated.
Coke, activated carbon, organoclay, and apatite can sequester hydrophobic organic contaminants and heavy metals and significantly improved the performance of traditional sand caps, and provide decades to centuries of reduced flux compared to uncapped sediments. Fe0 and bionsoil can be used to enhance contaminant degradation within the cap. Geotextiles can be used to accurately place thin (1.25-cm) layers of high value reactive materials within sediment caps in the field. Cap and treat options include in-cap scenarios where contaminants are degraded within the "active" cap layer, or sub-cap scenarios where the cap isolates contaminants for periods of decades to centuries while the contaminants are degraded within the underlying sediment. Impermeable caps, e.g., AquablokTM, can be combined with permeable reactive caps, e.g. Fe0 or bionsoil to create a "funnel and gate" system for in situ management/treatment of contaminated sediments. There is a high potential for coupling in situ capping/treatment with habitat restoration. The state of knowledge and implications from the Anacostia River demonstration project, and the ongoing research needs required to make cap and treat technologies robust, viable, and publicly accepted treatment alternatives for contaminated sediments is discussed. Example applications both in the United States and in Russia will be provided.