An assessment of the design of in situ management approaches for contaminated sediments
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Sediments serve as the ultimate sink for many hydrophobic organic compounds and thus present a residual environmental risk many years after sources of contamination are eliminated. Monitored natural attenuation and ex situ treatment processes are often ineffective for treatment; as such in situ remediation technologies (i.e., capping) are under review. A conventional in situ remediation technology for refractory sediment contaminants is placement of a clean layer of material as a cap. A series of design models was developed to predict the performance of caps composed of the traditional material, sand. A passive sampling method using polydimethylsiloxane (PDMS) fibers for evaluating the performance of caps was developed and tested in the laboratory. The results of the laboratory analysis showed the ability to measure pore water concentration profiles in caps, the consistency of profiles with design model predictions, and correlation of PDMS-derived concentrations with contaminant uptake in test organisms. Potentially more effective caps composed of permeable adsorptive materials (to retard contaminant migration) and impermeable materials (to divert groundwater flow) were placed along with a conventional sand cap in the Anacostia River in Washington DC in 2004. Field tests of this site showed the ability to measure in situ pore water concentration profiles in caps using a field-deployable version of the PDMS passive sampling device and demonstrated the necessity of pore water-based approaches for analyzing caps. A model for assessing the uptake rates of HOCs within PDMS fibers was developed and shown to predict the kinetics of HOC sorption into fibers. The model is based on external-mass transport processes, which through a series of analyses were shown to be more significant than internal diffusion in PDMS fibers. Using the PDMS approach, field bioaccumulation tests at the Anacostia site as well as at San Diego Bay and Hunters Point Naval Shipyard showed stronger correlation of PDMS-based pore water concentrations than solid-phase concentrations with observations of bioaccumulation in the field. The overall conclusions suggest that pore water concentrations can often be a better indicator of risk than bulk solid concentrations.