Current technologies for remediating contaminated sediments include removal followed by treatment and disposal; in situ isolation of the sediments from the environment by covering the sediment with a sand or gravel cap (i.e., passive capping); and monitored natural recovery, which involves monitoring processes that isolate, degrade, transform, and immobilize contaminated sediments under natural conditions. However, these remedial alternatives offer only temporary solutions, do not address a wide variety of organic and inorganic contaminants, may not be applicable for both marine and freshwater sediments, and may be destructive to the benthic environment.
Active or reactive capping is the application of a relatively thin layer of reactive material to the sediment to physically and chemically reduce contaminant mobility and/or bioavailability. This addressed high priority research needs related to developing/selecting active capping materials and cap designs for contaminant sequestration under a range of aquatic sediment conditions and assessing the ability of innovative amendments to immobilize a variety of organic and inorganic contamination and resist erosion in situ. The work was conducted in two phases. Phase I was focused on identifying and evaluating promising sequestering materials for active caps that stabilize sediment contaminants and resist physical disturbance (Final Report: Part I). The objectives of Phase II were 1) the development and evaluation of multiple amendment active caps (MAACs) technology developed under Phase I for sorption and desorption of contaminants, 2) prediction of contaminant release over time from MAAC formulations by numerical modeling, 3) evaluation of MAAC resistance to erosion, and 4) assessment of MAAC toxicity to aquatic organisms (Final Report: Part II).
The active capping technology under study consisted of the in situ application of phosphate materials, organoclays, and biopolymer products. The amendments were selected based on the proven ability of phosphate-based materials to stabilize metals, of organoclays to bind nonpolar pollutants such as polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs), and of biopolymers and their cross-link networks to act as plugging agents that bind contaminants. We theorized that phosphate amendments, organoclays, and the cross-link biopolymer products would complement each other to stabilize a wider range of organic and inorganic compounds than they could individually. This project included laboratory studies that researched fundamental aspects of active cap design followed by a pilot-scale field study that evaluated newly developed active caps under realistic conditions. Numerical simulations were used to determine how active caps composed of promising amendments and amendment mixtures affected the diffusive and advective transport of contaminants from the sediment surface into the water column. Procedures were developed for making biopolymer materials that contributed to erosion resistance.
This project identified beneficial active cap materials, active cap compositions, and the effects of active cap components on metal bioavailability, retention, toxicity, and erosion under laboratory and field conditions. Apatite, organoclay, and cross-linked biopolymers showed high potential for the design of an environmentally benign, multiple-amendment active cap that is effective for the remediation of organic and inorganic contaminants in fresh and salt water. In situ bioassays employing multiple organisms showed the advantages of providing realistic conditions of exposure and provided one of the first quantitative measures of the toxicity of active cap amendments. The results showed that apatite and organoclay at concentrations useful for remediation are acceptable to benthic organisms. Numerical modeling was used to evaluate the long term performance of active cap amendments. The results showed that reactive amendments were effective in delaying the release of contaminants compared to passive cap materials. The field deployment provided a realistic evaluation of the ability of the innovative capping technologies to control the movement, bioavailability, and environmental toxicity of contaminants commonly found at Department of Defense installations and other sites. Lastly, this project tested a new method, diffusion gradients in thin-films (DGT), for evaluating active caps in the field. Because of its ability to mimic the uptake of contaminants by biota (i.e., bioavailability), DGT may be able to more accurately assess the performance of active caps than other analytical techniques.
Active capping has the potential for in situ remediation of a variety of contaminants in a range of sediments (fresh and salt water), especially in the areas where dredging or passive capping may not be effective or practical. This project improved the understanding of active cap design and deployment, sequestration mechanisms occurring within active caps and underlying sediments, and the environmental impact of active caps. It also resulted in the development of a flexible multi-amendment active cap design that can be used to remediate a variety of contaminants and contaminant mixtures under a range of environmental conditions.